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copied!<p>It seems that you are trying to implement composition and at the same time extending the tree set, but that is not a good practice, you either use composition and implement the Set interface (backend with a TreeSet) or extends the tree set</p> <p>Extending the TreeSet</p> <pre><code> class MySet<E> extends TreeSet<E> { public void union(Set<E> s){ addAll(s); } public void intersection(Set<E> s){ retainAll(s); } } </code></pre> <p>using composition</p> <pre><code> class MySet<E> implements Set<E> { private TreeSet<E> set; public MySet(TreeSet<E> set) { this.set = new TreeSet<>(set); } public void union(Set<E> s){ set.addAll(s); } public void intersection(Set<E> s){ set.retainAll(s); } @Override public int size() { return set.size(); } @Override public boolean isEmpty() { return set.isEmpty(); } @Override public boolean contains(Object o) { return set.contains(o); } @Override public Iterator<E> iterator() { return set.iterator(); } @Override public Object[] toArray() { return set.toArray(); } @Override public <T> T[] toArray(T[] a) { return set.toArray(a); } @Override public boolean add(E e) { return set.add(e); } @Override public boolean remove(Object o) { return set.remove(o); } @Override public boolean containsAll(Collection<?> c) { return set.containsAll(c); } @Override public boolean addAll(Collection<? extends E> c) { return set.addAll(c); } @Override public boolean retainAll(Collection<?> c) { return set.retainAll(c); } @Override public boolean removeAll(Collection<?> c) { return set.removeAll(c); } @Override public void clear() { set.clear(); } @Override public boolean equals(Object o) { return set.equals(o); } @Override public int hashCode() { return set.hashCode(); } } </code></pre>

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