Read-Only Collections : Wrapped Collections « Collections « Java Tutorial

1. Java Tutorial
2. Collections
3. Wrapped Collections

public static Collection unmodifiableCollection(Collection c)
public static List unmodifiableList(List l)
public static Map unmodifiableMap(Map m)
public static Set unmodifiableSet(Set s)
public static SortedMap unmodifiableSortedMap(SortedMap m)
public static SortedSet unmodifiableSortedSet(SortedSet s)

To use these factory methods

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

public class MainClass {
  public static void main(String[] a) {
    Set s = new HashSet();
    s.add("A");
    s.add("B");
    s.add("C");
    s.add("D");
    s.add("E");
    s.add("F");
    s.add("H");

    Collections.unmodifiableSet(s);

    s = Collections.unmodifiableSet(s);

    s.clear();
  }

}

Exception in thread "main" java.lang.UnsupportedOperationException
  at java.util.Collections$UnmodifiableCollection.clear(Collections.java:1037)
  at MainClass.main(MainClass.java:20)