I made my own Collection based on TreeSet but I have troubles with existing test: it doesn’t see elements

Here is my collection. Here I try to make my own implemetation with special Comparator that sorts Integer elements by its absolute values.

class SortedByAbsoluteValueIntegerSet extends TreeSet {

private TreeSet<Integer> mySet;

public SortedByAbsoluteValueIntegerSet() {
    mySet = new TreeSet<Integer>(Comparator.comparing(Math::abs));
}

@Override
public boolean add(Object o) {
    mySet.add((Integer) o);
    return true;
}

@Override
public boolean addAll(Collection c) {
    for (Object o : c) {
        mySet.add((Integer) o);
    }
    return true;
}

public Iterator<Integer> iterator() {
    return mySet.iterator();
}

(and other methods)

Main class. Everything seems to work correct except toString() method. When I overwrite this method without lambdas it works. But! This method is in tests and I mustn’t change it. I just copied it to Main class trying to understand the problem. And problem I want to solve is somewhere in SortedByAbsoluteValueIntegerSet class.

public static void main(String[] args) {

    Set<Integer> set = new SortedByAbsoluteValueIntegerSet();

    Arrays.asList(1, 3, 5, 7, 9).forEach(set::add);
    set.addAll(Arrays.asList(-2, -4, -6, -8, -10));

    System.out.println("set.size() = " + set.size());  //OUTPUT:"set.size() = 10"
    System.out.println("set = " + set);  //OUTPUT:"set = [-10, -8, -6, -4, -2, 1, 3, 5, 7, 9]"
    System.out.println("toString(set) = " + toString(set)); //OUTPUT:"toString(set) = "

}

private static String toString(final Collection<Integer> collection) {
    return String.join(" ", collection.stream()
            .map(i -> Integer.toString(i))
            .toArray(String[]::new));
}

This is another realization that works good. So what’s the difference?

private static String toString(final Collection<Integer> collection) {
    List<String> list = new ArrayList<>();
    for (Integer i : collection) {
        String s = Integer.toString(i);
        list.add(s);
    }
    return String.join(" ", list.toArray(new String[0]));
}

Answer

You shouldn’t be extending TreeSet and having a TreeSet field. One or the other, but both makes no sense.

This is probably actually the cause of your issue: you have two different TreeSets associated with each SortedByAbsoluteValueIntegerSet, and the one you’re adding to and the one toString() is getting are different.

Extend AbstractSet instead.