HashSet包含自定义对象的问题 [英] HashSet contains problem with custom objects

问题描述

我的自定义类将由HashSet包含

My Custom class that will be contained by HashSet

public class Person {
    String name;
    int age;

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

    @Override
    public String toString() {
        return "Person{" +
                "hashcode='" + this.hashCode() + '\'' +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Person)) return false;

        Person person = (Person) o;

        if (age != person.age) return false;
        if (!name.equals(person.name)) return false;

        return true;
    }

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

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

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

我的HashSet测试失败

My HashSet test that fails

   public void hashSetTest() {
        Set<Person>  personSet = new HashSet<Person>();
        Person p1 = new Person("raghu", 12);
        Person p2 = new Person("rimmu", 21);

        personSet.add(p1);
        personSet.add(p2);


       p1.setName("raghus");
       p1.setAge(13);

       int i2 =p1.hashCode();
       System.out.println(personSet.size() + ": "+ p1.hashCode()+" : "+personSet.contains(p1)+ " : "+i2);
    }

我希望personSet.contains（p1）通过。为什么它返回假？
谢谢
sri

Iam expecting personSet.contains(p1) to pass. Why is it returning false? Thanks sri

推荐答案

因为 p1.hashCode（）修改 p1 时的更改，因此无法再在哈希表中的原始索引处找到它。永远不要让哈希值依赖于一个可变字段。

Because p1.hashCode() changes when you modify p1, so it can't be found at its original index in the hash table anymore. Never let a hash value depend on a mutable field.

（你很幸运，它在测试期间失败了;它可能也成功了，但只是失败了生产。）

(You're quite lucky that it fails during testing; it might just as well have succeeded, only to fail in production.)

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