在Java1.4及以前，子类方法如果要覆盖超类的某个方法，必须具有完全相同的方法签名，包括返回值也必须完全一样。
Java5.0放宽了这一限制，只要子类方法与超类方法具有相同的方法签名，或者子类方法的返回值是超类方法的子类型，就可以覆盖。
注意："协变返回(covariant return)"，仅在subclass的返回类型是superclass返回类型的extension时才被容许。
 

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;
 }
}