A Set is a Collection that cannot contain duplicate elements. It models the mathematical set abstraction.

Set have its implementation in various classes like HashSet, TreeSet, LinkedHashSet.

For example:

HashSet:

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

Here T can be String, Integer or any other object. HashSet allows for quick lookup of O(1) but does not sort the data added to it and loses the insertion order of items.

TreeSet:

It stores data in a sorted manner sacrificing some speed for basic operations which take O(lg(n)). It does not maintain the insertion order of items.

TreeSet<T> sortedSet = new TreeSet<T>();

LinkedHashSet:

It is a linked list implementation of HashSet Once can iterate over the items in the order they were added. Sorting is not provided for its contents. O(1) basic operations are provided, however there is higher cost than HashSet in maintaining the backing linked list.

LinkedHashSet<T> linkedhashset = new LinkedHashSet<T>();

What is a Set?

A set is a data structure which contains a set of elements with an important property that no two elements in the set are equal.

Types of Set:

1. HashSet: A set backed by a hash table (actually a HashMap instance)
2. Linked HashSet: A Set backed by Hash table and linked list, with predictable iteration order
3. TreeSet: A NavigableSet implementation based on a TreeMap.

Creating a set

Set<Integer> set = new HashSet<Integer>(); // Creates an empty Set of Integers

Set<Integer> linkedHashSet = new LinkedHashSet<Integer>(); //Creates a empty Set of Integers, with predictable iteration order

Adding elements to a Set

Elements can be added to a set using the add() method

set.add(12); //  - Adds element 12 to the set
 set.add(13); //  - Adds element 13 to the set

Our set after executing this method:

set = [12,13]

Delete all the elements of a Set

set.clear();  //Removes all objects from the collection.

After this set will be:

set = []

Check whether an element is part of the Set

Existence of an element in the set can be checked using the contains() method

set.contains(0);  //Returns true if a specified object is an element within the set.

Output: False

Check whether a Set is empty

isEmpty() method can be used to check whether a Set is empty.

set.isEmpty();  //Returns true if the set has no elements

Output: True

Remove an element from the Set

set.remove(0); // Removes first occurrence of a specified object from the collection

Check the Size of the Set

set.size(); //Returns the number of elements in the collection

Output: 0

Generally, sets are a type of collection which stores unique values. Uniqueness is determined by the equals() and hashCode() methods.

Sorting is determined by the type of set.

HashSet - Random Sorting

Set<String> set = new HashSet<> ();
set.add("Banana");
set.add("Banana");
set.add("Apple");
set.add("Strawberry");

// Set Elements: ["Strawberry", "Banana", "Apple"]

LinkedHashSet - Insertion Order

Set<String> set = new LinkedHashSet<> ();
set.add("Banana");
set.add("Banana");
set.add("Apple");
set.add("Strawberry");

// Set Elements: ["Banana", "Apple", "Strawberry"]

TreeSet - By compareTo() or Comparator

Set<String> set = new TreeSet<> ();
set.add("Banana");
set.add("Banana");
set.add("Apple");
set.add("Strawberry");

// Set Elements: ["Apple", "Banana", "Strawberry"]

Set<String> set = new TreeSet<> ((string1, string2) -> string2.compareTo(string1));
set.add("Banana");
set.add("Banana");
set.add("Apple");
set.add("Strawberry");

// Set Elements: ["Strawberry", "Banana", "Apple"]

You can create a new class that inherits from HashSet:

Set<String> h = new HashSet<String>() {{
    add("a");
    add("b");
}};

One line solution:

Set<String> h = new HashSet<String>(Arrays.asList("a", "b"));

Using guava:

Sets.newHashSet("a", "b", "c")

Using Streams:

Set<String> set3 = Stream.of("a", "b", "c").collect(toSet());

Using a new List

List<String> list = new ArrayList<String>(listOfElements);

Using List.addAll() method

   Set<String> set = new HashSet<String>();
    set.add("foo");
    set.add("boo");
    
    List<String> list = new ArrayList<String>();
    list.addAll(set);

Using Java 8 Steam API

List<String> list = set.stream().collect(Collectors.toList());

Suppose you have a collection elements, and you want to create another collection containing the same elements but with all duplicates eliminated:

Collection<Type> noDuplicates = new HashSet<Type>(elements);

Example:

List<String> names = new ArrayList<>(
        Arrays.asList("John", "Marco", "Jenny", "Emily", "Jenny", "Emily", "John"));
Set<String> noDuplicates = new HashSet<>(names);
System.out.println("noDuplicates = " + noDuplicates);

Output:

noDuplicates = [Marco, Emily, John, Jenny]