Lab 12 : Experimenting with Polymorphism

Advanced Programming - CS 239 Department of Computer Science

Getting Ready: Before going any further you should:

1. Make a directory on your N: drive for this lab.

2. Setup your development environment.

3. Download the file named Document.java

4. Download the file named FormattedDocument.java

5. Download the file named Driver1.java

6. Download the file named Driver2.java

7. Make sure you understand the classes you just copied. (Note: These classes are different from any earlier versions you may have seen or used.)

Part I: This part of the lab will help you learn how to find errors related to polymorphism.

8. Compile all of the classes and execute Driver1. Is the output correct?

9a. Execute Driver2. What is wrong with the output?

9b. Why is the output incorrect? (Hint: Add output statements that will help you debug the getLineCount() method.)

Part II: This part of the lab reviews some issues involving accessibility/visibility.

10a. Make the attribute, text, in the Document class private. Re-compile the Document and FormattedDocument classes (in that order). What compile-time errors are generated

10b. Why were they generated?

11a. Fix the problem (without changing the accessibility/visibility of text). (Hint: How can a FormattedDocument object get access to text if it's private?)

11b. What did you do to fix the problem?

12. Compile and execute Driver2. Is the output correct?

Part II: This part of the lab will help you gain a better understanding of polymorphism.

13. When the getLineCount() message is sent to the object named, formatted, what code is executed?

14. Replace the getLineCount() method with the contents of getLineCount.java and explain the changes .

15. When the getLineCount() message is sent to the object named formatted, what does this refer to?

16. Continuing with this same example, when the getText() message is sent to this, what code is executed?

17. Compile and run the driver. Is the output correct?

18. Compile and run the original driver (i.e., the version that uses a Document object). Is the output correct?