How can a method return an immutable (read-only) collection (list,map,set)?

return Collections.unmodifiableList(list);

return Collections.unmodifiableMap(map);

return Collections.unmodifiableSet(set);

References

http://download.oracle.com/javase/1,5.0/docs/api/java/util/Collections.html#unmodifiableSet(java.util.Set)