JavaHashSet操作与Person对象的equals和hashCode方法-优快云博客

本文链接：https://blog.youkuaiyun.com/gycaaa/article/details/130110974

代码示例展示了当Person类基于id和name重写equals和hashCode方法时，HashSet的行为。首先添加两个Person对象到集合中，然后修改一个对象的name属性，尝试移除该对象但失败，因为根据新的name属性已经找不到它。接着添加一个新的Person对象，虽然有相同的id和新的name，导致集合中有两个相同hashCode的对象。最后，尝试再次添加一个具有原始name的新对象，结果是集合中包含三个对象。

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已知: Person类按照 id 和 name 重写了 hashCode 和 equals 方法,问下面代码输出什么?

HashSet set = new HashSet(0);
Person p1 = new Person(1001,"AA");

Person p2 = new Person(1002,"BB");

set.add(p1);
set.add(p2);
p1.name = "CC";

set.remove(p1);
System.out.println(set);
set.add(new Person(1001,"CC"));System.out.println(set);
set.add(new Person(1001,"AA"));System.out.println(set);

public class Homework05 {
    public static void main(String[] args) {
        HashSet set = new HashSet(0);
        Person p1 = new Person(1001,"AA");
        Person p2 = new Person(1002,"BB");

        set.add(p1); //ok
        set.add(p2); //ok
        p1.name = "CC";

        set.remove(p1);
        System.out.println(set);
        set.add(new Person(1001,"CC"));
        System.out.println(set);
        set.add(new Person(1001,"AA"));
        System.out.println(set);
    }
}
class Person{
    public int id;
    public String name;

    public Person(int id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Person person = (Person) o;
        return id == person.id && Objects.equals(name, person.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, name);
    }
}

输出：

[Person{id=1001, name='CC'}, Person{id=1002, name='BB'}]
[Person{id=1001, name='CC'}, Person{id=1001, name='CC'}, Person{id=1002, name='BB'}]
[Person{id=1001, name='CC'}, Person{id=1001, name='CC'}, Person{id=1001, name='AA'}, Person{id=1002, name='BB'}]