Set in Java

08 Jul, 2022
The set interface is present in java.util package and extends the Collection interface is an unordered collection of objects in which duplicate values cannot be stored. It is an interface that implements the mathematical set. This interface contains the methods inherited from the Collection interface and adds a feature that restricts the insertion of the duplicate elements. There are two interfaces that extend the set implementation namely SortedSet and NavigableSet.
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: The Set interface is declared as:
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> (); 
Let us discuss methods present in the Set interface provided below in a tabular format below as follows:
Illustration: Sample Program to Illustrate Set interface
Java
// Java program Illustrating Set Interface  // Importing utility classesimport java.util.*;  // Main class public class GFG {        // Main driver method    public static void main(String[] args)    {        // Demonstrating Set using HashSet        // Declaring object of type String         Set<String> hash_Set = new HashSet<String>();          // Adding elements to the Set        // using add() method        hash_Set.add("Geeks");        hash_Set.add("For");        hash_Set.add("Geeks");        hash_Set.add("Example");        hash_Set.add("Set");          // Printing elements of HashSet object        System.out.println(hash_Set);    }}
[Set, Example, Geeks, For]
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:
Java
// Java Program Demonstrating Operations on the Set// such as Union, Intersection and Difference operations   // Importing all utility classesimport java.util.*;  // Main class public class SetExample {        // 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> union = new HashSet<Integer>(a);        union.addAll(b);        System.out.print("Union of the two Set");        System.out.println(union);          // To find intersection        Set<Integer> intersection = new HashSet<Integer>(a);        intersection.retainAll(b);        System.out.print("Intersection of the two Set");        System.out.println(intersection);          // To find the symmetric difference        Set<Integer> difference = new HashSet<Integer>(a);        difference.removeAll(b);        System.out.print("Difference of the two Set");        System.out.println(difference);    }}
Union 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]
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 elementsAccessing elementsRemoving elementsIterating elementsIterating through Set
Adding elements
Accessing elements
Removing elements
Iterating elements
Iterating through Set
Now let us discuss these operations individually as follows:
Operations 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  
Java
// Java Program Demonstrating Working of Set by// Adding elements using add() method   // Importing all utility classesimport java.util.*;  // Main classclass GFG {      // Main driver method    public static void main(String[] args)    {        // Creating an object of Set and         // declaring object of type String        Set<String> hs = new HashSet<String>();          // Adding elements to above object        // using add() method        hs.add("B");        hs.add("B");        hs.add("C");        hs.add("A");          // Printing the elements inside the Set object        System.out.println(hs);    }}
[A, B, C]
Operation 2: Accessing the Elements
After adding the elements, if we wish to access the elements, we can use inbuilt methods like contains().
Example 
Java
// Java code to demonstrate Working of Set by// Accessing the Elements og the Set object  // Importing all utility classesimport java.util.*;  // Main classclass GFG {      // Main driver method    public static void main(String[] args)    {        // Creating an object of Set and         // declaring object of type String        Set<String> hs = new HashSet<String>();          // Elements are added using add() method        // Later onwards we will show accessing the same          // Custom input elements        hs.add("A");        hs.add("B");        hs.add("C");        hs.add("A");          // Print the Set object elements        System.out.println("Set is " + hs);          // Declaring a string        String check = "D";          // Check if the above string exists in        // the SortedSet or not        // using contains() method        System.out.println("Contains " + check + " "                           + hs.contains(check));    }}
Set is [A, B, C]
Contains D false
Operation 3: Removing the Values
The values can be removed from the Set using the remove() method.
Example
Java
// Java Program Demonstrating Working of Set by// Removing Element/s from the Set  // Importing all utility classesimport java.util.*;  // Main classclass GFG {      // Main driver method    public static void main(String[] args)    {        // Declaring object of Set of type String        Set<String> hs = new HashSet<String>();          // Elements are added        // using add() method          // Custom input elements        hs.add("A");        hs.add("B");        hs.add("C");        hs.add("B");        hs.add("D");        hs.add("E");          // Printing initial Set elements        System.out.println("Initial HashSet " + hs);          // Removing custom element        // using remove() method        hs.remove("B");          // Printing Set elements after removing an element        // and printing updated Set elements        System.out.println("After removing element " + hs);    }}
Initial HashSet [A, B, C, D, E]
After removing element [A, C, D, E]
Operation 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 
Java
// Java Program to Demonstrate Working of Set by // Iterating through the Elements   // Importing utility classes import java.util.*;  // Main class class GFG {      // Main driver method    public static void main(String[] args)    {        // Creating object of Set and declaring String type        Set<String> hs = new HashSet<String>();          // Adding elements to Set          // using add() method          // Custom input elements         hs.add("A");        hs.add("B");        hs.add("C");        hs.add("B");        hs.add("D");        hs.add("E");          // Iterating through the Set        // via for-each loop         for (String value : hs)              // Printing all the values inside the object             System.out.print(value + ", ");                  System.out.println();    }}
A, B, C, D, E, 
Classes that implement the Set interface in Java Collections can be easily perceived from the image below as follows and are listed as follows:
HashSet
EnumSet
LinkedHashSet
TreeSet
 
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
Java
// Java program Demonstrating Creation of Set object// Using the Hashset class  // Importing utility classesimport java.util.*;  // Main classclass GFG {      // 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());    }}
[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
Java
// Java program to demonstrate the// creation of the set object// using the EnumSet classimport java.util.*;  enum Gfg { CODE, LEARN, CONTRIBUTE, QUIZ, MCQ };  public class GFG {      public static void main(String[] args)    {        // Creating a set        Set<Gfg> set1;          // Adding the elements        set1 = EnumSet.of(Gfg.QUIZ, Gfg.CONTRIBUTE,                          Gfg.LEARN, Gfg.CODE);          System.out.println("Set 1: " + set1);    }}
Set 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
Java
// Java program to demonstrate the// creation of Set object using// the LinkedHashset classimport java.util.*;  class GFG {      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());    }}
[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
Java
// Java Program Demonstrating Creation of Set object// Using the TreeSet class  // Importing utility classesimport java.util.*;  // Main classclass GFG {      // 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());    }}
[Australia, India, South Africa]
Set after removing Australia:[India, South Africa]
Iterating over set:
India
South Africa
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