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How to make ArrayList Read-Only in Java?

Making ArrayList Read-Only: Here, we are going to learn how to make ArrayList Read-Only in Java programming language?
Submitted by Preeti Jain, on September 10, 2019

Making ArrayList Read-Only

Given an ArrayList, and we have to make it Read-Only in Java.

Read-Only: If we make ArrayList as Read-Only i.e. we can only read ArrayList and we cannot perform other operations on ArrayList like delete, replace, add by using remove(), set(), add() methods, in read-only mode or in other words we cannot perform any modification in the ArrayList during Read-Only mode.

To make an ArrayList read-only, we use unmodifiableCollection() method of the Collections class.

unmodifiableCollection() method

  • unmodifiableCollection() method is available in java.util package.
  • unmodifiableCollection() method is used to make java collections (ArrayList) read-only.
  • unmodifiableCollection() method is used to returns the same ArrayList as we input (i.e. unmodifiable view).
  • unmodifiableCollection() method may throw an exception at the time of modification in unmodifiableCollection view.
    UnsupportedOperationException: In this exception, if we attempt to modify the collection.


    public static Collection unmodifiableCollection(Collection co){


co –represents the ArrayList collection object for which an unmodifiable view is to be returned.

Return value:

The return type of this method is Collection, it returns an unmodifiable view of the collection.


// Java program to demonstrate the example of 
// Java ArrayList make Read-Only by using 
// unmodifiableCollection() method of Collections class

import java.util.*;

public class ArrayListMakeReadOnly {
    public static void main(String[] args) {
        // ArrayList Declaration
        Collection arr_list = new ArrayList();

        // By using add() method to add few elements in 
        // ArrayList

        // Display ArrayList 
        System.out.println("Display ArrayList Elements");


        // By using unmodifiableCollection() method is used to make 
        // ArrayList Read-Only
        Collection al_ro = Collections.unmodifiableCollection(arr_list);

        // We will get an exception if we add element in Read-Only 
        // ArrayList i.e. Below statement is invalid

        // al_ro.add(60); 

        // We will get an exception if we delete element from Read-Only 
        // ArrayList i.e. Below statement is invalid

        // al_ro.remove(1); 

        // We will get an exception if we replace element in Read-Only 
        // ArrayList i.e. Below statement is invalid

        // al_ro.set(2,60); 


Display ArrayList Elements
[10, 20, 30, 40, 50]

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