// guts of Set.java

package java.util;

public interface Set<E> extends Collection<E> {

    // Query Operations
    int size();
    boolean isEmpty();
    boolean contains(Object o);
    Iterator<E> iterator();
    Object[] toArray();
    <T> T[] toArray(T[] a);

    // Modification Operations
    boolean add(E o);
    boolean remove(Object o);

    // Bulk Operations
    boolean containsAll(Collection<?> c);
    boolean addAll(Collection<? extends E> c);
    boolean retainAll(Collection<?> c);
    boolean removeAll(Collection<?> c);
    void clear();

    // Comparison and hashing
    boolean equals(Object o);
    int hashCode();
}

// Surprise! All same as Collection<E>, but different post-conditions
// specified for add and addAll supposed to prevent duplicate elements.
// Of course, programmers must honor this "contract" for it to work.