Mutable objects and hashCode
- by robert
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.
