Class or method modifier that indicates that some or all behavior must be implemented in a subclass.

refers to an abstract class containing at least one abstract method. If more than one method labeled abstract is present, the class must be itself labeled abstract. An abstract class is most often used to create other classes: abstract class Instrument { public abstract void play( Note n ); public String what() { return "Instrument"; } } class Wind extends Instrument { public void play( Note n ) { System.out.println("Wind.play() " + n); } public String what() { return "Wind";

A Java keyword used in a class definition to specify that a class is not to be instantiated, but rather inherited by other classes. An abstract class can have abstract methods that are not implemented in the abstract class, but in subclasses.

The term abstract refers (in Java) , generally, to something that can not be physically realized. You can preface a method declaration with the keyword abstract. This allows you to provide a method declaration without a definition (similar to how you declare any method within an interface). Doing so cuases the class itself to become an abstract class. This means that you can no longer create an instance of that class - it is no longer physically realizable. Any descendents of that abstract class, however, must either provide a definition for any abstract classes or, in turn, be abstract classes themselves (the second technique is seldomly used). Using abstract methods/classes can be very useful within an inheritance heirarchy.

The abstract keyword is used to declare abstract methods and classes. An abstract method has no implementation defined; it is declared with arguments and a return type as usual, but the body enclosed in curly braces is replaced with a semicolon. The implementation of an abstract method is provided by a subclass of the class in which it is defined. If an abstract method appears in a class, the class is also abstract.