java - HashSet contains problem with custom objects

Question

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;
    }
}

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);
    }

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

Answer 1

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.

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