public class TestEquals {
   
    class Cat {
        
        int color ;
        int weight ;
        int height ;
        
        public Cat (int color ,int weight ,int height) {
            
            this.color = color ;
            this.weight = weight ;
            this.height = height ;
        }
    }
   
    public boolean equals(Object obj) {
        
        if (obj == null) {
            return false ;
        }
        
        else {
            if (obj instanceof Cat) {
                Cat c = (Cat) obj ;
               
                if(c.color == this.color && c.weight == this.weight && c.height == this.height){
                    return true ;
                }
               
                else {
                    return false ;
                }
            }
        }
    }
   
   
    public static void main (String[] args) {
        
        Cat c1 = new Cat (1 ,2 ,3);
        Cat c2 = new Cat (1 ,2 ,3);
        
        //System.out.println (c1 == c2);
        System.out.println (c1.equals (c2) );
    }
}

第一类错误：
  这里不能想当然的用this 用个其他方法替代吧

第二类错误：
  改法1：  1.先创建一个对象 2.再用赋值语句赋值
 改法2：    color = ncolor ;
            weight = nweight ;
            height = nheight ;注意参数和数据成员的名称最好不一样
      这样你的new语句应该没问题了

public class TestEquals2 {
   
    class Cat {
        
        int color ;
        int weight ;
        int height ;
        
        public Cat (int _color ,int _weight ,int _height) {
            
            color = _color ;
            weight = _weight ;
            height = _height ;
        }
    }
   
    public boolean equals(Object obj) {
        
        if (obj == null) {
            return false ;
        }
        
        else {
            if (obj instanceof Cat) {
                Cat c = (Cat) obj ;
               
                if(c.color == _color && c.weight == _weight && c.height == _height){
                    return true ;
                }
               
                else {
                    return false ;
                }
            }
        }
    }
   
   
    public static void main (String[] args) {
        
        Cat c1 = new Cat (1 ,2 ,3);
        Cat c2 = new Cat (1 ,2 ,3);
        
        //System.out.println (c1 == c2);
        System.out.println (c1.equals (c2) );
    }
}

我把this和变量名改了，可是还是这样？

我知道错在哪里了，我定义的Cat类是内部类，要建立内部类对象才可以访问到。
或者把内部类提出来就可以了。

这是我修改后的代码，OK了。

public class TestEqualsKey {
     
    public static void main (String[] args) {
        
        Cat c1 = new Cat (1 ,2 ,3);
        Cat c2 = new Cat (1 ,2 ,3);
        
        //System.out.println (c1 == c2);
        System.out.println (c1.equals (c2) );
    }
}


class Cat {
        
    int color ;
    int weight ;
    int height ;
   
    public Cat (int color ,int weight ,int height) {
        
        this.color = color ;
        this.weight = weight ;
        this.height = height ;
    }
   
    public boolean equals(Object obj) {

        if (obj == null) {
            return false ;
        }
        
        else {
            if (obj instanceof Cat) {
                Cat c = (Cat) obj ;
               
                if(c.color == this.color && c.weight == this.weight && c.height == this.height){
                    return true ;
                }
               
                else {
                    return false ;
                }
            }
        }
        
        return false ;
    }
}