This is called "covariant return", and was introduced in 1.5.  An overriding
method may return a subtype ("covariant type") of the parent method's return
type.  You cannot have two different overrides that differ only in return
type, though.

In 1.4 and earlier (you do realize that 1.3 has been retired for some time
now, don't you?  And that 1.4 hits its "End-of-Life" this very year?)
covariant return types are not permitted.

<http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.4.8>
Specifically ss. 8.4.8.3.

Here's why it's allowed.  Consider

 public interface StatementCreator
 {
  java.sql.Statement createStatement() throws java.sql.SQLException;
 }

and an implementing class with

 public com.lewscanon.sql.Statement createStatement()
 throws java.sql.SQLException

where the return type implements java.sql.Statement.  From the point of view
of any interface-typed variable that invokes the method, it is getting a
java.sql.Statement back, so the contract is fulfilled.  It's a natural
concomitant of implicit widening conversions.  As long as what you want is
wider than what you got, you're fine.

The put side is the converse.  What you want to put has to be narrower, so the
method signature of the callee has to be wider.  This is "contravariant", or
supertype substitution, contrasted with the "covariant" or subtype
substitution in the return part of the method signature.  This is where the
concept of method overloads and the rules for finding the narrowest available
signature, i.e., contravariant type pattern apply.

The terms "contravariant" and "covariant" are more common in the context of
generics, where the wildcard "? extends" is a covariant type, and "? super" is
the contravariant.  You see the latter a lot in Comparables and the like:

 public static <T extends Comparable<? super T>> void sort(List<T> list)
<http://java.sun.com/javase/6/docs/api/java/util/Collections.html#sort(java.util.List)>

 public static <T> void sort(List<T> list, Comparator<? super T> c)
<http://java.sun.com/javase/6/docs/api/java/util/Collections.html#sort(java.util.
List,%20java.util.Comparator)>