More about inheritance - Objects First with Java: A Practical Introduction Using BlueJ - page 307

You should now have a situation corresponding to Figure 9.4, with display methods in three
classes. Compile your project. (If there are errors, remove them. This design should work.)

Before executing, predict which of the display methods will get called if you execute the
news feed's show method.

Try it out. Enter a message post and a photo post into the news feed and call the news feed's
show method. Which display methods were executed? Was your prediction correct? Try to
explain your observations.

Figure 9.4
Display, version 3:
display method
in subclasses and
superclass

NewsFeed

Post

GLVSOD\

MessagePost PhotoPost

GLVSOD\ GLVSOD\

9.3 Overriding

The next design we shall discuss is one where both the superclass and the subclasses have a
display method (Figure 9.4). The header of all the display methods is exactly the same.

Code 9.1 shows the relevant details of the source code of all three classes. Class Post has a
display method that prints out all the fields that are declared in Post (those common to mes-
sage posts and photo posts), and the subclasses MessagePost and PhotoPost print out the
fields specific to MessagePost and PhotoPost objects, respectively.

Code 9.1
Source code of the
display methods
in all three classes

public class Post
{

...
public void display()
{

System.out.println(username);
System.out.print(timeString(timestamp));