134. What is the difference between Swing and AWT components?
AWT components are heavy-weight, whereas Swing components are lightweight. Heavy weight components depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button.
135. Why Java does not support pointers?
Because pointers are unsafe. Java uses reference types to hide pointers and programmers feel easier to deal with reference types without pointers. This is why Java and C-sharp shine.
136. Parsers? DOM vs SAX parser
Parsers are fundamental xml components, a bridge between XML documents and applications that process that XML. The parser is responsible for handling xml syntax, checking the contents of the document against constraints established in a DTD or Schema.
DOM
1. Tree of nodes
2. Memory: Occupies more memory, preffered for small XML documents
3. Slower at runtime
4. Stored as objects
5. Programmatically easy
6. Ease of navigation
SAX
1. Sequence of events
2. Doesn't use any memory preferred for large documents
3. Faster at runtime
4. Objects are to be created
5. Need to write code for creating objects
6. Backward navigation is not possible as it sequentially processes the document
137. Can you declare a class as private?
Yes, we can declare a private class as an inner class. For example,
class MyPrivate {
private static class MyKey {
String key = "12345";
}
public static void main(String[] args) {
System.out.println(new MyKey().key);//prints 12345
}
}
138. What is the difference between shallow copy and deep copy?
Shallow copy shares the same reference with the original object like cloning, whereas the deep copy get a duplicate instance of the original object. If the shallow copy has been changed, the original object will be reflected and vice versa.
139. Can one create a method which gets a String and modifies it?
No. In Java, Strings are constant or immutable; their values cannot be changed after they are created, but they can be shared. Once you change a string, you actually create a new object. For example:
String s = "abc"; //create a new String object representing "abc"
s = s.toUpperCase(); //create another object representing "ABC"
140. Why is multiple inheritance not possible in Java?
It depends on how you understand "inheritance". Java can only "extends" one super class, but can "implements" many interfaces; that doesn't mean the multiple inheritance is not possible. You may use interfaces to make inheritance work for you. Or you may need to work around. For example, if you cannot get a feature from a class because your class has a super class already, you may get that class's feature by declaring it as a member field or getting an instance of that class. So the answer is that multiple inheritance in Java is possible.
141. Can Java code be compiled to machine dependent executable file?
Yes. There are many tools out there. If you did so, the generated exe file would be run in the specific platform, not cross-platform.
142. What is the relationship between synchronized and volatile keyword?
The JVM is guaranteed to treat reads and writes of data of 32 bits or less as atomic.(Some JVM might treat reads and writes of data of 64 bits or less as atomic in future) For long or double variable, programmers should take care in multi-threading environment. Either put these variables in a synchronized method or block, or declare them volatile.
143. This class (IncrementImpl) will be used by various threads concurrently; can you see the inherent flaw(s)? How would you improve it?
public class IncrementImpl {
private static int counter = 0;
public synchronized void increment() {
counter++;
}
public int getCounter() {
return counter;
}
}
The counter is static variable which is shared by multiple instances of this class. The increment() method is synchronized, but the getCounter() should be synchronized too. Otherwise the Java run-time system will not guarantee the data integrity and the race conditions will occur. The famous producer/consumer example listed at Sun's thread tutorial site will tell more.
one of solutions
public class IncrementImpl {
private static int counter = 0;
public synchronized void increment() {
counter++;
}
public synchronized int getCounter() {
return counter;
}
}
144. What are the drawbacks of inheritance?
Since inheritance inherits everything from the super class and interface, it may make the subclass too clustering and sometimes error-prone when dynamic overriding or dynamic overloading in some situation. In addition, the inheritance may make peers hardly understand your code if they don't know how your super-class acts and add learning curve to the process of development.
Usually, when you want to use a functionality of a class, you may use subclass to inherit such function or use an instance of this class in your class. Which is better, depends on your specification.
145. Is there any other way that you can achieve inheritance in Java?
There are a couple of ways. As you know, the straight way is to "extends" and/or "implements". The other way is to get an instance of the class to achieve the inheritance. That means to make the supposed-super-class be a field member. When you use an instance of the class, actually you get every function available from this class, but you may lose the dynamic features of OOP
146. Two methods have key words static synchronized and synchronized separately. What is the difference between them?
Both are synchronized methods. One is instance method, the other is class method. Method with static modifier is a class method. That means the method belongs to class itself and can be accessed directly with class name and is also called Singleton design. The method without static modifier is an instance method. That means the instance method belongs to its object. Every instance of the class gets its own copy of its instance method.
When synchronized is used with a static method, a lock for the entire class is obtained. When synchronized is used with a non-static method, a lock for the particular object (that means instance) of the class is obtained.
Since both methods are synchronized methods, you are not asked to explain what is a synchronized method. You are asked to tell the difference between instance and class method. Of course, your explanation to how synchronized keyword works doesn't hurt. And you may use this opportunity to show your knowledge scope.
147. How do you create a read-only collection?
The Collections class has six methods to help out here:
1. unmodifiableCollection(Collection c)
2. unmodifiableList(List list)
3. unmodifiableMap(Map m)
4. unmodifiableSet(Set s)
5. unmodifiableSortedMap(SortedMap m)
6. unmodifiableSortedSet(SortedSet s)
If you get an Iterator from one of these unmodifiable collections, when you call remove(), it will throw an UnsupportedOperationException.
148. Can a private method of a superclass be declared within a subclass?
Sure. A private field or method or inner class belongs to its declared class and hides from its subclasses. There is no way for private stuff to have a runtime overloading or overriding (polymorphism) features.
149. Why Java does not support multiple inheritance ?
This is a classic question. Yes or No depends on how you look at Java. If you focus on the syntax of "extends" and compare with C++, you may answer 'No' and give explanation to support you. Or you may answer 'Yes'. Recommend you to say 'Yes'.
Java DOES support multiple inheritance via interface implementation. Some people may not think in this way. Give explanation to support your point.
150. What is the difference between final, finally and finalize?
Short answer:
final - declares constant
finally - relates with exception handling
finalize - helps in garbage collection
If asked to give details, explain:
final field, final method, final class
try/finally, try/catch/finally
protected void finalize() in Object class
151. What kind of security tools are available in J2SE 5.0?
There are three tools that can be used to protect application working within the scope of security policies set at remote sites.
keytool -- used to manage keystores and certificates.
jarsigner -- used to generate and verify JAR signatures.
policytool -- used for managing policy files.
There are three tools that help obtain, list and manage Kerberos tickets.
kinit -- used to obtain Kerberos V5 tickets.
tklist -- used to list entries in credential cache and key tab.
ktab -- used to help manage entries in the key table.
152. How to make an array copy from System?
There is a method called arraycopy in the System class. You can do it:
System.arraycopy(sourceArray, srcOffset, destinationArray, destOffset, numOfElements2Copy);
When you use this method, the destinationArray will be filled with the elements of sourceArray at the length specified.
153. Can we use System.arraycopy() method to copy the same array?
Yes, you can. The source and destination arrays can be the same if you want to copy a subset of the array to another area within that array.
154. What is shallow copy or shallow clone in array cloning?
Cloning an array invloves creating a new array of the same size and type and copying all the old elements into the new array. But such copy is called shallow copy or shallow clone because any changes to the object would be reflected in both arrays.
155. When is the ArrayStoreException thrown?
When copying elements between different arrays, if the source or destination arguments are not arrays or their types are not compatible, an ArrayStoreException will be thrown.
156. How to check two arrays to see if contents have the same types and contain the same elements?
One of options is to use the equals() method of Arrays class.
Arrays.equals(a, b);
If the array types are different, a compile-time error will happen.
157. Do not use the String contatenation operator in lengthy loops or other places where performance could suffer. Is that true?
Yes.
158. The superclass constructor runs before the subclass constructor. The subclass's version of the overridable method will be invoked before the subclass's constructor has been invoked. If the subclass's overridable method depends on the proper initialization of the subclass (through the subclass constructor), the method will most likely fail. Is that true?
Yes. It is true
159. Why are the interfaces more flexible than abstract classes?
--An interface-defined type can be implemented by any class in a class hierarchy and can be extended by another interface. In contrast, an abstract-class-defined type can be implemented only by classes that subclass the abstract class.
--An interface-defined type can be used well in polymorphism. The so-called interface type vs. implementation types.
--Abstract classes evolve more easily than interfaces. If you add a new concrete method to an abstract class, the hierarchy system is still working. If you add a method to an interface, the classes that rely on the interface will break when recompiled.
--Generally, use interfaces for flexibility; use abstract classes for ease of evolution (like expanding class functionality).
160. What are new language features in J2SE 5.0?
Generally:
1. generics
2. static imports
3. annotations
4. typesafe enums
5. enhanced for loop
6. autoboxing/unboxing
7. varargs
8. covariant return types
161. What is covariant return type?
A covariant return type lets you override a superclass method with a return type that subtypes the superclass method's return type. So we can use covariant return types to minimize upcasting and downcasting.
class Parent {
Parent foo () {
System.out.println ("Parent foo() called");
return this;
}
}
class Child extends Parent {
Child foo () {
System.out.println ("Child foo() called");
return this;
}
}
class Covariant {
public static void main(String[] args) {
Child c = new Child();
Child c2 = c.foo(); // c2 is Child
Parent c3 = c.foo(); // c3 points to Child
}
}
162. What is the result of the following statement?
int i = 1, float f = 2.0f;
i += f; //ok, the cast done automatically by the compiler
i = i + f; //error
The compound assignment operators automatically include cast operations in their behaviors.
163. The following statement prints true or false, why?
byte[] a = { 1, 2, 3 };,
byte[] b = (byte[]) a.clone();
System.out.println(a == b);
The false will be printed out. Because the two arrays have distinctive memory addresses. Starting in Java 1.2, we can use java.util.Arrays.equals(a, b) to compare whether two arrays have the same contents.
164. Why do we need to use getSystemResource() and getSystemResources() method to load resources?
Because we want to look for resources strictly from the system classpath, These methods use the system ClassLoader to locate resources, which gives you stricter control of the resources used by the application.
165.ArithmeticException?
The ArithmeticException is thrown when integer is divided by zero or taking the remainder of a number by zero. It is never thrown in floating-point operations.
166. What is a transient variable?
A transient variable is a variable that may not be serialized.
167. Which containers use a border Layout as their default layout?
The window, Frame and Dialog classes use a border layout as their default layout.
168. Why do threads block on I/O?
Threads block on I/O (that is enters the waiting state) so that other threads may execute while the I/O Operation is performed.
169. How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
170. What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.
171. Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.
172. What's new with the stop(), suspend() and resume() methods in JDK 1.2??
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
173. Is null a keyword?
The null value is not a keyword.
174. What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the component to display normally.
175. What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout.
176. Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
177. What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.
178. What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.
179. which characters may be used as the second character of an identifier, but not as the first character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.
180. What is the List interface?
The List interface provides support for ordered collections of objects.
181. How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
182. What is the Vector class?
The Vector class provides the capability to implement a growable array of objects
183. What modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
184. What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.
185. What is the difference between the >> and >>> operators?
The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
186. Which method of the Component class is used to set the position and size of a component?
setBounds()
187. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.
188. What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.
189. Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.
190. Is sizeof a keyword?
The sizeof operator is not a keyword.
191. What are wrapper classes?
Wrapper classes are classes that allow primitive types to be accessed as objects.
192. Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.
193. What restrictions are placed on the location of a package statement within a source code file?
A package statement must appear as the first line in a source code file (excluding blank lines and comments).
194. Can an object's finalize() method be invoked while it is reachable?
An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects.
195. What is the immediate superclass of the Applet class?
Panel
196. What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
197. Name three Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.
198. What value does readLine() return when it has reached the end of a file?
The readLine() method returns null when it has reached the end of a file.
199. What is the immediate superclass of the Dialog class?
Window.
200. What is clipping?
Clipping is the process of confining paint operations to a limited area or shape.
201. What is a native method?
A native method is a method that is implemented in a language other than Java.
202. Can a for statement loop indefinitely?
Yes, a for statement can loop indefinitely. For example, consider the following:
for(;;) ;
203. What are order of precedence and associativity, and how are they used?
Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left
204. When a thread blocks on I/O, what state does it enter?
A thread enters the waiting state when it blocks on I/O.
205. To what value is a variable of the String type automatically initialized?
The default value of a String type is null.
206. What is the catch or declare rule for method declarations?
If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.
207. What is the difference between a MenuItem and a CheckboxMenuItem?
The CheckboxMenuItem class extends the MenuItem class to support a menu item that may be checked or unchecked.
208. What is a task's priority and how is it used in scheduling?
A task's priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.
209. What class is the top of the AWT event hierarchy?
The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.
210. When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.
211. Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.
212. What is the range of the short type?
The range of the short type is -(2^15) to 2^15 - 1.
213. What is the range of the char type?
The range of the char type is 0 to 2^16 - 1.
214. In which package are most of the AWT events that support the event-delegation model defined?
Most of the AWT-related events of the event-delegation model are defined in the java.awt.event package. The AWTEvent class is defined in the java.awt package.
215. What is the immediate superclass of Menu?
What is the immediate superclass of Menu?
MenuItem
216. What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
217. Which class is the immediate superclass of the MenuComponent class.
Object
218. What invokes a thread's run() method?
After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.
219. What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.
220. Name three subclasses of the Component class.
Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent
221. What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.
222. Which Container method is used to cause a container to be laid out and redisplayed?
validate()
223. What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.
224. How many times may an object's finalize() method be invoked by the
garbage collector?
An object's finalize() method may only be invoked once by the garbage collector.
225. What is the purpose of the finally clause of a try-catch-finally statement?
garbage collector?
The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.
226. What is the argument type of a program's main() method?
A program's main() method takes an argument of the String[] type.
227. Which Java operator is right associative?
The = operator is right associative.
228. What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
229. Can a double value be cast to a byte?
Yes, a double value can be cast to a byte.
230. What is the difference between a break statement and a continue statement?
A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.
231. What must a class do to implement an interface?
It must provide all of the methods in the interface and identify the interface in its implements clause.
232. What method is invoked to cause an object to begin executing as a separate thread?
The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.
233. Name two subclasses of the TextComponent class.
TextField and TextArea
234. What is the advantage of the event-delegation model over the earlier event-inheritance model?
The event-delegation model has two advantages over the event-inheritance model. First, it enables event handling to be handled by objects other than the ones that generate the events (or their containers). This allows a clean separation between a component's design and its use. The other advantage of the event-delegation model is that it performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to repeatedly process unhandled events, as is the case of the event-inheritance
model.
235. Which containers may have a MenuBar?
Frame
236. How are commas used in the initialization and iteration parts of a for statement?
Commas are used to separate multiple statements within the initialization and iteration parts of a for statement.
237. What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object's wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object's notify() or notifyAll() methods.
238. What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.
239. How are Java source code files named?
A Java source code file takes the name of a public class or interface that is defined within the file. A source code file may contain at most one public class or interface. If a public class or interface is defined within a source code file, then the source code file must take the name of the public class or interface. If no public class or interface is defined within a source code file, then the file must take on a name that is different than its classes and interfaces. Source code files use the .java extension.
240. What is the relationship between the Canvas class and the Graphics class?
A Canvas object provides access to a Graphics object via its paint() method.
241. What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.
242. What value does read() return when it has reached the end of a file?
The read() method returns -1 when it has reached the end of a file.
243. Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.
244. What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.
245. What is the difference between the String and StringBuffer classes?
String objects are constants. StringBuffer objects are not.
246. If a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within the class in which it is declared.
247. What is an object's lock and which objects have locks?
An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.
248. What is the Dictionary class?
The Dictionary class provides the capability to store key-value pairs.
249. How are the elements of a BorderLayout organized?
The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center of a container.
250. What is the % operator?
It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.
251. When can an object reference be cast to an interface reference?
An object reference be cast to an interface reference when the object implements the referenced interface.
252. What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can have a menu bar.
253. Which class is extended by all other classes?
The Object class is extended by all other classes.
254. Can an object be garbage collected while it is still reachable?
A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected..
255. Is the ternary operator written x : y ? z or x ? y : z ?
It is written x ? y : z.
256. What is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.
257. How is rounding performed under integer division?
The fractional part of the result is truncated. This is known as rounding toward zero.
258. What happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.
259. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.
260. What classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.
261. If a class is declared without any access modifiers, where may the class be accessed?
A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.
262. What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
263. What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.
264. Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its super classes.
265. For which statements does it make sense to use a label?
The only statements for which it makes sense to use a label are those statements that can enclose a break or continue statement.
266. What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.
267. Which TextComponent method is used to set a TextComponent to the read-only state?
setEditable()
268. How are the elements of a CardLayout organized?
The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.
269. Is &&= a valid Java operator?
No, it is not.
267. Name the eight primitive Java types.
The eight primitive types are byte, char, short, int, long, float, double, and boolean.
268. Which class should you use to obtain design information about an object?
The Class class is used to obtain information about an object's design.
269. What is the relationship between clipping and repainting?
When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.
270. Is "abc" a primitive value?
The String literal "abc" is not a primitive value. It is a String object.
271. What is the relationship between an event-listener interface and an event-adapter class?
An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event adapter provides a default implementation of an event-listener interface.
272. What restrictions are placed on the values of each case of a switch statement?
During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.
273. What modifiers may be used with an interface declaration?
An interface may be declared as public or abstract.
274. Is a class a subclass of itself?
A class is a subclass of itself.
275. What is the highest-level event class of the event-delegation model?
The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.
276. What event results from the clicking of a button?
The ActionEvent event is generated as the result of the clicking of a button.
277. How can a GUI component handle its own events?
A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.
278. What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
279. How are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.
280. What advantage do Java's layout managers provide over traditional windowing systems?
Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.
281. What is the Collection interface?
The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates.
282. What modifiers can be used with a local inner class?
A local inner class may be final or abstract.
283. What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.
284. What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.
285. What is the purpose of the File class?
The File class is used to create objects that provide access to the files and directories of a local file system.
286. Can an exception be rethrown?
Yes, an exception can be rethrown.
287. Which Math method is used to calculate the absolute value of a number?
The abs() method is used to calculate absolute values.
288. How does multithreading take place on a computer with a single CPU?
The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.
289. When does the compiler supply a default constructor for a class?
The compiler supplies a default constructor for a class if no other constructors are provided.
290. When is the finally clause of a try-catch-finally statement executed?
The finally clause of the try-catch-finally statement is always executed unless the thread of execution
terminates or an exception occurs within the execution of the finally clause.
291. Which class is the immediate superclass of the Container class?
Component
292. If a method is declared as protected, where may the method be accessed?
A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.
293. How can the Checkbox class be used to create a radio button?
By associating Checkbox objects with a CheckboxGroup.
294. Which non-Unicode letter characters may be used as the first character of an identifier?
The non-Unicode letter characters $ and _ may appear as the first character of an identifier
295. What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.
296. What happens when you invoke a thread's interrupt method while it is sleeping or waiting?
When a task's interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.
297. What is casting?
There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
298. What is the return type of a program's main() method?
A program's main() method has a void return type.
299. Name four Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane
300. What is the difference between a Choice and a List?
A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items.
301. What class of exceptions are generated by the Java run-time system?
The Java runtime system generates RuntimeException and Error exceptions.
302. What class allows you to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects from input streams.
303. What is the difference between a field variable and a local variable?
A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.
304. Under what conditions is an object's finalize() method invoked by the garbage collector?
The garbage collector invokes an object's finalize() method when it detects that the object has become unreachable.
305. How are this () and super () used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
306. What is the relationship between a method's throws clause and the exceptions that can be thrown during the method's execution?
A method's throws clause must declare any checked exceptions that are not caught within the body of the method.
307. What is the difference between the JDK 1.02 event model and the event-delegation model introduced with JDK 1.1?
The JDK 1.02 event model uses an event inheritance or bubbling approach. In this model, components are required to handle their own events. If they do not handle a particular event, the event is inherited by (or bubbled up to) the component's container. The container then either handles the event or it is bubbled up to its container and so on, until the highest-level container has been tried.
In the event-delegation model, specific objects are designated as event handlers for GUI components. These objects implement event-listener interfaces. The event-delegation model is more efficient than the event-inheritance model because it eliminates the processing required to support the bubbling of unhandled events.
308. How is it possible for two String objects with identical values not to be equal under the == operator?
The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.
309. Why are the methods of the Math class static?
So they can be invoked as if they are a mathematical code library.
310. What Checkbox method allows you to tell if a Checkbox is checked?
getState()
311. What state is a thread in when it is executing?
An executing thread is in the running state.
312. What are the legal operands of the instanceof operator?
The left operand is an object reference or null value and the right operand is a class, interface, or array type.
313. How are the elements of a GridLayout organized?
The elements of a GridBad layout are of equal size and are laid out using the squares of a grid.
314. What an I/O filter?
An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
315. If an object is garbage collected, can it become reachable again?
Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.
316. What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.
317. What classes of exceptions may be thrown by a throw statement?
A throw statement may throw any expression that may be assigned to the Throwable type.
318. What are E and PI?
E is the base of the natural logarithm and PI is mathematical value pi.
319. Are true and false keywords?
The values true and false are not keywords.
320. What is a void return type?
A void return type indicates that a method does not return a value.
321. What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.
322. What is the difference between the File and RandomAccessFile classes?
The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.
323. What happens when you add a double value to a String?
The result is a String object.
324. What is your platform's default character encoding?
If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1..
325. Which package is always imported by default?
The java.lang package is always imported by default.
326. What interface must an object implement before it can be written to a stream as an object?
An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.
327. How are this and super used?
this is used to refer to the current object instance. super is used to refer to the variables and methods of the superclass of the current object instance.
328. What is the purpose of garbage collection?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and
reused.
329. What is a compilation unit?
A compilation unit is a Java source code file.
330. What interface is extended by AWT event listeners?
All AWT event listeners extend the java.util.EventListener interface.
331. What restrictions are placed on method overriding?
Overridden methods must have the same name, argument list, and return type.
The overriding method may not limit the access of the method it overrides.
The overriding method may not throw any exceptions that may not be thrown
by the overridden method.
332. How can a dead thread be restarted?
A dead thread cannot be restarted.
333. What happens if an exception is not caught?
An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown.
334. What is a layout manager?
A layout manager is an object that is used to organize components in a container.
335. Which arithmetic operations can result in the throwing of an ArithmeticException?
Integer / and % can result in the throwing of an ArithmeticException.
336. What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its
(deprecated) suspend() method.
337. Can an abstract class be final?
An abstract class may not be declared as final.
338. What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.
339. What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement?
The exception propagates up to the next higher level try-catch statement (if any) or results in the program's termination.
340. What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. In numerical promotion, byte, char, and short values are converted to int
values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.
341. What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.
342. What is the difference between a public and a non-public class?
A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.
343. To what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.
344. Can try statements be nested?
Try statements may be tested.
345. What is the difference between the prefix and postfix forms of the ++ operator?
The prefix form performs the increment operation and returns the value of the increment operation. The postfix form returns the current value all of the expression and then performs the increment operation on that value.
346. What is the purpose of a statement block?
A statement block is used to organize a sequence of statements as a single statement group.
347. What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.
348. What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.
349. What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.
350. How does a try statement determine which catch clause should be used to handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed.
The remaining catch clauses are ignored.
351. Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.
352. When is an object subject to garbage collection?
An object is subject to garbage collection when it becomes unreachable to the program in which it is used.
353. What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
354. What methods are used to get and set the text label displayed by a Button object?
getLabel() and setLabel()
355. Which Component subclass is used for drawing and painting?
Canvas
356. What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement
357. What are the two basic ways in which classes that can be run as threads may be defined?
A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.
358. What are the problems faced by Java programmers who don't use layout managers?
Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system.
359. What is the difference between an if statement and a switch statement?
The if statement is used to select among two alternatives. It uses a boolean expression to decide which alternative should be executed. The switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed.
Java Question only
- What are the differences between a List and a Set?
- What are value objects and how are they used with EJBs?
- How do you code a Singleton in Java?
- What's a dynamic proxy class?
- When should you call SwingUtilities.invokeLater()?
- What are the differences between a Session Bean and an Entity Bean?
- How do you read an XML file in Java?
- Give an example of the Decorator pattern in the Java API.
- Difference between an abstract class and interface
- Inner classes
- Difference between an inner class and a normal class
- What is Entity Bean and Session Bean ?
- What are the methods of Entity Bean?
- How does Stateful Session bean store its state ?
- Why does Stateless Session bean not store its state even though it has ejbActivate and ejbPassivate ?
- What are the services provided by the container ?
- Types of transaction ?
- What is bean managed transaction ?
- Why does EJB needs two interface( Home and Remote Interface) ?
- What are transaction attributes ?
- What is the difference between Container managed persistent bean and Bean managed persistent entity bean ?
- What is J2EE ?
- What is JTS ?
- How many entity beans used and how many tables can u use in EJB project ?
- What is scalable,portability in J2EE?
- What is Connection pooling?Is it advantageous?
- Method and class used for Connection pooling ?
- How to deploy in J2EE(i.e Jar,War file) ?
- How is entity bean created using Container managed entity bean ?
- Sotware architechture of EJB ?
- In Entity bean will the create method in EJB home and ejbCreate in Entity bean have the same parameters ?
- What methods do u use in Servlet - Applet communication ?
- What are the types of Servlet ?
- Difference between HttpServlet and Generic Servlets ?
- Difference between doGet and doPost ?
- What are the methods in HttpServlet?
- What are the types of SessionTracking?
- What is Cookie ? Why is Cookie used ?
- If my browser does not support Cookie,and my server sends a cookie instance What will happen ?
- Why do u use Session Tracking in HttpServlet ?
- Can u use javaScript in Servlets ?
- What is the capacity the doGet can send to the server ?
- What are the type of protocols used in HttpServlet ?
- Difference between TCP/IP and IP protocol ?
- Why do you use UniCastRemoteObject in RMI ?
- How many interfaces are used in RMI?
- Can Rmi registry be written in the code, without having to write it in the command prompt and if yes where?
- Why is Socket used ?
- What are the types of JDBC drivers ?
- Explain the third driver(Native protocol driver) ?
- Which among the four driver is pure Java driver ?
- What are the Isolation level in JDBC transaction ?
- How do you connect with the database ?
- How do you connect without the Class.forName (" ") ?
- What does Class.forName return ?
- What are the types of statement ?
- Why is preparedStatement,CallableStatement used for?
- In real time project which driver did u use ?
- Difference between AWT and Swing compenents ?
- Is there any heavy weight component in Swings ?
- Can the Swing application if you upload in net, be compatible with your browser?
- What should you do get your browser compatible with swing components?
- What are the methods in Applet ?
- When is init(),start() called ?
- When you navigate from one applet to another what are the methods called ?
- What is the difference between Trusted and Untrusted Applet ?
- What is Exception ?
- What are the ways you can handle exception ?
- When is try,catch block used ?
- What is finally method in Exceptions ?
- What are the types of access modifiers ?
- What is protected and friendly ?
- What are the other modifiers ?
- Is synchronised modifier ?
- What is meant by polymorphism ?
- What is inheritance ?
- What is method Overloading ? What is this in OOPS ?
- What is method Overriding ? What is it in OOPS ?
- Does java support multi dimensional arrays ?
- Is multiple inheritance used in Java ?
- How do you send a message to the browser in JavaScript ?
- Does javascript support multidimensional arrays ?
- Why is XML used mainly?
- Do use coding for database connectivity in XML?
- What is DTD ?
- Is there any tool in java that can create reports ?
- Explain protected vs private vs public data members and methods?
- Explain overloading and overriding?
- What is polymorphism?
- What is inheritance?
- What is encapsulation?
- Give an example of where you might use a static method?
- Explain up casting and down casting?
- What is an abstract class?
- Explain multiple inheritance and interfaces?
- If you have a tree of derived classes, what order are the constructors run
- How do you deal with errors?
- What is a ClassLoader?
- How are ClassLoaders used, when and why?
- What is a fully qualified class name?
- What is the classpath, and how does it relate to classloaders?
- When does the JVM invoke a classloader?
- When invoked, how does a classloader handle the loading of a class?
- Can you pass classes between classloader, why/whynot?
- What is a ClassDefNotFoundError?
- What is a ClassCastException?
- What is a CircularityError?
- How are classes stored in the JVM?
- Can CORBA be integrated with XML?
- can I make my site completely safe by running the server in a "chroot" envirinment?
- Whats the "chroot" environment?
- How can I detect if my site's been broken into?
- what types of access restrictions are avilable?
- Isn't there a CGI Script to allow users to change their passwords online?
- What are: SSL, SHTTP, Shen? and Cybercash, SET and Open Market?
- Are there any "freeware" secure services?
- Can I use Personal Certificates to control server access?
- What are Perl taint checks? How do I turn them On?
- How do I untaint a variable?
- How can you push data from an Applet to Servlet ?
- How you can know about drivers and database information ?
- If you are truncated using JDBC, How can you know ..that how much data is truncated ?
- And What situation , each of the 4 drivers used ?
- In RMI, server object first loaded into the memory and then the stub reference is sent to the client ? or whether a stub reference is directly sent to the client ?
Suppose server object is not loaded into the memory, and the client request for it , what will happen?
- Can you load the server object dynamically? If so, what are the major 3 steps involved in it ?
- What is difference RMI registry and OSAgent ?
- What is the corresponding Layout for Card in Swing ?
- What is light weight component ?
- Can you run the product development on all operating systems ?
- What is difference in between Java Class and Bean ?
- Can we send object using Sockets ?
- How to communicate 2 threads each other ?
- What are the files generated after using IDL to Java Compilet ?
- Can I modify an object in CORBA ?
- What is the functionality stubs and skeletons ?
- What is the mapping mechanism used by Java to identify IDL language ?
- Suppose If we have variable ' I ' in run method, If I can create one or more thread each thread will occupy a separate copy or same variable will be shared ?
- In servlets, we are having a web page that is invoking servlets username and password ? which is cheks in the database ? Suppose the second page also If we want to verify the same information whethe it will connect to the database or it will be used previous information?
- What are session variable in Servlets ?
- How will you pass parameters in RMI ? Why u serialize?
- What is the exact difference in between Unicast and Multicast object ? Where we will use ?
- What is the main functionality of the Remote Reference Layer ?
- How do you download stubs from a Remote place ?
- I want to store more than 10 objects in a remote server ? Which methodology will follow ?
What is meant by Session ? Tell me something about HTTPSession Class ?
- Have you used threads in Servelet ?
- How do you sing an Applet ?
- In a Container there are 5 components. I want to display the all the components names, how will you do that one ?
- Why there are some null interface in java ? What does it mean ? Give me some null interfaces in JAVA ?
- Tell me the latest versions in JAVA related areas ?
- What is meant by class loader ? How many types are there? When will we use them ?
- How do you load an Image in a Servlet ?
- What is meant by flickering ?
- What is meant by distributed Application ? Why we are using that in our applications ?
- What is the functionality of the stub ?
- What is the latest version of JDBC ? What are the new features are added in that ?
- What is the role of the webserver ?
- What is meant by cookies ? Explain ?
- How OOPS concept is achieved in Java
- How does Java 2.0 differ from Java 1.0
- What are command line arguments
- Wrapper class. Is String a Wrapper Class
- What are the restriction for static method
- Purpose of the file class
- What is a virtual function in C++?
Simply put, the virtual keyword enables a function to be 'virtual' which then gives possibility for that function to be overridden (redefined) in one or more descendant classes. It is a good feature since the specific function to call is determined at run-time. In other words, a virtual function allows derived classes to replace the implementation provided by the base class.
- What is the difference between private, protected, and public?
These keywords are for allowing privilages to components such as functions and variables.
Public: accessible to all classes
Private: accessible only to the class to which they belong
Protected: accessible to the class to which they belong and any subclasses.
- What is a cartesian product in PL/SQL?
When a Join condition is not specified by the programmer or is invalid(fails), PL/SQL forms a Cartesian product.
In a Cartesian product, all combinations of rows will be displayed.
For example, All rows in the first table are joined to all rows in the second table. It joins a bunch of rows and it's result is rarely useful unless you have a need to combine all rows from all tables.
- What is mutual exclusion? How can you take care of mutual exclusion using Java threads?
Mutual exclusion is where no two processes can access critical regions of memory at the same time.
Java provides many utilities to deal with mutual exclusion with the use of threaded programming.
For mutual exclusion, you can simply use the synchronized keyword and explicitly or implicitly provide an Object, any Object, to synchronize on.
The runtime system/Java compiler takes care of the gruesome details for you. The synchronized keyword can be applied to a class, to a method, or to a block of code. There are several methods in Java used for communicating mutually exclusive threads such as wait( ), notify( ), or notifyAll( ). For example, the notifyAll( ) method wakes up all threads that are in the wait list of an object.
- What are some advantages and disadvantages of Java Sockets?
Some advantages of Java Sockets:
Sockets are flexible and sufficient. Efficient socket based programming can be easily implemented for general communications.
Sockets cause low network traffic. Unlike HTML forms and CGI scripts that generate and transfer whole web pages for each new request, Java applets can send only necessary updated information.
Some disadvantages of Java Sockets:
Security restrictions are sometimes overbearing because a Java applet running in a Web browser is only able to establish connections to the machine where it came from, and to nowhere else on the network
Despite all of the useful and helpful Java features, Socket based communications allows only to send packets of raw data between applications. Both the client-side and server-side have to provide mechanisms to make the data useful in any way.
Since the data formats and protocols remain application specific, the re-use of socket based implementations is limited.
- What is the difference between an Interface and an Abstract class?
An Abstract class declares have at least one instance method that is declared abstract which will be implemented by the subclasses. An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior.
- What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.
- Describe synchronization in respect to multithreading.?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.
- Explain different way of using thread?
The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.
- What are pass by reference and passby value?
Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.
- What is HashMap and Map?
Map is Interface and Hashmap is class that implements that.
- Difference between HashMap and HashTable?
The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is non synchronized and Hashtable is synchronized.
- Difference between Vector and ArrayList?
Vector is synchronized whereas arraylist is not.
- Difference between Swing and Awt?
AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.
- What is the difference between a constructor and a method?
A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.
- What is an Iterators?
Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.
- State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.?
public : Public class is visible in other packages, field is visible everywhere (class must be public too) private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature. protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature. default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.
- What is an abstract class?
Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
- What is static in java?
Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.
- What is final?
A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).
- Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol symbol : class ABCD location: package io import java.io.ABCD;
- Does importing a package imports the subpackages as well?
e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.
- What is the difference between declaring a variable and defining a variable?
In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization. e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.
- What is the default value of an object reference declared as an instance variable?
null unless we define it explicitly.
- Can a level class be private or protected?
No. A level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a level class can not be private. Same is the case with protected.
- What type of parameter passing does Java support?
In Java the arguments are always passed by value .
- Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.
- Objects are passed by value or by reference?
Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .
- What is serialization?
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
- How do I serialize an object to a file?
The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.
- Which methods of Serializable interface should I implement?
The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.
- How can I customize the seralization process?
i.e. how can one have a control over the serialization process?
Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.
- What is an abstract class?
Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
- What is the common usage of serialization?
Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.
- What is Externalizable interface?
Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
- What happens to the object references included in the object?
The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.
- What one should take care of while serializing the object?
One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.
- What if the main method is declared as private?
The program compiles properly but at runtime it will give "Main method not public." message.
- What if the static modifier is removed from the signature of the main method?
Program compiles. But at runtime throws an error "NoSuchMethodError".
- What if I write static public void instead of public static void?
Program compiles and runs properly.
- What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error "NoSuchMethodError".
- What is the first argument of the String array in main method?
The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.
- If I do not provide any arguments on the command line, then the String array of Main method will be empty of null?
It is empty. But not null.
- How can one prove that the array is not null but empty?
Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.
- What environment variables do I need to set on my machine in order to be able to run Java programs?
CLASSPATH and PATH are the two variables.
- Can an application have multiple classes having main method?
Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.
- Can I have multiple main methods in the same class?
No the program fails to compile. The compiler says that the main method is already defined in the class.
- Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.
- Can I import same package/class twice?
Will the JVM load the package twice at runtime?
One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.
- What are Checked and UnChecked Exception?
A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method
checked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.
- What is Overriding?
When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.
- What are different types of inner classes?
They are Nested -level classes, Member classes, Local classes, Anonymous classes
Nested -level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other -level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. -level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested -level variety.
Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested -level class. The primary difference between member classes and nested -level classes is that member classes have access to the specific instance of the enclosing class.
Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.
Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.
- Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol symbol : class ABCD location: package io import java.io.ABCD;
- Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.
- What is the difference between declaring a variable and defining a variable?
In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization. e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.
- What is the default value of an object reference declared as an instance variable?
null unless we define it explicitly.
- Can a level class be private or protected?
No. A level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access. If a level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a level class can not be private. Same is the case with protected.
- What type of parameter passing does Java support?
In Java the arguments are always passed by value .
- Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.
- Objects are passed by value or by reference?
Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .
- What is serialization?
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
- How do I serialize an object to a file?
The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.
- Which methods of Serializable interface should I implement?
The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.
- How can I customize the seralization process?
i.e. how can one have a control over the serialization process? Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.
- What is the common usage of serialization?
Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.
- What is Externalizable interface?
Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
- What happens to the object references included in the object?
The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.
- What one should take care of while serializing the object?
One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.
- What happens to the static fields of a class during serialization?
Are these fields serialized as a part of each serialized object? Yes the static fields do get serialized. If the static field is an object then it must have implemented Serializable interface. The static fields are serialized as a part of every object. But the commonness of the static fields across all the instances is maintained even after serialization.
- How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects. [Received from Venkateswara Manam]
- What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.
- How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
- Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection .
- What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
- When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.
- What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
- What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
- What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
- What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.
- How are this() and super() used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
- Explain the life cycle methods of a Servlet.
The javax.servlet.Servlet interface defines the three methods known as life-cycle method. public void init(ServletConfig config) throws ServletException public void service( ServletRequest req, ServletResponse res) throws ServletException, IOException public void destroy() First the servlet is constructed, then initialized wih the init() method. Any request from client are handled initially by the service() method before delegating to the doXxx() methods in the case of HttpServlet. The servlet is removed from service, destroyed with the destroy() methid, then garbaged collected and finalized.
- What is the difference between the getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface and javax.servlet.ServletContext interface?
The getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface accepts parameter the path to the resource to be included or forwarded to, which can be relative to the request of the calling servlet. If the path begins with a "/" it is interpreted as relative to the current context root.
The getRequestDispatcher(String path) method of javax.servlet.ServletContext interface cannot accepts relative paths. All path must sart with a "/" and are interpreted as relative to curent context root.
- Explain the directory structure of a web application.
The directory structure of a web application consists of two parts. A private directory called WEB-INF A public resource directory which contains public resource folder. WEB-INF folder consists of 1. web.xml
2. classes directory
3. lib directory
- What are the common mechanisms used for session tracking?
Cookies SSL sessions URL- rewriting
- Explain ServletContext.
ServletContext interface is a window for a servlet to view it's environment. A servlet can use this interface to get information such as initialization parameters for the web applicationor servlet container's version. Every web application has one and only one ServletContext and is accessible to all active resource of that application.
- What is preinitialization of a servlet?
A container doesnot initialize the servlets ass soon as it starts up, it initializes a servlet when it receives a request for that servlet first time. This is called lazy loading. The servlet specification defines the element, which can be specified in the deployment descriptor to make the servlet container load and initialize the servlet as soon as it starts up. The process of loading a servlet before any request comes in is called preloading or preinitializing a servlet.
- What is the difference between Difference between doGet() and doPost()?
A doGet() method is limited with 2k of data to be sent, and doPost() method doesn't have this limitation. A request string for doGet() looks like the following: http://www.allapplabs.com/svt1?p1=v1&p2=v2&...&pN=vN
doPost() method call doesn't need a long text tail after a servlet name in a request. All parameters are stored in a request itself, not in a request string, and it's impossible to guess the data transmitted to a servlet only looking at a request string.
- What is the difference between HttpServlet and GenericServlet?
A GenericServlet has a service() method aimed to handle requests.
HttpServlet extends GenericServlet and adds support for doGet(), doPost(), doHead() methods (HTTP 1.0) plus doPut(), doOptions(), doDelete(), doTrace() methods (HTTP 1.1). Both these classes are abstract.
- What is a output comment?
A comment that is sent to the client in the viewable page source.The JSP engine handles an output comment as uninterpreted HTML text, returning the comment in the HTML output sent to the client. You can see the comment by viewing the page source from your Web browser. JSP Syntax Example 1 Displays in the page source:
- What is a Hidden Comment?
A comments that documents the JSP page but is not sent to the client. The JSP engine ignores a hidden comment, and does not process any code within hidden comment tags. A hidden comment is not sent to the client, either in the displayed JSP page or the HTML page source. The hidden comment is useful when you want to hide or "comment out" part of your JSP page. You can use any characters in the body of the comment except the closing --%> combination. If you need to use --%> in your comment, you can escape it by typing --%\>. JSP Syntax Examples
- What is a Expression?
An expression tag contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file. Because the value of an expression is converted to a String, you can use an expression within text in a JSP file. Like You cannot use a semicolon to end an expression
- What is a Declaration?
A declaration declares one or more variables or methods for use later in the JSP source file. A declaration must contain at least one complete declarative statement. You can declare any number of variables or methods within one declaration tag, as long as they are separated by semicolons. The declaration must be valid in the scripting language used in the JSP file.
- What is a Scriptlet?
A scriptlet can contain any number of language statements, variable or method declarations, or expressions that are valid in the page scripting language.Within scriptlet tags, you can
1.Declare variables or methods to use later in the file (see also Declaration).
2.Write expressions valid in the page scripting language (see also Expression).
3.Use any of the JSP implicit objects or any object declared with a tag.
You must write plain text, HTML-encoded text, or other JSP tags outside the scriptlet. Scriptlets are executed at request time, when the JSP engine processes the client request. If the scriptlet produces output, the output is stored in the out object, from which you can display it.
- What are implicit objects? List them?
Certain objects that are available for the use in JSP documents without being declared first. These objects are parsed by the JSP engine and inserted into the generated servlet. The implicit objects re listed below request response pageContext session application out config page exception
- Difference between forward and sendRedirect?
When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completly with in the web container. When a sendRedirtect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completly new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward.
- What are the different scope valiues for the ?
The different scope values for are
1. page
2. request
3.session
4.application
- Explain the life-cycle mehtods in JSP?
THe generated servlet class for a JSP page implements the HttpJspPage interface of the javax.servlet.jsp package. Hte HttpJspPage interface extends the JspPage interface which inturn extends the Servlet interface of the javax.servlet package. the generated servlet class thus implements all the methods of the these three interfaces.
The JspPage interface declares only two mehtods - jspInit() and jspDestroy() that must be implemented by all JSP pages regardless of the client-server protocol.
However the JSP specification has provided the HttpJspPage interfaec specifically for the JSp pages serving HTTP requests.
This interface declares one method _jspService(). The jspInit()- The container calls the jspInit() to initialize te servlet instance.It is called before any other method, and is called only once for a servlet instance.
The _jspservice()- The container calls the _jspservice() for each request, passing it the request and the response objects.
The jspDestroy()- The container calls this when it decides take the instance out of service. It is the last method called n the servlet instance.
Sources :
DEVFYI - Developer Resource - FYI
TechGuider
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