HashSet contains problem with custom objects

问题

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

回答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.)

回答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.

回答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.

回答4:

Hashes are simple pairing of key and values. Here's how the state of your code would look before and after the renaming in pseudo-code:

Before:

personSet => {
    SOME_NUM1 => Person(name=>"raghu", 12),
    SOME_NUM2 => Person(name=>"rimmu", 21)
}

p1.setName("raghus"); #p1.hashcode() = SOME_NEW_NUM
p1.setAge(13);#p1.hashcode() = SOME_OTHER_NEW_NUM

After:

personSet => {
    SOME_NUM1 => Person(name=>"raghu", 13),
    SOME_NUM2 => Person(name=>"rimmu", 21)
}

Since you have direct access to the p1 the object within the HashSet is updated correctly, but HashSet does not pay attention to contained objects hashcodes being updated. When a call to personSet.contains(p1) is called, the HashSet looks for an entry with the new value of p1.hashcode().

The p1 object is associated with its previous hashcode at the time when it was added to the HashSet.

来源：https://stackoverflow.com/questions/5110376/hashset-contains-problem-with-custom-objects

标签

java

hashcode

hashset