Java LinkedHashSet

The LinkedHashSet class of the Java collections framework provides functionalities of both the hashtable and the linked list data structure.

It implements the Set interface.

Elements of LinkedHashSet are stored in hash tables similar to HashSet.

However, linked hash sets maintain a doubly-linked list internally for all of its elements. The linked list defines the order in which elements are inserted in hash tables.

Create a LinkedHashSet

In order to create a linked hash set, we must import the java.util.LinkedHashSet package first.

Once we import the package, here is how we can create linked hash sets in Java.

// LinkedHashSet with 8 capacity and 0.75 load factor
LinkedHashSet<Integer> numbers = new LinkedHashSet<>(8, 0.75);

Here, we have created a linked hash set named numbers.

Notice, the part new LinkedHashSet<>(8, 0.75). Here, the first parameter is capacity and the second parameter is loadFactor.

• capacity - The capacity of this hash set is 8. Meaning, it can store 8 elements.
• loadFactor - The load factor of this hash set is 0.6. This means, whenever our hash table is filled by 60%, the elements are moved to a new hash table of double the size of the original hash table.

Default capacity and load factor

It's possible to create a linked hash set without defining its capacity and load factor. For example,

// LinkedHashSet with default capacity and load factor
LinkedHashSet<Integer> numbers1 = new LinkedHashSet<>();

By default,

• the capacity of the linked hash set will be 16
• the load factor will be 0.75

Creating LinkedHashSet from Other Collections

Here is how we can create a linked hash set containing all the elements of other collections.

import java.util.LinkedHashSet;
import java.util.ArrayList;

class Main {
    public static void main(String[] args) {
        // Creating an arrayList of even numbers
        ArrayList<Integer> evenNumbers = new ArrayList<>();
        evenNumbers.add(2);
        evenNumbers.add(4);
        System.out.println("ArrayList: " + evenNumbers);

        // Creating a LinkedHashSet from an ArrayList
        LinkedHashSet<Integer> numbers = new LinkedHashSet<>(evenNumbers);
        System.out.println("LinkedHashSet: " + numbers);
    }
}

Output

ArrayList: [2, 4]
LinkedHashSet: [2, 4]

Methods of LinkedHashSet

The LinkedHashSet class provides methods that allow us to perform various operations on the linked hash set.

Insert Elements to LinkedHashSet

• add() - inserts the specified element to the linked hash set
• addAll() - inserts all the elements of the specified collection to the linked hash set

For example,

import java.util.LinkedHashSet;

class Main {
    public static void main(String[] args) {
        LinkedHashSet<Integer> evenNumber = new LinkedHashSet<>();

        // Using add() method
        evenNumber.add(2);
        evenNumber.add(4);
        evenNumber.add(6);
        System.out.println("LinkedHashSet: " + evenNumber);

        LinkedHashSet<Integer> numbers = new LinkedHashSet<>();
        
        // Using addAll() method
        numbers.addAll(evenNumber);
        numbers.add(5);
        System.out.println("New LinkedHashSet: " + numbers);
    }
}

Output

LinkedHashSet: [2, 4, 6]
New LinkedHashSet: [2, 4, 6, 5]

Access LinkedHashSet Elements

To access the elements of a linked hash set, we can use the iterator() method. In order to use this method, we must import the java.util.Iterator package. For example,

import java.util.LinkedHashSet;
import java.util.Iterator;

class Main {
    public static void main(String[] args) {
        LinkedHashSet<Integer> numbers = new LinkedHashSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(6);
        System.out.println("LinkedHashSet: " + numbers);

        // Calling the iterator() method
        Iterator<Integer> iterate = numbers.iterator();

        System.out.print("LinkedHashSet using Iterator: ");

        // Accessing elements
        while(iterate.hasNext()) {
            System.out.print(iterate.next());
            System.out.print(", ");
        }
    }
}

Output

LinkedHashSet: [2, 5, 6]
LinkedHashSet using Iterator: 2, 5, 6,

Note:

• hasNext() returns true if there is a next element in the linked hash set
• next() returns the next element in the linked hash set

Remove Elements from HashSet

• remove() - removes the specified element from the linked hash set
• removeAll() - removes all the elements from the linked hash set

For example,

import java.util.LinkedHashSet;

class Main {
    public static void main(String[] args) {
        LinkedHashSet<Integer> numbers = new LinkedHashSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(6);
        System.out.println("LinkedHashSet: " + numbers);

        // Using the remove() method
        boolean value1 = numbers.remove(5);
        System.out.println("Is 5 removed? " + value1);

        boolean value2 = numbers.removeAll(numbers);
        System.out.println("Are all elements removed? " + value2);
    }
}

Output

LinkedHashSet: [2, 5, 6]
Is 5 removed? true
Are all elements removed? true

Set Operations

The various methods of the LinkedHashSet class can also be used to perform various set operations.

Union of Sets

Two perform the union between two sets, we can use the addAll() method. For example,

import java.util.LinkedHashSet;

class Main {
    public static void main(String[] args) {
        LinkedHashSet<Integer> evenNumbers = new LinkedHashSet<>();
        evenNumbers.add(2);
        evenNumbers.add(4);
        System.out.println("LinkedHashSet1: " + evenNumbers);

        LinkedHashSet<Integer> numbers = new LinkedHashSet<>();
        numbers.add(1);
        numbers.add(3);
        System.out.println("LinkedHashSet2: " + numbers);

        // Union of two set
        numbers.addAll(evenNumbers);
        System.out.println("Union is: " + numbers);
    }
}

Output

LinkedHashSet1: [2, 4]
LinkedHashSet2: [1, 3]
Union is: [1, 3, 2, 4]

Intersection of Sets

To perform the intersection between two sets, we can use the retainAll() method. For example

import java.util.LinkedHashSet;

class Main {
    public static void main(String[] args) {
        LinkedHashSet<Integer> primeNumbers = new LinkedHashSet<>();
        primeNumbers.add(2);
        primeNumbers.add(3);
        System.out.println("LinkedHashSet1: " + primeNumbers);

        LinkedHashSet<Integer> evenNumbers = new LinkedHashSet<>();
        evenNumbers.add(2);
        evenNumbers.add(4);
        System.out.println("LinkedHashSet2: " + evenNumbers);

        // Intersection of two sets
        evenNumbers.retainAll(primeNumbers);
        System.out.println("Intersection is: " + evenNumbers);
    }
}

Output

LinkedHashSet1: [2, 3]
LinkedHashSet2: [2, 4]
Intersection is: [2]

Difference of Sets

To calculate the difference between the two sets, we can use the removeAll() method. For example,

import java.util.LinkedHashSet;

class Main {
    public static void main(String[] args) {
        LinkedHashSet<Integer> primeNumbers = new LinkedHashSet<>();
        primeNumbers.add(2);
        primeNumbers.add(3);
        primeNumbers.add(5);
        System.out.println("LinkedHashSet1: " + primeNumbers);

        LinkedHashSet<Integer> oddNumbers = new LinkedHashSet<>();
        oddNumbers.add(1);
        oddNumbers.add(3);
        oddNumbers.add(5);
        System.out.println("LinkedHashSet2: " + oddNumbers);

        // Difference between LinkedHashSet1 and LinkedHashSet2
        primeNumbers.removeAll(oddNumbers);
        System.out.println("Difference : " + primeNumbers);
    }
}

Output

LinkedHashSet1: [2, 3, 5]
LinkedHashSet2: [1, 3, 5]
Difference: [2]

Subset

To check if a set is a subset of another set or not, we can use the containsAll() method. For example,

import java.util.LinkedHashSet;

class Main {
    public static void main(String[] args) {
        LinkedHashSet<Integer> numbers = new LinkedHashSet<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        System.out.println("LinkedHashSet1: " + numbers);

        LinkedHashSet<Integer> primeNumbers = new LinkedHashSet<>();
        primeNumbers.add(2);
        primeNumbers.add(3);
        System.out.println("LinkedHashSet2: " + primeNumbers);

        // Check if primeNumbers is a subset of numbers
        boolean result = numbers.containsAll(primeNumbers);
        System.out.println("Is LinkedHashSet2 is subset of LinkedHashSet1? " + result);
    }
}

Output

LinkedHashSet1: [1, 2, 3, 4]
LinkedHashSet2: [2, 3]
Is LinkedHashSet2 is a subset of LinkedHashSet1? true

Other Methods Of LinkedHashSet

To learn more about LinkedHashSet methods, visit Java LinkedHashSet (official Java documentation).

LinkedHashSet Vs. HashSet

Both LinkedHashSet and HashSet implements the Set interface. However, there exist some differences between them.

• LinkedHashSet maintains a linked list internally. Due to this, it maintains the insertion order of its elements.
• The LinkedHashSet class requires more storage than HashSet. This is because LinkedHashSet maintains linked lists internally.
• The performance of LinkedHashSet is slower than HashSet. It is because of linked lists present in LinkedHashSet.

LinkedHashSet Vs. TreeSet

Here are the major differences between LinkedHashSet and TreeSet:

• The TreeSet class implements the SortedSet interface. That's why elements in a tree set are sorted. However, the LinkedHashSet class only maintains the insertion order of its elements.
• A TreeSet is usually slower than a LinkedHashSet. It is because whenever an element is added to a TreeSet, it has to perform the sorting operation.
• LinkedHashSet allows the insertion of null values. However, we cannot insert a null value to TreeSet.