LAB 12: EXPERIMENTING WITH POLYMORPHISM

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?

YESÏ

Output is:
ÏÏ§ÏThis document has 18 words and 2 lines
ÏÏ§Ï
ÏÏ§ÏGeorge is a little monkey, and all monkeys are curious.
ÏÏ§ÏBut no monkey is as curious as George.

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

The document has 18 words but it has more than 2 lines

Output is:Ï
ÏÏ§ÏThis document has 18 words and 2 lines
ÏÏ§Ï
ÏÏ§ÏGeorge is a little
ÏÏ§Ïmonkey, and all
ÏÏ§Ïmonkeys are
ÏÏ§Ïcurious.
ÏÏ§ÏBut no
ÏÏ§Ïmonkey is as curious
ÏÏ§Ïas George.

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

The getLineCount() method is using the text attribute of Document class method while the text is being printed by the FormattedDocument 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

«ÏjGRASP exec: javac A:\Lab12 -Polymorphism\FormattedDocument.java
Ï¼§ÏFormattedDocument.java:51: text has private access in Document
ÏÏ§Ïtokenizer = new StringTokenizer(text, " "); // Construct the tokenizer
ÏÏ§Ï1 error

10b. Why were they generated?

because FormattedDocument was denied access to parent’s text attribute (it doesn’t have its own text attribute.

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?)

have the child call the parent’s getText method

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

I stored the result of calling the parent’s getText() method in a temporary variable. the StringTokenizer then used the temporary variable (the copy) of the text

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

It compiles but the output is still not correct. It still says that there are 18 words and 2 lines (when there are 18 words and 7 lines).

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?

the getLineCount() method of the Document class using the unformatted text

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

a String called temp was declared (a new temporary String variable)

the current object’s getText() method was called.

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

the FormattedObject

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

the getText() method of the FormattedObject which calls the Document getText() method

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

Yes

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

Yes