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.)

Answer 2

HashSet implements Set. The ApiDoc specifies:

Note: Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set.

In your example this is the case, because changing name or age on p1 affects equal-comparison. So according the ApiDoc, the behavior of Set in your case is unspecified.

Answer 3

I think you need to have hashCode depends on mutable fields quite often indeed: when you override equals that depends on mutable fields.

From hashCode's contract: "If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result."

So, if you create two objects such that A.equals(B) is true, and then modify A such a way that you get A.equals(B) became false, you need to have hashCodes change too.

It's true that in hashCode's documentation is stated that "It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results.", but I don't know how this can help.