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 list object to Collections.unmodifiableList(). See the example below.

import java.util.*;

public class CollDemo
{
    public static void main(String[] argv) throws Exception
    {
        List stuff = Arrays.asList(new String[] { "a", "b" });
        List list = new ArrayList(stuff);
        list = Collections.unmodifiableList(list);
        Set set = new HashSet(stuff);
        set = Collections.unmodifiableSet(set);
        Map map = new HashMap();
        map = Collections.unmodifiableMap(map);
        System.out.println("Collection is read-only now.");
    }
}