CLASSES

Method Modifiers

8.4.3

An instance method that is not

abstract

 can be overridden by an

abstract

method. For example, we can declare an

abstract

 class

Point

 that requires its

subclasses to implement

toString

 if they are to be complete, instantiable classes:

abstract class Point {

int x, y;

public abstract String toString();

}

This

abstract

 declaration of

toString

 overrides the non 

abstract toString

method of class

Object

 ( 20.1.2). (Class

Object

 is the implicit direct superclass

of class

Point

.) Adding the code:

class ColoredPoint extends Point {

int color;

public String toString() {

return super.toString() + ": color " + color; //

error

}

}

results in a compile time error because the invocation

super.toString()

 refers

to method

toString

 in class

Point

, which is

abstract

 and therefore cannot be

invoked. Method

toString

 of class

Object

 can be made available to class

ColoredPoint

only if class

Point

 explicitly makes it available through some

other method, as in:

abstract class Point {

int x, y;

public abstract String toString();

protected String objString() { return super.toString(); }

}

class ColoredPoint extends Point {

int color;

public String toString() {

return objString() + ": color " + color;

//

correct

}

}

8.4.3.2

static

Methods

A method that is declared

static

 is called a

class method

. A class method is

always invoked without reference to a particular object. An attempt to reference

the current object using the keyword

this

 or the keyword

super

 in the body of a

class method results in a compile time error. It is a compile time error for a

static

 method to be declared

abstract

.

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