How to make an ArrayList read only in Java

A read only ArrayList means that no modification operations is allowed to be performed on ArrayList. This list cannot be modified by adding, removing or updating any element in the ArrayList. This is to prevent any modification be done to the ArrayList. The only operations that are allowed to be performed on read only ArrayList are search and read.

Operations Not allowed on Read Only ArrayList:

Adding of new element using add() method.
Removing any existing element using remove() method.
Changing any existing element using set() method.

Operations allowed on Read Only ArrayList:

Search
Read

Program to make ArrayList read only

import java.util.*;
public class JavaExample {
  public static void main(String[] args) {
    //An ArrayList of sports
    List<String> sports = new ArrayList<String>();
    sports.add("basketball");
    sports.add("tennis");
    sports.add("baseball");
    sports.add("golf");
    sports.add("volleyball");

    //Making ArrayList read only
    List<String> readOnlyList= Collections.unmodifiableList(sports);

    //We made the ArrayList "sports" read only
    //The following add() operation is not allowed on read only lists
    readOnlyList.add("Cricket");
    System.out.println(sports);
  }
}

Output: The programs throws following error because we are trying to add an element to a read only ArrayList. The important note here is that the list returned by the Collections.unmodifiableList() method is read only, which is readOnlyList in this example. This doesn’t make the original ArrayList read only which means you can still add, update and remove elements from sports ArrayList, however you would not be able to do the same in readOnlyList.
Java ArrayList read only error

Search and Read operations on read only ArrayList

Let’s see if we can perform search and read operations on the read only list. Here, we are not making any change in the unmodifiable list so the program runs fine.

import java.util.*;
public class JavaExample {
  public static void main(String[] args) {
    //An ArrayList of sports
    List<String> sports = new ArrayList<String>();
    sports.add("basketball"); //index 0
    sports.add("tennis"); //index 1
    sports.add("baseball"); //index 2
    sports.add("golf"); //index 3
    sports.add("volleyball"); //index 4

    //Making ArrayList read only
    List<String> readOnlyList= Collections.unmodifiableList(sports);

    //Read operation, reading element with index 1
    System.out.println(readOnlyList.get(1));

    //Search operation, finding the index of "golf" element
    System.out.println(readOnlyList.indexOf("golf"));
  }
}

Output:

tennis
3

Program to make ArrayList read only

Search and Read operations on read only ArrayList

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