Make a collection an immutable collection

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

public class Main {

  public static void main(String args[]) {
    List<Character> list = new ArrayList<Character>();

    list.add('X');

    System.out.println("Element added to list: " + list.get(0));

    Collection<Character> immutableCol = Collections.unmodifiableCollection(list);

    immutableCol.add('Y');
  }
}