CLASSES

Class Modifiers

8.1.2

8.1.2.1

abstract

Classes

An

abstract

 class is a class that is incomplete, or to be considered incomplete.

Only

abstract

 classes may have

abstract

 methods ( 8.4.3.1,  9.4), that is,

methods that are declared but not yet implemented. If a class that is not

abstract

contains an

abstract

 method, then a compile time error occurs. A class has

abstract

 methods if any of the following is true:

It explicitly contains a declaration of an

abstract

 method ( 8.4.3).

It inherits an

abstract

 method from its direct superclass ( 8.1.3).

A direct superinterface ( 8.1.4) of the class declares or inherits a method

(which is therefore necessarily

abstract

) and the class neither declares nor

inherits a method that implements it.

In the example:

abstract class Point {

int x = 1, y = 1;

void move(int dx, int dy) {

x += dx;

y += dy;

alert();

}

abstract void alert();

}

abstract class ColoredPoint extends Point {

int color;

}

class SimplePoint extends Point {

void alert() { }

}

a class

Point

 is declared that must be declared

abstract

, because it contains a

declaration of an

abstract

 method named

alert

. The subclass of

Point

 named

ColoredPoint

 inherits the

abstract

 method

alert

, so it must also be declared

abstract

. On the other hand, the subclass of

Point

 named

SimplePoint

 pro 

vides an implementation of

alert

, so it need not be

abstract

.

A compile time error occurs if an attempt is made to create an instance of an

abstract

 class using a class instance creation expression ( 15.8). An attempt to

instantiate an

abstract

 class using the

newInstance

 method of class

Class

( 20.3.6) will cause an

InstantiationException

 ( 11.5.1) to be thrown. Thus,

continuing the example just shown, the statement:

Point p = new Point();

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