How do I get access to an unmodifiable collection

In this Q&A, we'll go over how to get access to unmodifiable collection.

You can use Collections.unmodifiable<Set|Map|List> methods to get access to an unmodifiable collection.

Any write operations on the unmodifiable collection object will throw UnsupportedOperationException.

Please note the following

• unmodifiable collection only maintains the structural integrity of the collection.  Mutable objects in collection can still be mutated.  Please see code below
• collection can still be modified using a modifiable reference 
• unmodifiable class is a static nested class within Collections class.  It is not exposed via the collections framework

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

public class Unmodifiable {

    static class inner {
        private int i;

        inner(int i) {
            this.i = i;
        }

        public int getI() {
            return i;
        }

        public void setI( int _i) {
            i = _i;
        }
    }
    public static void main(String[] args ) {
        List<inner> l = new ArrayList<>(1000);

        System.out.println( "In setup");
        for(int i=0; i < 1000; ++i) {
            l.add(new inner(i));
        }

        // Note that return value is List not UnmodifiableList
        List<inner> ul = Collections.unmodifiableList(l);

        // element values can be changed
        ul.get(0).setI(2);        
        assert(ul.get(0).getI() == 2);

        try {
            ul.add(new inner(1001));
        }
        catch( Exception e) {
            // Cannot add/remove elements to unmodifiable list
            System.out.println(e.toString());
            assertEquals("caught exception", true, true);
        }

    }
}