A Set is a Collection which cannot contain duplicated elements. In other words, the same object cannot occur more than once in a Set.


A Set interface contains only methods, inherited from Collection interface + it adds it’s own methods:

public interface Set<E> extends Collection<E> {
    int size();
    boolean isEmpty();
    boolean contains(Object o);
    Iterator<E> iterator();
    Object[] toArray();
    <T> T[] toArray(T[] a);
    boolean add(E e);
    boolean remove(Object o);
    boolean containsAll(Collection<?> c);
    boolean addAll(Collection<? extends E> c);
    boolean retainAll(Collection<?> c);
    boolean removeAll(Collection<?> c);
    void clear();
    boolean equals(Object o);
    int hashCode();
    default Spliterator<E> spliterator() { /* ... */ }

When to use

Set should be used in situations when you don’t want to see duplicated elements.


Set interface has three basic implementations, available in Collections framework: HashSet, TreeSet and LinkedHashSet:

How to create

The default way of creating sets is following:

Set<String> hashSet = new HashSet<>();
Set<String> treeSet = new TreeSet<>();
Set<String> linkedHashSet = new LinkedHashSet<>();

Basic operations

Here are the basic operations described:

Set<String> set = new HashSet<>();


set.addAll(Arrays.asList("element1", "element2"));
set.size(); // 2
set.contains("element1"); // true
set.containsAll(Arrays.asList("element1", "element2"));

set.isEmpty(); // false

set.size(); // 3
set.removeIf(item -> "element3".equals(item));
set.size(); // 2

set.stream() // stream operations
        .forEach(item -> System.out.println(item));


set.isEmpty(); // true

Bulk operations

There are bulk operations, available for Set interface:

Set<String> set = new HashSet<>();

set.addAll(Arrays.asList("element1", "element2", "element3")); // [element1, element2, element3]
set.containsAll(Arrays.asList("element1", "element2")); // true
set.retainAll(Arrays.asList("element1", "element2")); // [element1, element2]
set.removeAll(Arrays.asList("element1", "element2")); // []
set.size(); // 0