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Altering hashCode of object inside of HashSet / HashMap

I am relatively new to Java and am puzzled about the following thing: I usually add objects to an ArrayList before setting its content. I.e.,

List<Bla> list = new ArrayList<>();
Bla bla = new Bla();
list.add(bla);
bla.setContent(); // content influences hashCode

This approach works great. I am concerned whether this approach will give me trouble when used with HashSets or HashMaps. The internal hash table get set at the time the object is added. What will happen if setContent() gets called after the object was added to HashSet or HashMap (and its hashCode changes)?

Should I fully set the (hashCode influencing) content before adding / putting into HashSets or HashMaps? Is it generally recommended to finish building objects before adding them?

Thank you very much for your insights.

Solution

  • What will happen if setContent() gets called after the object was added to HashSet or HashMap (and its hashCode changes)?

    Disaster.

    Should I fully set the (hashCode influencing) content before adding / putting into HashSets or HashMaps? Is it generally recommended to finish building objects before adding them?

    Yes.

    The relevant line of documentation is on java.util.Set:

    Note: Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set. A special case of this prohibition is that it is not permissible for a set to contain itself as an element.

    Generally speaking, this sort of error will manifest itself with elements being both "in" and "not in" your collection, with different methods disagreeing. You may get lucky and your elements may appear to still be in the collection, or they may not; this may happen essentially at random.

    This is one of the many, many reasons why it's excellent practice for most of your objects to be immutable -- completely impossible to modify in the first place after construction.