When does the compiler insist that the class must be abstract

If one or more methods of the class are abstract. If class inherits one or more

abstract methods from the parent abstract class and no implementation is provided for

that method If class implements an interface and provides no implementation for

those methods

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.

What is Marker interface? How is it used in Java?

The marker interface is a design pattern, used with languages that provide run-time type information about objects. It provides a way to associate metadata with a class where the language does not have explicit support for such metadata. To use this pattern, a class implements a marker interface, and code that interact with instances of that class test for the existence of the interface. Whereas a typical interface specifies methods that an implementing class must support, a marker interface does not do so. The mere presence of such an interface indicates specific behavior on the part of the implementing class. There can be some hybrid interfaces, which both act as markers and specify required methods, are possible but may prove confusing if improperly used. Java utilizes this pattern very well and the example interfaces are:

io.Serializable – Serializability of a class is enabled by the class implementing the java.io.Serializable interface. The Java Classes that do not implement Serializable interface will not be able to serialize or deserializ their state. All subtypes of a serializable class are themselves serializable. The serialization interface has no methods or fields and serves only to identify the semantics of being serializable.
rmi.Remote – The Remote interface serves to identify interfaces whose methods may be invoked from a non-local virtual machine. Any object that is a remote object must directly or indirectly implement this interface. Only those methods specified in a “remote interface”, an interface that extends java.rmi.Remote are available remotely.
lang.Cloneable – A class implements the Cloneable interface to indicate to the Object.clone() method that it is legal for that method to make a field-for-field copy of instances of that class. Invoking Object’s clone method on an instance that does not implement the Cloneable interface results in the exception CloneNotSupportedException being thrown.
servlet.SingleThreadModel – Ensures that servlets handle only one request at a time. This interface has no methods.
util.EvenListener – A tagging interface that all event listener interfaces must extend.
The “instanceof” keyword in java can be used to test if an object is of a specified type. So this keyword in combination with Marker interface can be used to take different actions based on type of interface an object implements.

How can we implement polymorphism in Java ?

Polymorphism is the capability of an action or method to do different things based on the object that it is acting upon.

There are two types of polymorphism:-

Method Polymorphism through overloading.
Object polymorphism by inheritance / interfaces.

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

What is the List interface?

The List interface provides support for ordered collections of objects.

What is RMI and steps involved in developing an RMI object

Remote Method Invocation (RMI) allows java object that executes on one machine and to invoke the method of a Java object to execute on another machine. The steps involved in developing an RMI object are: a) Define the interfaces b) Implementing these interfaces c) Compile the interfaces and their implementations with the java compiler d) Compile the server implementation with RMI compiler e) Run the RMI registry f) Run the application

How are Java source code files named

within the file. A source code file may contain at most one public class or interface. If a public class or interface is defined A Java source code file takes the name of a public class or interface that 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 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.

What is the Map interface

The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.