Java ArrayList retainAll() method

The retainAll () method of Java ArrayList class keeps only elements in the original list that are contained in the specified collection. In other words, it replaces the original list with the specified list.

Syntax:

Parameter:

"c": is the Collection that contains elements to be retained in this list.

Return:

Return "true": if the collection "c" has been retained successfully in the original list.

Exception:

java.lang.ClassCastException: if the class of an element of this list is incompatible with the specified collection.

java.lang.NullPointerException: If the specified collection "c" contains null element and the specified collection does not permit null elements, or the specified collection is null.

Example 1

import java.util.ArrayList;
public class ArrayListRetainAllExample1
{
ArrayList<String> numbers = new ArrayList<>();
numbers.add("10");
System.out.println(numbers);	// [10]
ArrayList<String> numbers1 = new ArrayList<>();
numbers1.add("1");
numbers1.add("2");
numbers1.add("3");
System.out.println(numbers1);	// [1, 2, 3]
System.out.println(numbers1.retainAll(numbers)); // true
System.out.println(numbers1);	// [10]

}

Test it Now

Output:

[10]
[1, 2, 3]
true
[10]

Example 2

import java.util.ArrayList;
public class ArrayListRetainAllExample2
{
ArrayList<String> numbers = null;
System.out.println(numbers);		// null
ArrayList<String> retainedNumbers = new ArrayList<>();
retainedNumbers.add("1");
retainedNumbers.add("2");
System.out.println(retainedNumbers);	// [1, 2]
retainedNumbers.retainAll(numbers); 	// throw  NullPointerException

}

Test it Now

Output:

null
[1, 2]

Example 3

import java.util.ArrayList;
public class ArrayListRetainAllExample3
{
ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(2);
numbers.add(null);
System.out.println(numbers);	// [2, null]
ArrayList<String> retainedNumbers = new ArrayList<>();
retainedNumbers.add("1");
retainedNumbers.add("3");
System.out.println(retainedNumbers);	// [1, 3]
System.out.println(retainedNumbers.retainAll(numbers)); // true
System.out.println(retainedNumbers);	// []
}

Test it Now

Output: