The Set Interface is present in java.util package and extends the Collection interface. It is an unordered collection of objects in which duplicate values cannot be stored. It is an interface that implements the mathematical set. This interface adds a feature that restricts the insertion of duplicate elements.
- No Specific Order: Does not maintain any specific order of elements (Exceptions: LinkedHashSet and TreeSet).
- Allows One Null Element: Most Set implementations allow a single null element.
- Implementation Classes: HashSet, LinkedHashSet, and TreeSet.
- Thread-Safe Alternatives: For thread-safe operations, use
ConcurrentSkipListSet or wrap a set using Collections.synchronizedSet().
Two interfaces extend the set implementation, that are, SortedSet and NavigableSet.
Example: This example demonstrates how to create an empty HashSet.
Java // Java Program to Implementing Set Interface import java.util.HashSet; import java.util.Set; public class Geeks { public static void main(String args[]) { // Create a Set using HashSet Set<String> s = new HashSet<>(); // Displaying the Set System.out.println("Set Elements: " + s); } } Explanation: In the above example, HashSet will appear as an empty set, as no elements were added. The order of elements in HashSet is not guaranteed, so the elements will be displayed in a random order if any are added.
Hierarchy of Java Set interface
The image below demonstrates the hierarchy of Java Set interface.
In the above image, the navigable set extends the sorted set interface. Since a set doesn't retain the insertion order, the navigable set interface provides the implementation to navigate through the Set. The class which implements the navigable set is a TreeSet which is an implementation of a self-balancing tree. Therefore, this interface provides us with a way to navigate through this tree.
Declaration of Set Interface
The declaration of Set interface is listed below:
public interface Set extends Collection
Creating Set Objects:
Since Set is an interface, objects cannot be created of the typeset. We always need a class that extends this list in order to create an object. And also, after the introduction of Generics in Java 1.5, it is possible to restrict the type of object that can be stored in the Set. This type-safe set can be defined as:
// Obj is the type of the object to be stored in Set
Set<Obj> set = new HashSet<Obj> ();
Methods
Let us discuss methods present in the Set interface provided below in a tabular format below as follows:
| Method | Description |
|---|
| add(element) | This method is used to add a specific element to the set. The function adds the element only if the specified element is not already present in the set else the function returns False if the element is already present in the Set. |
| addAll(collection) | This method is used to append all of the elements from the mentioned collection to the existing set. The elements are added randomly without following any specific order. |
| clear() | This method is used to remove all the elements from the set but not delete the set. The reference for the set still exists. |
| contains(element) | This method is used to check whether a specific element is present in the Set or not. |
| containsAll(collection) | This method is used to check whether the set contains all the elements present in the given collection or not. This method returns true if the set contains all the elements and returns false if any of the elements are missing. |
| hashCode() | This method is used to get the hashCode value for this instance of the Set. It returns an integer value which is the hashCode value for this instance of the Set. |
| isEmpty() | This method is used to check whether the set is empty or not. |
| iterator() | This method is used to return the iterator of the set. The elements from the set are returned in a random order. |
| remove(element) | This method is used to remove the given element from the set. This method returns True if the specified element is present in the Set otherwise it returns False. |
| removeAll(collection) | This method is used to remove all the elements from the collection which are present in the set. This method returns true if this set changed as a result of the call. |
| retainAll(collection) | This method is used to retain all the elements from the set which are mentioned in the given collection. This method returns true if this set changed as a result of the call. |
| size() | This method is used to get the size of the set. This returns an integer value which signifies the number of elements. |
| toArray() | This method is used to form an array of the same elements as that of the Set. |
Example: Sample Program to Illustrate Set interface
Java // Java program Illustrating Set Interface // Importing utility classes import java.util.*; // Main class public class Geeks { // Main driver method public static void main(String[] args) { // Demonstrating Set using HashSet // Declaring object of type String Set<String> s = new HashSet<String>(); // Adding elements to the Set // using add() method s.add("Geeks"); s.add("For"); s.add("Geeks"); s.add("Example"); s.add("Set"); // Printing elements of HashSet object System.out.println(s); } } Output[Set, Example, Geeks, For]
Explanation: In the above example, we can see the elements are in random order as HashSet does not maintain any order and duplicate elements are automatically ignored in HashSet.
Operations on the Set Interface
The set interface allows the users to perform the basic mathematical operation on the set. Let's take two arrays to understand these basic operations. Let set1 = [1, 3, 2, 4, 8, 9, 0] and set2 = [1, 3, 7, 5, 4, 0, 7, 5]. Then the possible operations on the sets are:
1. Intersection: This operation returns all the common elements from the given two sets. For the above two sets, the intersection would be:
Intersection = [0, 1, 3, 4]
2. Union: This operation adds all the elements in one set with the other. For the above two sets, the union would be:
Union = [0, 1, 2, 3, 4, 5, 7, 8, 9]
3. Difference: This operation removes all the values present in one set from the other set. For the above two sets, the difference would be:
Difference = [2, 8, 9]
Now, let us implement the following operations as defined above as follows:
Example: This example demonstrates how to perform various operations on set.
Java // Java Program Demonstrating Operations on the Set // such as Union, Intersection and Difference operations // Importing all utility classes import java.util.*; // Main class public class Geeks { // Main driver method public static void main(String args[]) { // Creating an object of Set class // Declaring object of Integer type Set<Integer> a = new HashSet<Integer>(); // Adding all elements to List a.addAll(Arrays.asList( new Integer[] { 1, 3, 2, 4, 8, 9, 0 })); // Again declaring object of Set class // with reference to HashSet Set<Integer> b = new HashSet<Integer>(); b.addAll(Arrays.asList( new Integer[] { 1, 3, 7, 5, 4, 0, 7, 5 })); // To find union Set<Integer> u = new HashSet<Integer>(a); u.addAll(b); System.out.print("Union of the two Set"); System.out.println(u); // To find intersection Set<Integer> i = new HashSet<Integer>(a); i.retainAll(b); System.out.print("Intersection of the two Set"); System.out.println(i); // To find the symmetric difference Set<Integer> d = new HashSet<Integer>(a); d.removeAll(b); System.out.print("Difference of the two Set"); System.out.println(d); } } OutputUnion of the two Set[0, 1, 2, 3, 4, 5, 7, 8, 9] Intersection of the two Set[0, 1, 3, 4] Difference of the two Set[2, 8, 9]
Performing Various Operations on SortedSet
After the introduction of Generics in Java 1.5, it is possible to restrict the type of object that can be stored in the Set. Since Set is an interface, it can be used only with a class that implements this interface. HashSet is one of the widely used classes which implements the Set interface. Now, let’s see how to perform a few frequently used operations on the HashSet. We are going to perform the following operations as follows:
- Adding elements
- Accessing elements
- Removing elements
- Iterating elements
- Iterating through Set
Now let us discuss these operations individually as follows:
1. Adding Elements
In order to add an element to the Set, we can use the add() method. However, the insertion order is not retained in the Set. Internally, for every element, a hash is generated and the values are stored with respect to the generated hash. the values are compared and sorted in ascending order. We need to keep a note that duplicate elements are not allowed and all the duplicate elements are ignored. And also, Null values are accepted by the Set.
Example: This example demonstrates how HashSet stores unique element and does not maintain any insertion order.
Java // Java Program Demonstrating Working of Set by // Adding elements using add() method // Importing all utility classes import java.util.*; // Main class class Geeks { // Main driver method public static void main(String[] args) { // Creating an object of Set and // declaring object of type String Set<String> s = new HashSet<String>(); // Adding elements to above object // using add() method s.add("B"); s.add("B"); s.add("C"); s.add("A"); // Printing the elements inside the Set object System.out.println(s); } }
2. Accessing the Elements
After adding the elements, if we wish to access the elements, we can use inbuilt methods like contains().
Example: This example demonstrates how to check if a specified element exists or not using the contains() method.
Java // Java code to demonstrate Working of Set by // Accessing the Elements of the Set object // Importing all utility classes import java.util.*; // Main class class Geeks { // Main driver method public static void main(String[] args) { // Creating an object of Set and // declaring object of type String Set<String> h = new HashSet<String>(); // Elements are added using add() method // Later onwards we will show accessing the same // Custom input elements h.add("A"); h.add("B"); h.add("C"); h.add("A"); // Print the Set object elements System.out.println("Set is " + h); // Declaring a string String s = "D"; // Check if the above string exists in // the SortedSet or not // using contains() method System.out.println("Contains " + s + " " + h.contains(s)); } } OutputSet is [A, B, C] Contains D false
3. Removing the Values
The values can be removed from the Set using the remove() method.
Example: This example demonstrates how to remove element from a HashSet using remove() method
Java // Java Program Demonstrating Working of Set by // Removing Element/s from the Set // Importing all utility classes import java.util.*; // Main class class Geeks { // Main driver method public static void main(String[] args) { // Declaring object of Set of type String Set<String> h = new HashSet<String>(); // Elements are added // using add() method // Custom input elements h.add("A"); h.add("B"); h.add("C"); h.add("B"); h.add("D"); h.add("E"); // Printing initial Set elements System.out.println("Initial HashSet " + h); // Removing custom element // using remove() method h.remove("B"); // Printing Set elements after removing an element // and printing updated Set elements System.out.println("After removing element " + h); } } OutputInitial HashSet [A, B, C, D, E] After removing element [A, C, D, E]
4. Iterating through the Set
There are various ways to iterate through the Set. The most famous one is to use the enhanced for loop.
Example: This example demonstrates how to iterate through a HashSet.
Java // Java Program to Demonstrate Working of Set by // Iterating through the Elements // Importing utility classes import java.util.*; // Main class class Geeks { // Main driver method public static void main(String[] args) { // Creating object of Set and declaring String type Set<String> h = new HashSet<String>(); // Adding elements to Set // using add() method // Custom input elements h.add("A"); h.add("B"); h.add("C"); h.add("B"); h.add("D"); h.add("E"); // Iterating through the Set // via for-each loop for (String value : h) // Printing all the values inside the object System.out.print(value + ", "); System.out.println(); } } Classes that implement the Set interface in Java Collections can be easily perceived from the image below as follows and are listed as follows:
Hierarchy of Java Collections
Class 1: HashSet
HashSet class which is implemented in the collection framework is an inherent implementation of the hash table data structure. The objects that we insert into the HashSet do not guarantee to be inserted in the same order. The objects are inserted based on their hashcode. This class also allows the insertion of NULL elements. Let’s see how to create a set object using this class.
Example: This example demonstrates how to add, remove, and iterate through elements in a HashSet.
Java // Java program Demonstrating Creation of Set object // Using the Hashset class // Importing utility classes import java.util.*; // Main class class Geeks { // Main driver method public static void main(String[] args) { // Creating object of Set of type String Set<String> h = new HashSet<String>(); // Adding elements into the HashSet // using add() method // Custom input elements h.add("India"); h.add("Australia"); h.add("South Africa"); // Adding the duplicate element h.add("India"); // Displaying the HashSet System.out.println(h); // Removing items from HashSet // using remove() method h.remove("Australia"); System.out.println("Set after removing " + "Australia:" + h); // Iterating over hash set items System.out.println("Iterating over set:"); // Iterating through iterators Iterator<String> i = h.iterator(); // It holds true till there is a single element // remaining in the object while (i.hasNext()) System.out.println(i.next()); } } Output[South Africa, Australia, India] Set after removing Australia:[South Africa, India] Iterating over set: South Africa India
Class 2: EnumSet
EnumSet class which is implemented in the collections framework is one of the specialized implementations of the Set interface for use with the enumeration type. It is a high-performance set implementation, much faster than HashSet. All of the elements in an enum set must come from a single enumeration type that is specified when the set is created either explicitly or implicitly. Let's see how to create a set object using this class.
Example: This example demonstrates how to create and initialize a set using EnumSet.
Java // Java program to demonstrate the // creation of the set object // using the EnumSet class import java.util.*; enum Gfg { CODE, LEARN, CONTRIBUTE, QUIZ, MCQ } ; public class Geeks { public static void main(String[] args) { // Creating a set Set<Gfg> s1; // Adding the elements s1 = EnumSet.of(Gfg.QUIZ, Gfg.CONTRIBUTE, Gfg.LEARN, Gfg.CODE); System.out.println("Set 1: " + s1); } } OutputSet 1: [CODE, LEARN, CONTRIBUTE, QUIZ]
Class 3: LinkedHashSet
LinkedHashSet class which is implemented in the collections framework is an ordered version of HashSet that maintains a doubly-linked List across all elements. When the iteration order is needed to be maintained this class is used. When iterating through a HashSet the order is unpredictable, while a LinkedHashSet lets us iterate through the elements in the order in which they were inserted. Let's see how to create a set object using this class.
Example: This example demonstrates how to use a LinkedHashSet to maintain insertion.
Java // Java program to demonstrate the // creation of Set object using // the LinkedHashset class import java.util.*; class Geeks { public static void main(String[] args) { Set<String> lh = new LinkedHashSet<String>(); // Adding elements into the LinkedHashSet // using add() lh.add("India"); lh.add("Australia"); lh.add("South Africa"); // Adding the duplicate // element lh.add("India"); // Displaying the LinkedHashSet System.out.println(lh); // Removing items from LinkedHashSet // using remove() lh.remove("Australia"); System.out.println("Set after removing " + "Australia:" + lh); // Iterating over linked hash set items System.out.println("Iterating over set:"); Iterator<String> i = lh.iterator(); while (i.hasNext()) System.out.println(i.next()); } } Output[India, Australia, South Africa] Set after removing Australia:[India, South Africa] Iterating over set: India South Africa
Class 4: TreeSet
TreeSet class which is implemented in the collections framework and implementation of the SortedSet Interface and SortedSet extends Set Interface. It behaves like a simple set with the exception that it stores elements in a sorted format. TreeSet uses a tree data structure for storage. Objects are stored in sorted, ascending order. But we can iterate in descending order using the method TreeSet.descendingIterator(). Let's see how to create a set object using this class.
Example: This example demonstrates how to use a TreeSet to store elements in a sorted order.
Java // Java Program Demonstrating Creation of Set object // Using the TreeSet class // Importing utility classes import java.util.*; // Main class class Geeks { // Main driver method public static void main(String[] args) { // Creating a Set object and declaring it of String // type // with reference to TreeSet Set<String> ts = new TreeSet<String>(); // Adding elements into the TreeSet // using add() ts.add("India"); ts.add("Australia"); ts.add("South Africa"); // Adding the duplicate // element ts.add("India"); // Displaying the TreeSet System.out.println(ts); // Removing items from TreeSet // using remove() ts.remove("Australia"); System.out.println("Set after removing " + "Australia:" + ts); // Iterating over Tree set items System.out.println("Iterating over set:"); Iterator<String> i = ts.iterator(); while (i.hasNext()) System.out.println(i.next()); } } Output[Australia, India, South Africa] Set after removing Australia:[India, South Africa] Iterating over set: India South Africa