Set<E> interface

Sets are collections that allow only one object with a given value in them. The java.util.Set interface is a subinterface of Collection. There are two very useful concrete classes that implement the Set interface:

• java.util.HashSet is implemented with a hash table. Access time is O(1). Entries are unsorted.
• java.util.TreeSet is implemented as a balanced binary tree. Access time is O(log N). Entries are sorted.

A set is an unordered collection of elements that has no duplicate members. elements retrieved by identity, not index.

Set Interface

Set Implementations

We have following as implementing classes:

public interface Set {
    // Basic Operations
    int size();
    boolean isEmpty();
    boolean contains(Object element);
    boolean add(Object element);    // Optional
    boolean remove(Object element); // Optional
    Iterator iterator();

    // Bulk Operations
    boolean containsAll(Collection c);
    boolean addAll(Collection c);    // Optional
    boolean removeAll(Collection c); // Optional
    boolean retainAll(Collection c); // Optional
    void clear();                    // Optional        

    // Array Operations
    Object[] toArray();
    Object[] toArray(Object a[]);
}