浅谈java重写equals方法的种种坑

x.equals(x) = x.equals(x) (好像很傻)
x.equals(y) = y.equals(x）(这也是最容易出现问题的地方）
x.equals(y) = y.equals(z) 那么x.equals(z) == true

public class CaseInsensitiveString {

 private final String s;//存有不可变字符串s

 public CaseInsensitiveString(String s) {
  this.s = s;
 }

 @Override //重写equals方法，请注意参数为Object
 public boolean equals(Object o) {
  if (o instanceof CaseInsensitiveString)//判断传入的是同类型的参数
  {
   return s.equalsIgnoreCase(((CaseInsensitiveString) o).s);
  }
  if (o instanceof String) {
   return s.equalsIgnoreCase(((String) o));
  }
  return false;
 }
}

 //也确实看起来实现了功能
 public static void main(String[] args)
 {
  //不区分大小写的字符串Point
  CaseInsensitiveString cis = new CaseInsensitiveString("Point");
  String s = "point";
  System.out.printf(cis.equals(s)+"");//true
  
 }

 //s.equals(cis)却返回了false
 public static void main(String[] args)
 {
  CaseInsensitiveString cis = new CaseInsensitiveString("Point");
  String s = "point";
  System.out.printf(cis.equals(s)+"");//true
  System.out.printf(s.equals(cis)+"");//false
 }

public class CaseInsensitiveString {

 private final String s;//存有不可变字符串s

 public CaseInsensitiveString(String s) {
  this.s = s;
 }

 @Override //重写equals方法，请注意参数为Object
 public boolean equals(Object o) {
  if (o instanceof CaseInsensitiveString)//判断传入的是同类型的参数
  {
   return s.equalsIgnoreCase(((CaseInsensitiveString) o).s);
  }
  /*删除掉的与String互操作的代码*/
  return false;
 }

}

public class Point {
 private final int x;
 private final int y;
 public Point(int x,int y)
 {
  this.x=x;
  this.y=y;
 }
 @Override
 public boolean equals(Object o )
 {
  if(o instanceof Point)
  {
   Point p = (Point)o;
   return (this.x == p.x && this.y==p.y);
  }
  return false;
 }
}

public class ColorPoint extends Point{
 private final Color color;
 public ColorPoint(int x,int y,Color color)
 {
  super(x,y);
  this.color=color;
 }
}

public class ColorPoint extends Point{
 private final Color color;
 public ColorPoint(int x,int y,Color color)
 {
  super(x,y);
  this.color=color;
 }
 @Override
 public boolean equals(Object o)
 {
  if(o instanceof ColorPoint)
  {
   return (super.equals(o)&& this.color==
   ( (ColorPoint) o ).color);//使用父类的equals方法
  }
  return false;
 }
}

 public static void main(String[] args)
 {
  Point p = new Point(1,2);
  ColorPoint cp = new ColorPoint(1,2,Color.RED);
  System.out.printf(p.equals(cp)+"");//true
  System.out.printf(cp.equals(p)+"");//false

 }

public class ColorPoint{
 private final Color color;
 private final Point point;
 public ColorPoint(int x,int y,Color color)
 {
  point = new Point(x,y);
  this.color=color;
 }
 /*view*/
 public Point asPoint()
 {
  return point;
 }
 
 @Override
 public boolean equals(Object o)
 {
  if(o instanceof ColorPoint)
  {
   ColorPoint cp =(ColorPoint)o;
   return this.point.equals(cp.point)&&
   this.color.equals(cp.color);//调用Point以及Color类的equals方法
  }
  //那么有色点与无色点会被判断为不相同而返回false
  return false;
 }
}