Object interaction - Objects First with Java: A Practical Introduction Using BlueJ - page 88

We can now use the Step button repeatedly to step to the end of the method. This allows us to
see the path the execution takes. This is especially interesting in conditional statements: we can
clearly see which branch of an if statement is executed and use this to see whether it matches
our expectations.

Exercise 3.39 Call the same method ( printNextMailItem ) again. Step through the
method again, as before. What do you observe? Explain why this is.

3.13.3 Stepping into methods

When stepping through the printNextMailItem method, we have seen two method calls
to objects of our own classes. The line

MailItem item = server.getNextMailItem(user);
includes a call to the getNextMailItem method of the server object. Checking the instance
variable declarations, we can see that the server object is declared of class MailServer .

The line

item.print();
calls the print method of the item object. We can see in the first line of the printNextMail
Item method that item is declared to be of class MailItem .

Using the Step command in the debugger, we have used abstraction: we have viewed the print
method of the item class as a single instruction, and we could observe that its effect is to print
out the details (sender, recipient, and message) of the mail item.

If we are interested in more detail, we can look further into the process and see the print method
itself execute step by step. This is done by using the Step Into command in the debugger instead
of the Step command. Step Into will step into the method being called and stop at the first line
inside that method.

Exercise 3.40 Set up the same test situation as we did before. That is, send a message
from Sophie to Juan. Then invoke the printNextMailItem message of Juan's mail client
again. Step forward as before. This time, when you reach the line

item.print();

use the Step Into command instead of the Step command. Make sure you can see the text
terminal window as you step forward. What do you observe? Explain what you see.

3.14 Method calling revisited

In the experiments in Section 3.13, we have seen another example of object interaction
similar to one we saw before: objects calling methods of other objects. In the printNext-
MailItem method, the MailClient object made a call to a MailServer object to retrieve
the next mail item. This method ( getNextMailItem ) returned a value—an object of type