make ArrayList Read only

Question

In Java, how can you make an ArrayList read-only (so that no one can add elements, edit, or delete elements) after initialization?

Answer 1

Pass the collection object to its equivalent unmodifiable function of Collections class. The following code shows use of unmodifiableList

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;


public class Temp {

    public static void main(String[] args) {

        List objList = new ArrayList();
        objList.add(4);
        objList.add(5);
        objList.add(6);
        objList.add(7);

        objList = Collections.unmodifiableList(objList);
        System.out.println("List contents " + objList);

        try {
            objList.add(9);
        } catch(UnsupportedOperationException e) {
            e.printStackTrace();
            System.out.println("Exception occured");
        }
        System.out.println("List contents " + objList);
    }

}

same way you can create other collections unmodifiable as well

• Collections.unmodifiableMap(map)
• Collections.unmodifiableSet(set)