在Java1.4及以前，子類方法如果要覆蓋超類的某個方法，必須具有完全相同的方法簽名，包括返回值也必須完全一樣。
Java5.0放寬了這一限制，只要子類方法與超類方法具有相同的方法簽名，或者子類方法的返回值是超類方法的子類型，就可以覆蓋。
注意："協變返回(covariant return)"，僅在subclass的返回類型是superclass返回類型的extension時才被容許。
 

package com.eric.news;

public class Point2D {

 protected int x, y;
 
 public Point2D() {
  this.x = 0;
  this.y = 0;
 }
 public Point2D(int x, int y) {
  this.x = x;
  this.y = y;
 }
 
}

package com.eric.news;

public class Point3D extends Point2D {

 protected int z;
 
 public Point3D(int x, int y) {
  this(x, y, 0);
 }
 public Point3D(int x, int y, int z) {
  this.x = x;
  this.y = y;
  this.z = z;
 }
}

package com.eric.news;

public class Position2D {
 Point2D location;
 
 public Position2D() {
  this.location = new Point2D();
 }
 
 public Position2D(int x, int y) {
  this.location = new Point2D(x, y);
 }
 
 public Point2D getLocation() {
  return location;
 }
}

package com.eric.news;

public class Position3D extends Position2D {

 Point3D location;
 
 public Position3D(int x, int y, int z) {
  this.location = new Point3D(x, y, z);
 }
 
 public Point3D getLocation() {
  return location;
 }
}