In this guide, we’re going to take a look at the HashSet class from Java Collections Framework.

We will learn all the APIs that HashSet implementation offers.

HashSet class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.

HashSet implementation is not synchronized - If multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally.

Example:

   Set s = Collections.synchronizedSet(new HashSet(...));

This guide covers important HashSet implementation API with examples. All the APIs are referred from HashSet JavaDoc.

1. Set Interface Overview

The Set interface contains only methods inherited from the Collection interface and adds the restriction that duplicate elements are prohibited.
A Set that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element.
As implied by its name, this interface models the mathematical set abstraction.
Set does not guarantee an insertion order.

2. HashSet class Overview

Java HashSet class is used to create a collection that uses a hash table for storage. It inherits the AbstractSet class and implements the Set interface.

The important points about the Java HashSet class are:

HashSet stores the elements by using a mechanism called hashing.
HashSet contains unique elements only.

3. Create a HashSet and Add new Elements

add()

The example below shows how to create a HashSet using the HashSet() constructor, and add new elements to it using the add() method.

// Creating a HashSet
Set<String> daysOfWeek = new HashSet<>();

// Adding new elements to the HashSet
daysOfWeek.add("Monday");
daysOfWeek.add("Tuesday");
daysOfWeek.add("Wednesday");
daysOfWeek.add("Thursday");
daysOfWeek.add("Friday");
daysOfWeek.add("Saturday");
daysOfWeek.add("Sunday");

// Adding duplicate elements will be ignored
daysOfWeek.add("Monday");

System.out.println(daysOfWeek);

addAll()

Creating HashSet from Collection Example

List<Integer> list = new ArrayList<>();
list.add(5);
list.add(10);
list.add(15);
list.add(20);
list.add(25);

List<Integer> list2 = new ArrayList<>();
list2.add(3);
list2.add(6);
list2.add(9);
list2.add(12);
list2.add(15);

// Creating a HashSet from another collection (ArrayList)
Set<Integer> set = new HashSet<>(list);

// Adding all the elements from an existing collection to a HashSet
set.addAll(list2);

System.out.println(set);

4. HashSet remove APIs with Example

remove(Object o)

Remove an element from a HashSet (The remove() method returns false if the element does not exist in the HashSet)

Set<Integer> numbers = new HashSet<>();
numbers.add(2);
numbers.add(3);
numbers.add(4);
numbers.add(5);
numbers.add(6);
numbers.add(7);
numbers.add(8);
numbers.add(9);
numbers.add(10);

System.out.println("numbers : " + numbers);

boolean isRemoved = numbers.remove(10);
System.out.println("After remove(10) => " + numbers);

removeAll(Collection<?> c)

Remove all elements belonging to a given collection from a HashSet

Set<Integer> numbers = new LinkedHashSet<>();
numbers.add(2);
numbers.add(3);
numbers.add(4);
numbers.add(5);
numbers.add(6);
numbers.add(7);
numbers.add(8);
numbers.add(9);
numbers.add(10);
List<Integer> perfectSquares = new ArrayList<>();
perfectSquares.add(4);
perfectSquares.add(9);

numbers.removeAll(perfectSquares);
System.out.println("After removeAll(perfectSquares) => " + numbers);

removeIf(Predicate<? super Integer> filter)

Remove all elements matching a given predicate

numbers.removeIf(num -> num % 2 == 0);
System.out.println("After removeIf() => " + numbers);

clear()

Remove all elements from HashSet (clear it completely)

numbers.clear();
System.out.println("After clear() => " + numbers);

5. Demonstration of How the Set contains duplicates and null values?

public class SetInterfaceHashImpl {

    public static void main(String[] args) {
        nullValueDemo();
        duplicateValueDemo();
        //bulkOperationDemo();
    }

    // Set can contain one null value
    private static void nullValueDemo() {
        Set < String > set = new HashSet < > ();
        set.add(null);
        set.add(null);
        System.out.println(set.toString());
    }

    // it is not contain duplicate elements
    private static void duplicateValueDemo() {
        Set < String > set = new HashSet < > ();
        set.add("element1");
        set.add("element1");
        // displays only one element
        System.out.println(set.toString());
    }
}

Output :

[null]

[element1]

6. HashSet Bulk Operations APIs with Examples

Let's write below bulk operation methods with examples

addAll()
retainAll()
containsAll()
removeAll()

// Set bulk operations
private static void bulkOperationDemo() {
    Set < String > set = new HashSet < > ();
    set.add("element1");
    set.add("element2");
    set.add("element3");
    set.add("element4");

    // Appends all of the elements in the specified collection to the end of
    // this list,
    // in the order that they are returned by the specified collection's
    // iterator (optional operation).
    Set < String > union = new HashSet < String > ();
    union.addAll(set);
    printMessage(union, "addALL operation example ");

    // Retains only the elements in this list that are contained in
    // the specified collection (optional operation).
    Set < String > intersection = new HashSet < String > ();
    intersection.add("element 1");
    intersection.add("element 2");
    intersection.add("element 3");
    intersection.add("element 4");
    System.out.println("retainAll -- > " + intersection.retainAll(set));

    // Removes from this list all of its elements that are
    // contained in the specified collection (optional operation).
    Set < String > difference = new HashSet < String > ();
    difference.add("element 1");
    difference.add("element 2");
    difference.add("element 3");
    difference.add("element 4");
    System.out.println("removeAll operation example ---> " + difference.removeAll(set));
    printMessage(difference, "removeAll operation example ");

    Set < String > checking = new HashSet < String > ();
    checking.add("element 1");
    checking.add("element 2");
    checking.add("element 3");
    checking.add("element 4");
    System.out.println("containsAll operation example ---- > " + checking.containsAll(set));
}

private static void printMessage(Set < String > difference, String msg) {
    difference.forEach(key - > System.out.println(msg + key));
}

7. Iterating over a HashSet

The following example shows different ways of iterating over a HashSet

Iterate over a HashSet using Java 8 forEach and lambda expression.
Iterate over a HashSet using iterator().
Iterate over a HashSet using iterator() and Java 8 forEachRemaining() method.
Iterate over a HashSet using a simple for-each loop.

iterator()

Set<String> list = new HashSet<>();
list.add("element 1");
list.add("element 2");
list.add("element 3");
list.add("element 4");

// using Iterator
Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
    String str = iterator.next();
    System.out.println(" only forward direction ---" + str);
}

Advance for() loop

// Using advanced for loop
for (String str : list) {
    System.out.println(" only forward direction ---" + str);
}

forEachRemaining()

// Java 8
list.forEachRemaining(str -> System.out.println(" only forward direction ---" + str));

forEach()

// Java 8
list.forEach(str -> System.out.println(" only forward direction ---" + str));

8. HashSet with User-defined objects

This example shows how to create a HashSet of user-defined objects.

import java.util.HashSet;
import java.util.Objects;
import java.util.Set;

class Customer {
    private long id;
    private String name;

    public Customer(long id, String name) {
        this.id = id;
        this.name = name;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    // Two customers are equal if their IDs are equal
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Customer customer = (Customer) o;
        return id == customer.id;
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }

    @Override
    public String toString() {
        return "Customer{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

public class HashSetUserDefinedObjectExample {
    public static void main(String[] args) {
        Set<Customer> customers = new HashSet<>();
        customers.add(new Customer(101, "Rajeev"));
        customers.add(new Customer(102, "Sachin"));
        customers.add(new Customer(103, "Chris"));

        /*
          HashSet will use the `equals()` & `hashCode()` implementations 
          of the Customer class to check for duplicates and ignore them
        */
        customers.add(new Customer(101, "Rajeev"));

        System.out.println(customers);
    }
}

Output

[Customer{id=101, name='Rajeev'}, Customer{id=102, name='Sachin'}, Customer{id=103, name='Chris'}]

9. How to make HashSet thread-safe?

This class implementation is not synchronized so it is not thread-safe. If multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally.HashSet must be synchronized externally.
Example :

// HashSet is not synchronized
private static void synchronizedHashSetDemo() {
    Set<String> set = new HashSet<>();
    Set<String> synchronizedSet = Collections.synchronizedSet(set);
}

Related Collections Framework API Guides

References

Collections Framework Core Java Java Collections Guide java.util Package

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