8.2.1

Examples of Inheritance

CLASSES

package points;

public class Point {

int x, y;

public void move(int dx, int dy) { x += dx; y += dy; }

}

and:

package points;

public class Point3d extends Point {

int z;

public void move(int dx, int dy, int dz) {

x += dx; y += dy; z += dz;

}

}

and a third compilation unit, in another package, is:

import points.Point3d;

class Point4d extends Point3d {

int w;

public void move(int dx, int dy, int dz, int dw) {

x += dx; y += dy; z += dz; w += dw; //

compile time errors

}

}

Here both classes in the

points

 package compile. The class

Point3d

 inherits the

fields

x

 and

y

 of class

Point

, because it is in the same package as

Point

. The

class

Point4d

, which is in a different package, does not inherit the fields

x

 and

y

of class

Point

 or the field

z

 of class

Point3d

, and so fails to compile.

A better way to write the third compilation unit would be:

import points.Point3d;

class Point4d extends Point3d {

int w;

public void move(int dx, int dy, int dz, int dw) {

super.move(dx, dy, dz); w += dw;

}

}

using the

move

 method of the superclass

Point3d

 to process

dx

,

dy

, and

dz

. If

Point4d

 is written in this way it will compile without errors.

8.2.1.2

Inheritance with

public

and

protected

Given the class

Point

:

package points;

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