Improving structure with inheritance - Objects First with Java: A Practical Introduction Using BlueJ - page 286

8.4.1 Inheritance and access rights

To objects of other classes, MessagePost or PhotoPost objects appear just like all other
types of objects. As a consequence, members defined as public in either the superclass or sub-
class portions will be accessible to objects of other classes, but members defined as private
will be inaccessible.

In fact, the rule on privacy also applies between a subclass and its superclass: a subclass
cannot access private members of its superclass. It follows that if a subclass method
needed to access or change private fields in its superclass, then the superclass would
need to provide appropriate accessor and/or mutator methods. However, an object of a
subclass may call any public methods defined in its superclass as if they were defined lo-
cally in the subclass—no variable is needed, because the methods are all part of the same
object.

This issue of access rights between super- and subclasses is one we will discuss further in
Chapter 9 when we introduce the protected modifier.

Exercise 8.4 Open the project network-v2. This project contains a version of the network
application, rewritten to use inheritance, as described above. Note that the class diagram
displays the inheritance relationship. Open the source code of the MessagePost class and
remove the “ extends Post ” phrase. Close the editor. What changes do you observe in the
class diagram? Add the “ extends Post ” phrase again.

Exercise 8.5 Create a MessagePost object. Call some of its methods. Can you call the
inherited methods (for example, addComment )? What do you observe about the inherited
methods?

Exercise 8.6 In order to illustrate that a subclass can access non-private elements of
its superclass without any special syntax, try the following slightly artificial modification to
the MessagePost and Post classes. Create a method called printShortSummary in
the MessagePost class. Its task is to print just the phrase “Message post from NAME ”,
where NAME should show the name of the author. However, because the username field
is private in the Post class, it will be necessary to add a public getUserName method
to Post . Call this method from printShortSummary to access the name for printing.
Remember that no special syntax is required when a subclass calls a superclass method.
Try out your solution by creating a MessagePost object. Implement a similar method in the
PhotoPost class.

8.4.2 Inheritance and initialization

When we create an object, the constructor of that object takes care of initializing all object
fields to some reasonable state. We have to look more closely at how this is done in classes that
inherit from other classes.

When we create a MessagePost object, we pass two parameters to the message post's con-
structor: the name of the author and the message text. One of these contains a value for a field