Java Reference
In-Depth Information
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.
Example 8.2-3. Inheritance of
public
and
protected
Class Members
Given the class
Point
:
package points;
public class Point {
public int x, y;
protected int useCount = 0;
static protected int totalUseCount = 0;
public void move(int dx, int dy) {
x += dx; y += dy; useCount++; totalUseCount++;
}
}
the
public
and
protected
fields
x
,
y
,
useCount
, and
totalUseCount
are inherited in all sub-
classes of
Point
.
Therefore, this test program, in another package, can be compiled successfully:
class Test extends points.Point {
public void moveBack(int dx, int dy) {
x -= dx; y -= dy; useCount++; totalUseCount++;
}
}
