Improving structure with inheritance - Objects First with Java: A Practical Introduction Using BlueJ - page 290

inherits from Posts directly. MessagePost objects inherit everything from both superclasses
and have the same fields and methods as before. Objects of class EventPost will inherit the
username and timestamp , but not the comments.

Figure 8.8
Adding more post
types to network

This is a very common situation in designing class hierarchies. When the hierarchy does not
seem to fit properly, we have to refactor the hierarchy.

Classes that are not intended to be used to create instances, but whose purpose is exclusively to
serve as superclasses for other classes (such as Post and CommentedPost ), are called abstract
classes. We shall investigate this in more detail in Chapter 10.

Exercise 8.8 Open the network-v2 project. Add a class for event posts to the project.
Create some event-post objects and test that all methods work as expected.

8.6 Advantages of inheritance (so far)

We have seen several advantages of using inheritance for the network application. Before
we explore other aspects of inheritance, we shall summarize the general advantages we have
encountered so far:

■ Avoiding code duplication The use of inheritance avoids the need to write identical or very
similar copies of code twice (or even more often).