Java Collection retainAll() method

The retainAll() method of Java Collection Interface keeps only those elements in this queue that are present in the specified collection.

Syntax

Parameters

The parameter 'c' represents the collection having elements to be kept in this collection.

Return Value

The retainAll() method returns true if this collection changes as a result of the call.

Throws

The retainAll() method throws:

UnsupportedOperationException- if the retainAll operation is not supported by this collection.

ClassCastException- if the types of one or more elements in this collection are not compatible with the specified collection.

NullPointerException- if this collection is null or it contains one or more null elements and the invoked collection does not allow null elements.

Example 1

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.ConcurrentLinkedQueue;
public class JavaCollectionRetainAllExample1 {
    public static void main(String[] args) {
        Collection<Character> collection = new ConcurrentLinkedQueue<Character>();
        char i;
        for(i='a';i<='z';++i){
            collection.add(i);
        }
        System.out.println("collection : "+collection);
        List<Character> list = new ArrayList<Character>();
        list.add('a');
        list.add('e');
        list.add('i');
        list.add('o');
        list.add('u');
        //retains only those elements in the collection which are present in list
        collection.retainAll(list);
        System.out.println("Vowels : "+collection);
    }
}

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Output:

collection : [a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z]
Vowels : [a, e, i, o, u]

Example 2

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.ConcurrentLinkedQueue;
public class JavaCollectionRetainAllExample2 {
    public static void main(String[] args) {
        Collection<Integer> collection = new ConcurrentLinkedQueue<Integer>();
        List<Integer> list = new ArrayList<Integer>();
        for (int i=1;i<21;i++){
            collection.add(i);
        }
        System.out.println("Total no : "+ collection);
        for (int i=1;i<21;i++) {
            int j=i%2;

            if (j == 0) {
                list.add(i);
            }
        }
        //retains only those elements in the collection which are present in list
        collection.retainAll(list);
        System.out.println(" Table of 2 : "+collection);
    }
}

Test it Now

Output:

Total no : [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
 Table of 2 : [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

Example 3

import java.util.ArrayDeque;
import java.util.Collection;

public class JavaCollectionRetainAllExample3 {
    public static void main(String[] args) {
        Collection<String> collection=new ArrayDeque<>();
        collection.add("Himanshu");
        collection.add("Sham");
        collection.add("Rita");
        Collection<Boolean> collection1=new ArrayDeque<>();
        //it will give an exception as it do not support null elements.
        collection1.retainAll(null);
        System.out.println(collection.containsAll(collection1));
    }
}

Test it Now

Output:

Exception in thread "main" java.lang.NullPointerException
	at java.util.Objects.requireNonNull(Objects.java:203)
	at java.util.AbstractCollection.retainAll(AbstractCollection.java:405)
	at com.javaTpoint.JavaCollectionRetainAllExample4.main(JavaCollectionRetainAllExample4.java:17)

Java Collection retainAll() Method

The retainAll() method of Java Collection class retains or keeps only the elements in this collection that are contained in the invoked collection and removes all the elements that are not contained in the specified collection.

Syntax

Parameters

The parameter 'c' represents the collection which contains the elements to be retained in this collection.

Return Value:

The retainAll () method returns a Boolean value 'true' if this collection has changed as a result of this call.

Throws

UnsupportedOperationException- if the retainAll method is not supported by this collection.

ClassCastException- if the types of one or more elements in this collection are not compatible with the invoked collection.

NullPointerException- if this collection is null or it contains one or more null elements and the specified collection does not allow null elements.

Example 1

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.ConcurrentLinkedQueue;
public class JavaCollectionRetainAllExample4 {
    public static void main(String[] args) {
        Collection<Character> collection = new ConcurrentLinkedQueue<Character>();
        char c;
        for(c='A';c<='Z';++c){
            // collection does not support null elements
            collection.add(c);
        }
        System.out.println("collection : "+collection);
        List<Character> list = new ArrayList<Character>();
        list.add('A');
        list.add('E');
        list.add('I');
        list.add('O');
        list.add('U');
        //retains only those elements in the collection which are present in list
        collection.retainAll(list);
        System.out.println("Vowels : "+collection);
    }
}

Test it Now

Output:

collection : [A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z]
Vowels : [A, E, I, O, U]

Example 2

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.ConcurrentLinkedQueue;
public class JavaCollectionRetainAllExample5 {
    public static void main(String[] args) {
        Collection<Integer> collection = new ConcurrentLinkedQueue<Integer>();
        int i;
        for(i=1;i<=50;++i){
            collection.add(i);
        }
        System.out.println("collection : "+collection);
        List<Integer> list = new ArrayList<Integer>();
      for ( i=1; i<=50;++i){
          if(i%5==0){
              list.add(i);
          }
      }
        //retains only those elements in the collection which are present in list
        collection.retainAll(list);
        System.out.println("Vowels : "+collection);
    }
}

Test it Now

Output:

collection : [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 
22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 
44, 45, 46, 47, 48, 49, 50]
Vowels : [5, 10, 15, 20, 25, 30, 35, 40, 45, 50]

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