JMU
Lab: Experimenting with Accessibility/Visibility


Instructions: Answer as many of the following questions as you can during the lab period. If you are unable to complete the assignment during the lab period it is strongly recommended that you complete it on your own.

Getting Ready: Before going any further, you should:

  1. Make a directory for this lab.
  2. Setup your development environment.
  3. Download the following files:
    Counter.java
    Document.java
    FormattedDocument.java
    Driver.java
    worksheet.txt
    to the appropriate directory/directories. (In most browsers, the easiest way to do this is by right-clicking on each of the links above.)

1. Review: This part of the lab is a review of material from earlier in the semester.
  1. Compile all of the classes and execute Driver.
  2. What output was generated? 
    Contains 18 word(s).

George is a little monkey, and all monkeys are curious. But no monkey is as cur
ious as George.
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  3. In Driver, don't change the declaration of the variable named doc but change the line containing doc = new Document(text); to doc = new String(text);.
  4. Compile Driver.
  5. What error was generated? Why? 
    Driver.java:24: incompatible types
found   : java.lang.String
required: Document
        doc = new String(text);
              ^
1 error

    This error was generated because a String object (the right operand of the assignment operator) is not a Document (the left operand of the assignment operator).

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  6. In Driver, don't change the declaration of the variable named doc but change the line that now contains doc = new String(text); to doc = new FormattedDocument(text);.
  7. Compile Driver.
  8. What error was generated? Why? 
    Driver.java:24: cannot find symbol
symbol  : constructor FormattedDocument(java.lang.String)
location: class FormattedDocument
        doc = new FormattedDocument(text);
              ^
1 error

    The FormattedDocument class does not have a constructor with a single String parameter.

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  9. Does the parent class of FormattedDocument have a constructor with a single String parameter?
    Yes.
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  10. Does the FormattedDocument class inherit constructors from its parent?
    No. In Java, constructors are not inherited and this is a good example of why they aren't. A FormattedDocument cannot do its job without knowing the maxWidth so it doesn't make sense to allow someone to construct a FormattedDocument without one.
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  11. In Driver, Don't change the declaration of the variable named doc but change the line that now contains doc = new FormattedDocument(text); to doc = new FormattedDocument(text, 20);.
  12. Compile Driver.
  13. Why did it compile even though there appear to be incompatible types?
    It compiles because a FormattedDocument (the right operand of the assignment operator) "is a" Document (the left operand of the assignment operator) since FormattedDocument extends Document.
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  14. Execute the driver.
  15. What output was generated? 
    This document has 18 words and at least 4 lines.

George is a little
monkey and all
monkeys are curious
But no monkey is as
curious as George
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  16. Why were the getDescription() and getText() methods in the FormattedDocument class used even though doc is declared to be a Document?

    doc behaved like a FormattedDocument even though it was declared to be a Document because doc contains a reference to a FormattedDocument. We will explore these kinds of issues more fully when we discuss polymorphism.

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  17. The getText() method in the FormattedDocument class contains the line temp = super.getText();. Explain this line of code.
    The getText() method in the FormattedDocument class is calling the method getText() method in the Document class.
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  18. Replace the line temp = super.getText(); in the FormattedDocument class with the line temp = getText();.
  19. Compile FormattedDocument and execute Driver.
  20. What output/error message was generated and why? 
    java.lang.StackOverflowError
	at FormattedDocument.getText(FormattedDocument.java:77)

    This (run-time) error was generated becase the getText() method calls itself over and over, never stopping. This is called an infinite recursion.

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2. Accessibility/Visibility Basics: In this part of the lab you will experiment with some of the basics of accessibility/visibility concepts.
  1. Compile Counter.java
  2. What error was generated? 
    Counter.java:13: cannot find symbol
symbol  : variable i
location: class Counter
       return i;
              ^
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  3. Why was this error generated?
    Because i has block scope and is not accessible outside of the for statement.
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  4. Add the following line to the top of the Counter class. 
    
public int    i;
  5. Compile Counter.java
  6. Why was no error generated?
    Because the i in the return statement refers to the attribute, not the local variable.
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  7. Change the declaration of the attribute i so that it is now private.
  8. Compile Counter.java
  9. Why was no error generated?
    Because the attribute i is accessible to all objects of the Counter class.
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  10. Assuming no other changes, what value will be returned by the firstNonNull() method when it is called.
    0
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  11. Why?
    Because the value of the attribute i is 0.
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  12. Delete the declaration of the attribute i.
  13. Add the following line to the top of the firstNonNull() method without making any other changes. 
    
private int    i;
  14. Compile Counter.java
  15. What error was generated? 
    Counter.java:7: illegal start of expression
       private int i;
       ^
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  16. Why was this error generated?
    Because a local variable can not have an accessibility modifier.
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  17. Fix firstNonNull() so that it works propery.
  18. What changes did you make?
    I removed private from the declaration of the local variable i and removed the declaration of i from the for statement.
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  19. Compile Counter.java to make sure you have didn't make any syntax errors.
  20. Remove the initialization of i from the for statement (but leave the semi-colon).
  21. Compile Counter.java,
  22. What error was generated? 
    Counter.java:9: variable i might not have been initialized
       for (; i<values.length; i++)
              ^
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3. Some Accessibility/Visibility Examples: In this part of the lab you will experiment with changing the accessibility/visibility of attributes and methods.
  1. Make the accessibility of the getDelimiters() method in the Document class private.
  2. Compile only the Document class.
  3. Why are no error messages generated?
    Because private methods are still available/visible within the class.
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  4. Compile only the FormattedDocument class.
  5. What error is generated and why? 
    FormattedDocument.java:78: getDelimiters() has private access in Document
        delim = super.getDelimiters();

    This error is generated because the getDelimiters() method is no longer available by any other classes, including derived classes.

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  6. Make the getDelimiters() method in the Document class protected.
  7. Compile all of the classes.
  8. Why are no errors generated?
    The getDelimiters() method in the Document class is now accessible by subclasses and no other classes use it.
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  9. What's the difference between the public version and the protected version? Which is better? Why?
    The first is visible to all classes, the second is visible only to subclasses.

    In this case it is probably better for it to be protected. First, it is hard to imagine why any class other than an ancestor would need to get the delimiters. Second, when it is public the API for the class is more complicated (i.e., there are more methods to try and understand).

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  10. Perform the same experiments with the getWordCount() method.
  11. In this case, which version is better? Why?
    In this case, the getWordCount() method is likely to be used by a whole variety of classes. Hence, it makes sense for it to be public.
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  12. Change the accessibility of the delimiters and text attributes in the Document class to protected.
  13. What changes can you now make to the FormattedDocument class? (Hint: Think about how the FormattedDocument class accesses these attributes.)
    You no longer need to use super.getText() and super.getDelimiters() in the subclass. Instead, you can directly use text and delimiters.
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  14. Do you like these changes? Why or why not?
    This is a hard one. At first glance it seems a lot simpler to use the attributes directly rather than the accessor methods. However, in subsequent labs we will see how this can get you into trouble.
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  15. Now that the delimiters attribute is protected, do you still need the getDelimiters() method?
    Certainly not the protected version.
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  16. Should either the getDescription() or getText() methods in the Document class be protected? Why or why not?
    Definitely not since they are used by other classes.
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Department of Computer Science

Copyright 2015