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

This is interesting, so here are some tests with output:

static void test(String... args) {
    Set s =new TreeSet(String.CASE_INSENSITIVE_ORDER);
    s.addAll(Arrays.asList( "a","b","c"));
    s.removeAll(Arrays.asList(args));
    System.out.println(s);
}

public static void main(String[] args) {
    test("C");          output: [a, b]
    test("C", "A");     output: [b]
    test("C", "A","B"); output: [a, b, c]
    test("B","C","A");  output: [a, b, c]
    test("K","C");      output: [a, b]
    test("C","K","M");  output: [a, b, c] !!
    test("C","K","A");  output: [a, b, c] !!
}

Now without the comparator it works just like a sorted HashSet():

    static void test(String... args) {
    Set s = new TreeSet();//
    s.addAll(Arrays.asList( "a","b","c"));
    s.removeAll(Arrays.asList(args));
    System.out.println(s);
}

public static void main(String[] args) {
    test("c");          output: [a, b]
    test("c", "a");     output: [b]
    test("c", "a","b"); output: []
    test("b","c","a");  output: []
    test("k","c");      output: [a, b]
    test("c","k","m");  output: [a, b]
    test("c","k","m");  output: [a, b]
}

Now from the documentation:

public boolean removeAll(Collection c)

Removes from this set all of its elements that are contained in the specified collection (optional operation). If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.

This implementation determines which is the smaller of this set and the specified collection, by invoking the size method on each. If this set has fewer elements, then the implementation iterates over this set, checking each element returned by the iterator in turn to see if it is contained in the specified collection. If it is so contained, it is removed from this set with the iterator's remove method. If the specified collection has fewer elements, then the implementation iterates over the specified collection, removing from this set each element returned by the iterator, using this set's remove method.

Source