Wrapper class for adding elements to HashSet

Question

I have many object of class Test. I want to be sure that among them, there are no two objects o1 and o2 where o1==o2. In order to achieve that, I want to add them all to a HashSet and check if hashSet.size() == numberOfInitialObjects .

In order to ensure that the potential future changes in equals method of the Test class will not affect my implementation, I wrote a simple wrapper so that instead of adding Test objects to the HashSet, the wrapper objects will be used. The code as such:

private class HashSetWrapper {

        private final Test component;

        public HashSetWrapper(Test component) {
            this.component = component;
        }

        @Override
        public int hashCode() {
            int hash = 7;
            hash = 89 * hash + Objects.hashCode(this.component);
            return hash;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj == null) {
                return false;
            }
            if (getClass() != obj.getClass()) {
                return false;
            }
            final HashSetWrapper other = (HashSetWrapper) obj;
            return this.component == other.component;
        }
    }

Is my approach OK, or are there more suitable ways to achieve my goal?

Answer 1

Why build your own implementation when you can use the features available in the Java standard libraries?

IdentityHashMap gives you all the features you need, and a Java Stream/collector will allow you to extract the map easily from your collection....

Collection<Test> tests = .....
Map<Test,Test> uniques = tests.stream().collect(Collectors.toMap(
     t -> t,
     t -> t,
     (t,u) -> t,
     IdentityHashMap::new))

Now, if the size of the uniques is the same as the tests, you're good. The uniques Map is an IdentityHashMap where the key values are all identical instances (using ==). It supports a null key/value so that should be fine too.

Answer 2

A general improvement on "add everything then count" is to fail as soon as a duplicate is found.

  public static <T> boolean uniqueInstances(Collection<T> items) {
    Set<T> set = Collections.newSetFromMap(new IdentityHashMap<>()); // Stand-in for (non-existent) IdentitySet
    return items.stream().allMatch(set::add); // add() returns false if element already existed
  }