【Java】重载equals和hashCode方法，以保证程序以雇员的id和姓名相同作为雇员实例相同的条件

如果场景中要求对象属性相同即判定为相等，那不能用==，因为==实际是在比地址，只有引用地址相同的实例才会返回true；而需要重载equals和hashCode方法，书写自己的比较规则。

代码：

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Objects;

public class Emp {
    private long id;
    private String name;
    
    public Emp(long id,String name) {
        this.id=id;
        this.name=name;
    }
    
    public Emp() {
        this(-1,null);
    }
    
    @Override
    public boolean equals(Object o) {
        if(this==o) {
            return true;
        }
        
        if(o==null || this.getClass()!=o.getClass()) {
            return false;
        }
        
        Emp emp=(Emp)o;
        
        return id==emp.id && Objects.equals(name, emp.name);
    }
    
    @Override
    public int hashCode() {
        return Objects.hash(id,name);
    }
    
    public String toString() {
        return "#"+id+" "+name;
    }
    
    public long getId() {
        return id;
    }
    public void setId(long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    
    public static void main(String[] args) {
        List ls=new ArrayList<>();
        ls.add(new Emp(1,"Andy"));
        ls.add(new Emp(1,"Andy"));
        ls.add(new Emp(2,"Bill"));
        ls.add(new Emp(2,"Bill"));
        ls.add(new Emp(2,"Bill"));
        ls.add(new Emp(3,"Cindy"));
        ls.add(new Emp(3,"Cindy"));
        ls.add(new Emp(3,"Cindy"));
        ls.add(new Emp(3,"Cindy"));
        
        Map map=new HashMap<>();
        for(Emp emp:ls) {
            if(map.containsKey(emp)) {
                int count=map.get(emp);
                count++;
                map.put(emp, count);
            }else {
                map.put(emp, 1);
            }
        }
        
        for(Map.Entry it:map.entrySet()) {
            System.out.println(it.getKey()+" took place "+it.getValue()+" times.");
        }
    }
}

输出：

#2 Bill took place 3 times.
#3 Cindy took place 4 times.
#1 Andy took place 2 times.

END