Java Reference
In-Depth Information
class Point {
int x = 0, y = 0, color;
void move(int dx, int dy) { x += dx; y += dy; }
int getX() { return x; }
int getY() { return y; }
}
class RealPoint extends Point {
float x = 0.0f, y = 0.0f;
void move(int dx, int dy) { move((float)dx, (float)dy); }
void move(float dx, float dy) { x += dx; y += dy; }
float getX() { return x; }
float getY() { return y; }
}
Here, the class
Point
provides methods
getX
and
getY
that return the values of its fields
x
and
y
; the class
RealPoint
then overrides these methods by declaring methods with the
same signature. The result is two errors at compile time, one for each method, because
the return types do not match; the methods in class
Point
return values of type
int
, but
the wanna-be overriding methods in class
RealPoint
return values of type
float
.
This program corrects the errors of the preceding program:
class Point {
int x = 0, y = 0;
void move(int dx, int dy) { x += dx; y += dy; }
int getX() { return x; }
int getY() { return y; }
int color;
}
class RealPoint extends Point {
float x = 0.0f, y = 0.0f;
void move(int dx, int dy) { move((float)dx, (float)dy); }
void move(float dx, float dy) { x += dx; y += dy; }
int getX() { return (int)Math.floor(x); }
int getY() { return (int)Math.floor(y); }
}
Here, the overriding methods
getX
and
getY
in class
RealPoint
have the same return
types as the methods of class
Point
that they override, so this code can be successfully
compiled.
Consider, then, this test program:
