Sets in Java

Overview

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

Interface

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.

Implementations

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

HashSet, a default implementation, backed by HashMap. Gives no guarantee about ordering.
TreeSet, order set structure, gives guarantee about ordering.
LinkedHashSet should be used if you want to maintain insertion order, without duplicates.

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.add("element1");

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

set.isEmpty(); // false

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

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

set.clear();

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

ivanursul