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 basically used to pass around the collections and manipulate them where the maximum generality is desired.

There are many methods defined in the Collection interface. These are as follows:

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.

Example 1

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
public class JavaCollectionExample1 {
    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);
    }
}

Test it Now

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

import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import java.util.concurrent.ConcurrentLinkedQueue;
public class JavaCollectionExample2 {
    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);
    }
}

Test it Now

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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