Java Collection Interface

Last Updated : 3 Apr 2026

Java Collection interface is the root interface of the Java Collection Framework that represents a group of objects, known as elements.

In this chapter, you will learn about the Collection interface, its purpose, and how it is used to work with groups of objects in Java.

What is Java Collection Interface?

Collection is a group of objects, which are known as elements. It is the root interface in the collection hierarchy. This interface is mainly used to pass collections and perform operations on them where maximum generality is required.

Collection Interface Declaration

The following syntax shows the declaration of the Collection interface:

Constructors of Collection Interface

The Collection interface does not have any constructors because it is an interface. Constructors are provided by the classes that implement this interface.

Methods of the Collection Interface

The Collection interface provides various methods to perform operations on a group of elements such as adding, removing, and checking elements.

The following table lists the commonly used methods of the Collection interface along with their descriptions.

Method	Description
add()	This method returns a Boolean value true if it inserts the specified element in this collection.
addAll()	This method returns a Boolean value true if it adds all the elements of specified collection in the invoking collection.
clear()	It removes all the elements automatically from this collection.
contains()	It returns a Boolean value true if this queue contains the specified element.
containsAll()	It returns a Boolean value true if this collection contains all the elements in the specified collection.
equals()	This method returns a boolean value true if the specified object is equal with this collection.
hashCode()	It returns a hash code value for this collection.
isEmpty()	This method returns true if this collection contains no elements or is empty.
iterator()	It returns an iterator over the elements in proper sequence.
remove()	It removes the specified element from this queue, if it is present in the collection.
removeAll()	It removes all the elements of this collection which are also present in the specified collection.
removeIf()	It removes all the elements of this collection that satisfy the given predicate filter.
retainAll()	This method retains only those elements in this collection that are present in the specified collection.
size()	It returns the total number of the elements in this collection.
spliterator()	It returns a spliterator over the elements in this collection.
toArray()	It returns an array containing all the elements of this collection which are in proper sequence.

Examples of Collection Interface

Here are the examples of the Collection interface that demonstrate how to perform operations like adding, removing, iterating, and checking elements.

Example 1: Basic Collection Operations

This example demonstrates how to add elements, check size, verify existence, and clear a collection.

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
public class Main {
    public static void main(String[] args) {
        Set<Integer> set = new HashSet<>();
        // inserts the specified element in this collection
        set.add(2);
        set.add(5);
        System.out.println("Initial collection :"+set);
        //it adds all the elements of specified collection in the invoking collection.
        Collections.addAll(set, 11, 12, 13, 14, 15);
        System.out.println("Final Collection : "+set);
        //returns the total size of the collection
        int size =set.size();
        System.out.println("Size of Collection : "+size);
        //It returns a Boolean value true if this queue contains the specified element.
        Boolean val=set.contains(5);
        if (val){
            System.out.println("5 is present in the collection");
        }
        else{
            System.out.println("5 is not present in the collection");
        }
        //It removes all the elements automatically from this collection.
        set.clear();
        System.out.println("Elements in collection : "+set);
    }
}

Compile and Run

Output:

Initial collection :[2, 5]
Final Collection : [2, 5, 11, 12, 13, 14, 15]
Size of Collection : 7
5 is present in the collection
Elements in collection : []

Example 2: Iteration and retainAll() Operation

This example demonstrates how to iterate over a collection and filter elements using the retainAll() method.

import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import java.util.concurrent.ConcurrentLinkedQueue;
public class Main {
    public static void main(String[] args) {
        ConcurrentLinkedQueue<Integer> queue = new ConcurrentLinkedQueue<Integer>();
        Set<Integer> set = new HashSet<>();
        //it adds all the elements of specified collection in the invoking collection.
        Collections.addAll(set, 11, 12, 13, 14, 15);
        System.out.println("Collection : "+set);
        //It returns an iterator over the elements in proper sequence.
        Iterator<Integer> iterator = set.iterator();
        while(iterator.hasNext()){
            System.out.println(iterator.next());
        }
       set.clear();
        //checks whether the set is empty or not
        Boolean b1= set.isEmpty();
        if (b1){
            System.out.println("Queue is empty");
        }
        else{
            System.out.println("Queue is not empty");
        }
        for (int i=1;i<21;i++){
            queue.add(i);
        }
        System.out.println(" Elements in the set : " + queue);
        for (int i = 1; i < 11; i++) {
            int j = i *5;
            set.add(j);
        }
        //will give the elements in the queue which are present in set
        queue.retainAll(set);
        System.out.println(" Multiple of 5 : " + queue);
    }
}

Compile and Run

Output:

Collection : [11, 12, 13, 14, 15]
11
12
13
14
15
Queue is empty
 Elements in the set : [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
 Multiple of 5 : [5, 10, 15, 20]

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Java Collection Interface

What is Java Collection Interface?

Collection Interface Declaration

Constructors of Collection Interface

Methods of the Collection Interface

Examples of Collection Interface

Example 1: Basic Collection Operations

Example 2: Iteration and retainAll() Operation

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Java Collection Interface

What is Java Collection Interface?

Collection Interface Declaration

Constructors of Collection Interface

Methods of the Collection Interface

Examples of Collection Interface

Example 1: Basic Collection Operations

Example 2: Iteration and retainAll() Operation

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