Java J2ee Planet

What is the difference between interface and an abstract class?  This is the first question most of the java interviewers start in interviews. Ofcourse it seems a silly question and can answer. But if interview goes depth of the concept of abstract class and interface, most of the candidates not able to answer. To overcome that ignorance just read the points that I have highlighted below. 1. Abstract class is a class which contain one or more abstract methods, which has to be implemented by sub classes. An abstract class can contain no abstract methods also i.e. abstract class may contain concrete methods. A Java Interface can contain only method declarations and public static final constants and doesn't contain their implementation. The classes which implement the Interface must provide the method definition for all the methods present. 2. Abstract class definition begins with the keyword "abstract" keyword followed by Class definition. An Interface definition be...

According to JLS: 8.1.3 Inner Classes and Enclosing Instances , inner classes may not declare static initializers or member interfaces. Inner classes may not declare static members, unless they are compile-time constant fields. According to 8.5.2 Static Member Type Declarations , "Member interfaces are always implicitly static. It is permitted but not required for the declaration of a member interface to explicitly list the static modifier". They are always top-level, not inner. Therefore, an interface can't be defined in an inner class. The member interface can only be defined in inside a top-level class or interface. For example, interface OuterInterface { // Top-level interface interface NestedInterface {// top-level nested interface } }

In design, you want the base class to present only an interface for its derived classes. This means, you don't want anyone to actually instantiate an object of the base class. You only want to upcast to it (implicit upcasting, which gives you polymorphic behavior), so that its interface can be used. This is accomplished by making that class abstract using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents it. The interface keyword takes this concept of an abstract class a step further by preventing any method or function implementation at all. You can only declare a method or function but not provide the implementation. The class, which is implementing the interface, should provide the actual implementation. The interface is a very useful and commonly used aspect in OO design, as it provides the separation of interface and implementation and enables you to: