mutable fields for objects in a Java Set

Question

Am I correct in assuming that if you have an object that is contained inside a Java Set<> (or as a key in a Map<> for that matter), any fields that are used to determi

Answer 1

Yes, that will cause bad things to happen.

// Given that the Toy class has a mutable field called 'name' which is used
// in equals() and hashCode():
Set toys = new HashSet();
Toy toy = new Toy("Fire engine", ToyType.WHEELED_VEHICLE, Color.RED);
toys.add(toy);
System.out.println(toys.contains(toy)); // true
toy.setName("Fast truck");
System.out.println(toys.contains(toy)); // false