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The Java.util.concurrent.CopyOnArrayList.retainAll() method in Java is used to retain only the elements in the list that are contained in specific collection. Syntax:

 public boolean retainAll(Collection col)

Parameters: This method accepts a mandatory parameter col which is of the type of collection. This is the collection of elements to be retained in this vector. Return Type: This method returns a boolean value true if the list is changed at all as a result of call else false. Exception: This method throws a NullPointerException if the specified collection is NULL. Below Programs illustrate the retainAll() method of CopyOnArrayList in Java: Program 1: This program involves CopyOnArraylist retainAll() method of Integer Type:

Java

// Java Program to illustrate CopyOnArrayList
// retainAll() method
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.*;

public class GFG {
    public static void main(String[] args)
    {
        // create object of CopyOnWriteArrayList
        CopyOnWriteArrayList<Integer> ArrLis1
            = new CopyOnWriteArrayList<Integer>();

        // Add elements
        ArrLis1.add(34);
        ArrLis1.add(66);
        ArrLis1.add(72);
        ArrLis1.add(10);

        // print CopyOnWriteArrayList
        System.out.println("CopyOnWriteArrayList1: "
                           + ArrLis1);

        // create object of CopyOnWriteArrayList
        CopyOnWriteArrayList<Integer> ArrLis2
            = new CopyOnWriteArrayList<Integer>();

        // Add elements
        ArrLis2.add(10);
        ArrLis2.add(66);
        ArrLis2.add(97);

        // print CopyOnWriteArrayList
        System.out.println("CopyOnWriteArrayList2: "
                           + ArrLis2);

        // Applying the function
        ArrLis2.retainAll(ArrLis1);

        // print CopyOnWriteArrayLists

        System.out.println("After applying retainAll()"
                           + " method on ArrLis2");
        System.out.println("CopyOnWriteArrayList1: "
                           + ArrLis1);
        System.out.println("CopyOnWriteArrayList2: "
                           + ArrLis2);
    }
}

Output:

CopyOnWriteArrayList1: [34, 66, 72, 10]
CopyOnWriteArrayList2: [10, 66, 97]
After applying retainAll() method on ArrLis2
CopyOnWriteArrayList1: [34, 66, 72, 10]
CopyOnWriteArrayList2: [10, 66]

Program 2: This program involves CopyOnArraylist retainAll() method of String Type:

Java

// Java Program to illustrate CopyOnArrayList
// retainAll() method

import java.util.concurrent.CopyOnWriteArrayList;
import java.util.*;

public class GFG {
    public static void main(String[] args)
    {
        CopyOnWriteArrayList<String> ArrLis1
            = new CopyOnWriteArrayList<String>();

        // Add elements
        ArrLis1.add("geeks");
        ArrLis1.add("shaan");
        ArrLis1.add("gfg");
        ArrLis1.add("programming");

        // print CopyOnWriteArrayList
        System.out.println("CopyOnWriteArrayList1: "
                           + ArrLis1);

        // create object of CopyOnWriteArrayList
        CopyOnWriteArrayList<String> ArrLis2
            = new CopyOnWriteArrayList<String>();

        // Add elements
        ArrLis2.add("geeks");
        ArrLis2.add("gfg");
        ArrLis2.add("qwe");

        // print CopyOnWriteArrayList
        System.out.println("CopyOnWriteArrayList2: "
                           + ArrLis2);

        // Applying the function
        ArrLis2.retainAll(ArrLis1);

        // print CopyOnWriteArrayLists

        System.out.println("After applying retainAll()"
                           + " method on ArrLis2");
        System.out.println("CopyOnWriteArrayList1: "
                           + ArrLis1);
        System.out.println("CopyOnWriteArrayList2: "
                           + ArrLis2);
    }
}

Output:

CopyOnWriteArrayList1: [geeks, shaan, gfg, programming]
CopyOnWriteArrayList2: [geeks, gfg, qwe]
After applying retainAll() method on ArrLis2
CopyOnWriteArrayList1: [geeks, shaan, gfg, programming]
CopyOnWriteArrayList2: [geeks, gfg]

CopyOnWriteArrayList retainAll() method in Java with Examples

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