Java ArrayList retainAll()

In this tutorial, we will learn about the Java ArrayList retainAll() method, and learn how to use this method to retain only the elements in this ArrayList that are contained in the specified collection, with the help of examples.

Java ArrayList retainAll() method

ArrayList retainAll() retains only the elements in this ArrayList that are contained in the specified collection.

Syntax

The syntax of retainAll() method is

retainAll(Collection<?> collection)

where

Parameter	Description
collection	The collection containing elements to be retained in this ArrayList.

Returns

The method returns boolean value.

1. retainAll(collection) – Retain elements in the ArrayList that are present in given collection

In this example, we will take an ArrayList of strings with the elements: "a", "b", "c", "d", "e", "d", "c", "f". We will also take a collection "c" ,"a", "d". We will retain only the elements of this ArrayList which match any of the element in the collection using ArrayList retainAll() method.

Java Program

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Example {
	public static void main(String[] args) {
		ArrayList<String> arrayList = new ArrayList<String>(
				Arrays.asList("a", "b", "c", "d", "e", "d", "c", "f"));		
		List<String> collection = Arrays.asList("c" ,"a", "d");
		
		System.out.println("Original ArrayList           : " + arrayList);
		boolean value = arrayList.retainAll(collection);
		System.out.println("Returned value : " + value);
		System.out.println("ArrayList after retainAll()  : " + arrayList);
	}
}

Output

Original ArrayList           : [a, b, c, d, e, d, c, f]
Returned value : true
ArrayList after retainAll()  : [a, c, d, d, c]

Conclusion

In this Java Tutorial, we have learnt the syntax of Java ArrayList retainAll() method, and also learnt how to use this method with the help of examples.