HashSet, LinkedHashSet And TreeSet in Java

         A Set is a collection which does not contain duplicate element. HashSet, LinkedHashSet and TreeSet implements Set interface.

Let's understand all these classes:

1. HashSet:

        HashSet actually backed by Hash Table. And the iteration of elements could be different each time.

      - HashSet offers constant time performance for basic operations such as add, remove, contains and size.

Note: HashSet is not synchronized and can contain null values.

Syntax:

            HashSet<E> set = new HashSet<E>();

            OR

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

Some of the useful methods of HashSet described below:

• add(element): adds element in the set

               Example:

                            set.add(12);

                            set.add(2);

                            set.add(5);

                            System.out.println(set); //output: [2, 5, 12] here order of elements could vary

                            set.add(5); //will not throw any error, but since 5 already exists, it won't add value 5 again in the set

                            System.out.println(set); //output: [2, 5, 12]

• isEmpty(): returns true if set is empty otherwise returns false

                            System.out.println(set.isEmpty()); //false, because array contains some elements

• contains(Object o): returns true if the object exists otherwise returns false

                             System.out.println(set.contains(5)); //true

• remove(Object o): removes object from the list if exists, otherwise throws an error

                            set.remove(5);  

                            System.out.println(set); //[2, 12]

• size(): returns the number of elements present in the set

                            System.out.println(set.size()); //returns 3, since array contains 3 elements 12, 2,5

• clear(): removes all the elements from the set

                            set.clear();

                            System.out.println(set); //output: [ ]

2. LinkedHashSet:

            LinkedHashSet is a combination of LinkedList and HashTable. It preserves the order in which elements are inserted.

Syntax:

            Set<E> set = new LinkedHashSet<E>();

            OR
            LinkedHashSet<E> set = new LinkedHashSet<E>();

Note: It also contains unique values like Set and can contain null values. It is non-synchronized.

The above methods of HashSet will also work for LinkedHashSet.

3. TreeSet:

           This class also implements Set interface and uses a tree for storage. Like Set, TreeSet contains only unique elements but in their natural order.(By default in ascending order)

Syntax:

        Set<E> set = new TreeSet<E>();

        OR

        TreeSet<E> set = new TreeSet<E>();

All the methods of HashSet will also work for TreeSet, since they all implements Set interface.

Contiune Learning: HashMap And TreeMap