A Quick Tour - The Java Programming Language - page 39

1.11.1. Invoking Methods of the Superclass
To make Pixel do the correct "clear" behavior, we provide a new im-
plementation of clear that first invokes its superclass's clear using the
super reference. The super reference is a lot like the this reference de-
scribed previously except that super references things from the super-
class, whereas this references things from the current object.
The invocation super.clear() looks to the superclass to execute clear as
it would for an object of the superclassnamely, Point . After invoking su-
per.clear() to clear out the Point part of the object, we add new func-
tionality to set color to a reasonable empty value. We choose null , a
reference to no object.
What would happen had we not invoked super.clear() in the previous
example? Pixel 's clear method would set color to its null value, but the
x and y variables that Pixel inherited from Point would not be set to any
"cleared" values. Not clearing all the values of a Pixel object, including
its Point parts, is probably a bug.
When you invoke super. method () , the runtime system looks back up the
inheritance hierarchy to the first superclass that contains the required
method . If Point didn't have a clear method, for example, the runtime
system would look at Point 's superclass for such a method and invoke
that, and so on.
For all other references, invoking a method uses the actual class of the
object, not the type of the object reference. Here is an example:
Point point = new Pixel();
point.clear(); // uses Pixel's clear()
In this example, Pixel 's version of clear is invoked even though the vari-
able that holds the Pixel is declared as a Point reference. But if we in-
voke super.clear() inside one of Pixel 's methods, that invocation will use
the Point class's implementation of the clear method.