Question: Have the following class:

```
public class Member {
private int x;
private long y;
private double d;

public Member(int x, long y, double d) {
    this.x = x;
    this.y = y;
    this.d = d;
}

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + x;
    result = (int) (prime * result + y);
    result = (int) (prime * result + Double.doubleToLongBits(d));
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj) {
        return true;
    }
    if (obj instanceof Member) {
        Member other = (Member) obj;
        return other.x == x && other.y == y
                && Double.compare(d, other.d) == 0;
    }
    return false;
}

public static void main(String[] args) {
    Set<Member> test = new HashSet<Member>();
    Member b = new Member(1, 2, 3);
    test.add(b);
    System.out.println(b.hashCode());
    b.x = 0;
    System.out.println(b.hashCode());
    Member first = test.iterator().next();
    System.out.println(test.contains(first));
    System.out.println(b.equals(first));
           System.out.println(test.add(first));

}

```

}

It produces the following results:  

`30814
29853
false
true
true`

Because the hashCode depends of the state of the object it can no longer by retrieved properly, so the check for containment fails. The HashSet in no longer working properly. A solution would be to make Member immutable, but is that the only solution? Should all classes added to HashSets be immutable? Is there any other way to handle the situation?

Regards.

Answer: 
Objects in hashsets should *either* be immutable, *or* you need to exercise discipline in not changing them after they've been used in a hashset (or hashmap).

In practice I've rarely found this to be a problem - I rarely find myself needing to use complex objects as keys or set elements, and when I do it's usually not a problem just not to mutate them. Of course if you've exposed the references to other code by this time, it can become harder.
Theoretically (and more often than not practically too) your class either:

1. has a natural immutable identity that can be inferred from a subset of its fields, in which case you can use those fields to generate the `hashCode` from.
2. has no natural identity, in which case using a `Set` to store them is unnecessary, you could just as well use a `List`.