TreeSet giving incorrect output - Java8

Question

While working with a tree set, I found very peculiar behavior.

As per my understanding following program should print two identical lines:

public class T         


        

Answer 1

Well, this surprised me, I don't know if I'm correct, but look at this implementation in AbstractSet:

public boolean removeAll(Collection c) {
  Objects.requireNonNull(c);
  boolean modified = false;

  if (size() > c.size()) {
    for (Iterator i = c.iterator(); i.hasNext(); )
      modified |= remove(i.next());
  } else {
    for (Iterator i = iterator(); i.hasNext(); ) {
      if (c.contains(i.next())) {
        i.remove();
        modified = true;
      }
    }
  }
  return modified;
}

Basically in your example, the size of set is equal to the size of arguments you want to remove, so the else condition is invoked. In that condition there is a check if your collection of arguments to remove contains the current element of iterator, and that check is case sensitive, so it checks if c.contains("a") and it returns false, because c contains "A", not "a", so the element is not removed. Notice that when you add an element to your set s.addAll(Arrays.asList("a", "b", "d")); it works correctly, because size() > c.size() is now true, thus there is no contains check anymore.