Java HashSet with a custom equality criteria? [duplicate]

Question

I was looking for something akin to the Java TreeSet's ability to receive a custom comparator at instantiation time, so I needed not to use the object's default equality (and hash code) criteria.

The closest I could come up with was to wrap my objects in a private custom class, but that seems hacky :( This ends up being a kind of recurring theme when programming, so I was wondering if there's something already available for us to use. Maybe in the commons libraries?

Thanks

Answer 1

Nope, you've found exactly the solution you're supposed to use.

Even for TreeSet, it's frowned upon to use comparison criteria that aren't compatible with equals:

Note that the ordering maintained by a sorted set (whether or not an explicit comparator is provided) must be consistent with equals if the sorted set is to correctly implement the Set interface.

I don't know about Apache Commons, but Guava specifically rejected requests for this sort of thing, although you can achieve what you want using Guava Equivalence:

Equivalence<T> equivalence = new Equivalence<T>() {     @Override     protected boolean doEquivalent(T a, T b) {         return CustomComparator.equals(a, b);     }      @Override     protected int doHash(T item) {         return CustomHashCodeGenerator.hashCode(item);     } }; List<T> items = getItems(); Set<Equivalence.Wrapper<T>> setWithWrappedObjects = items.stream()     .map(item -> equivalence.wrap(item))     .collect(Collectors.toSet()); 

Answer 2

There are a couple of third-party collections frameworks that allows custom equality logic. This is perfect for overriding equality for objects that you can't alter the source.

• Trove

Trove's maps/sets support the use of custom hashing strategies, allowing you to tune collections based on characteristics of the input data. This feature also allows you to define hash functions when it is not feasible to override Object.hashCode().

• HE-Collection

To achive this, any type that needs a standard correction, must implement the HE-Collection interface EqualsAndHashCorrection. This interface defines the methods hashCodeInHeCollection() and equalsInHeCollection(Object), that serve as correction for the incorrect implemented methods hashCode() and equals(Object).