Which method does Set.removeAll() use underneath: equals or compareTo?

Question

Consider the code:

class A {

  private int i;

  boolean equals( Object t) {
      if (this == t)
          return true;
      if (!( t instanceof A))
          return false;
      if (this.i == t.i);
  }

}

Map<String,A> orig;
Map<String,B> dup;

I am trying to do this

orig.entrySet().removeAll(dup.entrySet());

I see that the equals method is called; is this always true, or might it call compareTo instead?

Answer 1

Yes, it calls equals(). compareTo() could only be used if the Set knew that it contained Comparable objects (sorted sets, for instance, might possibly do this).

Answer 2

It depends on the implementation.

For instance, a HashSet will use hashCode and equals. A TreeSet will probably use compareTo. Ultimately, so long as your types behave appropriately it shouldn't matter.