CODEWITHSUNDEEP
javahashmap
zeds
zeds

Reputation: 137

Changing Hashcode in runtime

Everybody knows that it's better to use an immutable class as key in HashMap, and if we change object state then the JVM recalculates the hash code for that object.

Can someone make example where after changing the state of an object its hashcode changes? (in runtime)

Upvotes: 0

Views: 2050

Answers (3)

Eran
Eran

Reputation: 393771

Suppose Person class is mutable, and hashCode and equals are computed based on two properties - firstName and lastName.

Person p1 = new Person ("John", "Smith");
Person p2 = new Person ("John", "Doe");
Set<Person> persons = new HashSet<Person>();
persons.add (p1);
System.out.println(persons.contains(p2)); // prints false
p2.setLastName("Smith");
System.out.println(persons.contains(p2)); // prints true

And even worse :

Person p1 = new Person ("John", "Smith");
Person p2 = new Person ("John", "Doe");
Set<Person> persons = new HashSet<Person>();
persons.add (p1);
System.out.println(persons.contains(p2)); // prints false
persons.add (p2);
p2.setLastName("Smith");
System.out.println(persons.size()); // prints 2
System.out.println(p1.equals(p2)); // prints true

The HashSet is broken - it contains two equal instances.

The same example would apply for HashMaps where the key is Person.

Upvotes: 0

MeBigFatGuy
MeBigFatGuy

Reputation: 28568

If you use an object of the following class in a HashMap, and call setState while it's in the map, you will have problems.

Class MutableExample {
    private int state;

    public void setState(int s) {
        state = s;
    }

    public int hashCode() {
        return state;
    }

    public boolean equals(Object o) {
        if (!(o instanceof MutableExample)) {
            return false;
        }

        return ((MutableExample) o).state == state;
    }
}

Upvotes: 1

chiastic-security
chiastic-security

Reputation: 20520

public class Something {

    public String blah;

    public int hashCode() {
        return blah.hashCode();
    }
}

Obviously you wouldn't quite code things like this (you'd have a getter and a setter, and the hashCode would be a little more complicated, and you'd have an equals() too) but this meets the requirements of your question. If you change the blah field, then the hash code will change.

Upvotes: 1

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