Extending Classes - The Java Programming Language - page 127

This is no different from having overloaded forms of a method declared
in the same class.
Overriding a method means that you have replaced its implementation
so that when the method is invoked on an object of the subclass, it
is the subclass's version of the method that gets invoked. In the Col-
orAttr class, we overrode the Attr class's setValue(Object) by providing
a new setValue(Object) method in the ColorAttr class that uses the su-
per keyword to invoke the superclass's implementation and then invokes
decodeColor . The super reference can be used in method invocations to
access methods from the superclass that are overridden in this class.
You'll learn about super in detail on page 89 .
When you're overriding methods, the signature must be the same as in
the superclassif they differ then it is an overload, not an override. The
return type of an overriding method is allowed to vary in a specific way:
If the return type is a reference type then the overriding method can
declare a return type that is a subtype of that declared by the super-
class method. Because a reference to a superclass can always hold a
reference to an instance of a subclass, this variation is perfectly safeit is
referred to as being a covariant return type. We'll see a good example
of this when we look at object cloning on page 101 . If the return type is
a primitive type, then the return type of the overriding method must be
identical to that of the superclass method. It is an error if two methods
differ only in return type and the compiler will reject your class.
For overriding in a varargs method, as with overloading (see page 60 ) ,
a sequence parameter of type T... is treated the same as a parameter
of type T[] . This means that an overriding method can "convert" the last
array parameter to a sequence, without changing the signature of the
method. This allows clients of the subclass to invoke that method with a
variable number of arguments. Defining an overriding method that re-
places a sequence with an array is allowed, but is strongly discouraged.
This is confusing and not useful, so don't do it. [1]
[1] In the JDK 5.0 release the javac compiler issues a (somewhat misleading) warning in both cases.
It is expected that the warning when converting to a sequence will be removed. It is possible that in a
future release converting from a sequence to an array will be an error.