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: 
Yes. While maintaining your class mutable, you can compute the hashCode and the equals methods based on immutable values of the class ( perhaps a generated id ) to adhere to the [hashCode](http://download.oracle.com/javase/6/docs/api/java/lang/Object.html#hashCode()http://download.oracle.com/javase/6/docs/api/java/lang/Object.html#hashCode()) contract defined in Object class:

> 
> * Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
> * 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.
> * 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. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.
> 
> 
> 

Depending on your situation this may be easier or not.

```
class Member { 
    private static long id = 0;

    private long id = Member.id++;
    // other members here... 

    public int hashCode() { return this.id; }
    public boolean equals( Object o ) { 
        if( this == o ) { return true; }
        if( o instanceOf Member ) { return this.id == ((Member)o).id; }
        return false;
     }
     ...
 }

```

If you need a thread safe attribute, you may consider use: [AtomicLong](http://download.oracle.com/javase/6/docs/api/java/util/concurrent/atomic/AtomicLong.html) instead, but again, it depends on how are you going to use your object.
Jon Skeet has listed all alternatives. As for why the keys in a Map or Set must not change:

The contract of a Set implies that at any time, there are no two objects o1 and o2 such that

```
o1 != o2 && set.contains(o1) && set.contains(o2) && o1.equals(o2)

```

Why that is required is especially clear for a Map. From the contract of Map.get():

> 
> More formally, if this map contains a mapping from a key
>  `k` to a value `v` such that `(key==null ? k==null : key.equals(k))`, then this method returns `v`, otherwise it returns `null`. (There can be at most one such mapping.)
> 
> 
> 

Now, if you modify a key inserted into a map, you might make it equal to some other key already inserted. Moreover, the map can not know that you have done so. So what should the map do if you then do `map.get(key)`, where `key` is equal to several keys in the map? There is no intuitive way to define what that would mean - chiefly because our intuition for these datatypes is the mathematical ideal of sets and mappings, which don't have to deal with changing keys, since their keys are mathematical objects and hence immutable.