重新equals有必要重写hashCode吗

￡神魔★判官ぃ 2022-03-15 05:40 207阅读 0赞

答案是有的，为啥？

因为equlas一样的时候，hashCode一定一样

当hashCode一样的时候，equals不一定一样

如果你不重写的话如何保证hashCode也一样？是吧

下面举个栗子

public class People {
        private int id;
    
        private String name;
    
        public int getId() {
            return id;
        }
    
        public void setId(int id) {
            this.id = id;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (!(o instanceof People)) return false;
            People people = (People) o;
            return getId() == people.getId();
        }
    
        /*@Override
        public int hashCode() {
            return Objects.hash(getId());
        }*/
    
        public static void main(String[] args) {
    
            People people=new People();
            people.setId(1);
            people.setName("大鸡腿");
    
            People people1=new People();
            people1.setId(1);
            people1.setName("ooo");
    
            System.out.println(people.equals(people1));
            System.out.println(people.hashCode()+" "+people1.hashCode());
    
            Map<People,String> map=new HashMap<People,String>();
            map.put(people,"1");
            map.put(people1,"2");
            for (People m:map.keySet()){
                System.out.println(m.getName());
            }
    
        }
    }

输出：

true  
815033865 1555093762  
ooo  
大鸡腿

可以看到equals一样，而hashCode不一样哦

所以重写

@Override
        public int hashCode() {
            return Objects.hash(getId());
        }