CLASSES

Examples of Inheritance

8.2.1

class Point {

int x, y;

private Point() { reset(); }

Point(int x, int y) { this.x = x; this.y = y; }

private void reset() { this.x = 0; this.y = 0; }

}

class ColoredPoint extends Point {

int color;

void clear() { reset(); }

//

error

}

class Test {

public static void main(String[] args) {

ColoredPoint c = new ColoredPoint(0, 0); //

error

c.reset();

//

error

}

}

causes four compile time errors:

An error occurs because

ColoredPoint

 has no constructor declared with two

integer parameters, as requested by the use in

main

. This illustrates the fact

that

ColoredPoint

 does not inherit the constructors of its superclass

Point

.

Another error occurs because

ColoredPoint

 declares no constructors, and

therefore a default constructor for it is automatically created ( 8.6.7), and this

default constructor is equivalent to:

ColoredPoint() { super(); }

which invokes the constructor, with no arguments, for the direct superclass of

the class

ColoredPoint

. The error is that the constructor for

Point

 that takes

no arguments is

private

, and therefore is not accessible outside the class

Point

, even through a superclass constructor invocation ( 8.6.5).

Two more errors occur because the method

reset

 of class

Point

 is

private

,

and therefore is not inherited by class

ColoredPoint

. The method invoca 

tions in method

clear

 of class

ColoredPoint

 and in method

main

 of class

Test

 are therefore not correct.

8.2.1   Examples of Inheritance

This section illustrates inheritance of class members through several examples.

8.2.1.1

Example: Inheritance with Default Access

Consider the example where the

points

 package declares two compilation units:

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