java中如何重写hashCode()和equals()方法

结论：我们都知道，如果是引用类型要作为hashmap的key或者set的值，需要重写hashCode()和equals方法。

那么如何重写hashCode()和equals方法以及技巧是什么呢？

1.hashCode()和equals方法在java中每个类都默认有，因为所有的类都是Object类的子类，可以查看Object的hashCode()和equals方法的默认实现，代码如下：

    public native int hashCode();

    public boolean equals(Object obj) {
        return (this == obj);    
    }



    

hashCode()方法是调用c底层的原生方法，equals()默认表示两个对象的存储地址是否相同。

2.测试

准备实体类：

static class User{
        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getIdNum() {
            return idNum;
        }

        public void setIdNum(String idNum) {
            this.idNum = idNum;
        }

        private int age;
        private String name;
        private String idNum;

        @Override
        public boolean equals(Object obj) {
            if (obj instanceof User) {
                User user = (User) obj;

                if (user.getIdNum().equals(this.idNum) && user.getName().equals(this.name) && user.getAge() == this.age) {
                    return true;
                } else {
                    return false;
                }
            } else {
                return false;
            }
        }
        }

 用equals测试是否相等：

public static void main(String[] args){
        User user1 = new User();
        user1.setAge(10);
        user1.setName("Alice");
        user1.setIdNum("000000000001");

        User user2 = new User();
        user2.setAge(10);
        user2.setName("Alice");
        user2.setIdNum("000000000001");


        System.out.println("user1.equals(user2) == "+user1.equals(user2));
    }

输出：

user1.equals(user2) == true

用Map和Set测试是否能作为Key:

public static void main(String[] args){
        User user1 = new User();
        user1.setAge(10);
        user1.setName("Alice");
        user1.setIdNum("000000000001");

        User user2 = new User();
        user2.setAge(10);
        user2.setName("Alice");
        user2.setIdNum("000000000001");


        System.out.println("user1.equals(user2) == "+user1.equals(user2));

        Map map = new HashMap<>();

        Set set = new HashSet<>();

        map.put(user1,10);
        map.put(user2,10);

        set.add(user1);
        set.add(user2);

        System.out.println("map.keySet().size() is: "+map.keySet().size());

        System.out.println("set.size() is: "+set.size());
    }

输出：

user1.equals(user2) == true
map.keySet().size() is: 2
set.size() is: 2

由此可以得出结论，引用类作为map的key需要重写hashCode

实体类User类重写hashCode后为：

static class User{
        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getIdNum() {
            return idNum;
        }

        public void setIdNum(String idNum) {
            this.idNum = idNum;
        }

        private int age;
        private String name;
        private String idNum;

        @Override
        public int hashCode(){
            int value = name.hashCode();
            value = value*31 + idNum.hashCode();
            value = value*31 + age;
            return value;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj instanceof User) {
                User user = (User) obj;

                if (user.getIdNum().equals(this.idNum) && user.getName().equals(this.name) && user.getAge() == this.age) {
                    return true;
                } else {
                    return false;
                }
            } else {
                return false;
            }
        }



        }

测试后size结果都是1了。