Java: set <E>, where elements are identified by its class

Question

I need a Set collection where its elements will be identified by element classes. Something like ReferenceIdentityMapfrom Application Collections , but in the scope of a class, that is, two different instances of the same class should be identified as the same in this collection.

You know, this is a violation of the principle of identity equals()/hashCode(), but if used it makes sense.

I did this in simple class support using Map<Class<? extends E>, E>, but because of simplicity, it does not implement Set<E>. Perhaps there will be a more elegant solution, any decorator Set<E>will be great.

Is there any implementation of such a collection there (Apache / Google / something / ... Collections)?

Answer 1

You want to override the equals () / hashCode () value for your set members. The cleanest way to do this, I suppose, is to use a wrapper class:

class Wrapper<E> {

  private final E item;

  Wrapper(E item) {
    this.item = item;
  }

  E getItem() {
    return item;
  }

  public boolean equals(Object o) {
    if (!(o instanceof Wrapper)) {
      return false;
    }
    return getClass().equals(o.getClass());
  }

  public int hashCode() {
    return getClass().hashCode();
  }

}

Then you will create Set<Wrapper<E>>.

Answer 2

How about expanding HashSetand overriding only the method add(..), placing object.getClass()instead of the object itself in the internal Set<Class<? extends E>>, and if it succeeds, adding the element itself. Sort of

public class ClassSet<E> extends HashSet<E> {
    private Set<Class<? extends E>> classSet = new HashSet<Class<? extends E>>();

    @Override
    public boolean add(E element) {
        if (classSet.add((Class<E>) element.getClass())) {
            return super.add(element); // this actually should always return true
        }
        return false;
    }
}

Answer 3

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Check out the constructor here .