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Java Interview Questions and Answers


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.

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.

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.

Name two subclasses of the TextComponent class.
TextField and TextArea

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.

Which containers may have a MenuBar?
Frame

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.

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.

What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.

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.

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.

What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.

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.

Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.

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.

What is the difference between the String and StringBuffer classes?
String objects are constants. StringBuffer objects are not.

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. 

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