More about inheritance - Objects First with Java: A Practical Introduction Using BlueJ - page 313

9.6 Method polymorphism

What we have just discussed in the previous sections (Sections 9.2-9.5) is yet another form of
polymorphism. It is what is known as polymorphic method dispatch (or method polymorphism
for short).

Concept:

Method polymor-
phism. Method
calls in Java are
polymorphic. The
same method call
may at different
times invoke dif-
ferent methods,
depending on the
dynamic type of
the variable used to
make that call.

Remember that a polymorphic variable is one that can store objects of varying types (every
object variable in Java is potentially polymorphic). In a similar manner, Java method calls are
polymorphic, because they may invoke different methods at different times. For instance, the
statement

post.display();

could invoke the MessagePost 's display method at one time and the PhotoPost 's dis-
play method at another, depending on the dynamic type of the post variable.

9.7 Object methods: toString

In Chapter 8, we mentioned that the universal superclass, Object , implements some methods
that are then part of all objects. The most interesting of these methods is toString , which we
introduce here (if you are interested in more detail, you can look up the interface for Object in
the standard library documentation).

Concept:

Exercise 9.4 Look up toString in the library documentation. What are its parameters?
What is its return type?

Every object in Java
has a toString
method that can
be used to return
a string repre-
sentation of itself.
Typically, to make
it useful, an object
should override this
method.

The purpose of the toString method is to create a string representation of an object. This is
useful for any objects that are ever to be textually represented in the user interface, but also helps
for all other objects; they can then easily be printed out for debugging purposes, for instance.

The default implementation of toString in class Object cannot supply a great amount of detail.
If, for example, we call toString on a PhotoPost object, we receive the string similar to this:

PhotoPost@6acdd1
The return value simply shows the object's class name and a magic number. 2

Exercise 9.5 You can easily try this out. Create an object of class PhotoPost in your
project, and then invoke the toString method from the Object submenu in the object's
pop-up menu.

2 The magic number is in fact the memory address where the object is stored. It is not very useful except
to establish identity. If this number is the same in two calls, we are looking at the same object. If it is
different, we have two distinct objects.