CS Labs: Lab 4

Programmer-Defined Classes

There are 9 checkpoints () in this lab. If you need help with any exercise, raise your hand.

Get the Lab 04 program files and put them in your Lab 04 directory. Download program files.

Implementing and Testing Methods

Go into the subdirectory ConvertDistance. You will use the Demo class (in the file Demo.java) to test the MetricConverter class (in the file MetricConverter.java). Examine the implementation of MetricConverter carefully.

The MetricConverter class has several methods, some unfinished. To answer some of these questions, we need to explain how to identify a program stub. A stub is a method which is not finished, but has enough code to be compiled and work with other methods. When one or more programmers are writing code, they frequently use stubs as placeholders until the programmer responsible has finished the work. In a stub, the body of the method usually contains only a comment saying 'this is a stub'. If the method needs to return a value in order to compile successfully, the programmer will add a line to return a dummy value such as 0.

Answer these questions for your next checkpoint:

1. According to the comments, how many methods (including constructors) are supposed to be defined in this class?
2. How many of these methods are fully implemented? What are their names?
3. How many are partially defined as stubs? What are their names?
4. How many of the methods listed in the comments are not defined at all, not even as stubs? What are their names?

As you have seen from the earlier checkpoint question, the centimetersToInches() method is not yet implemented. Using the inchesToCentimeters() method as a model, implement the centimetersToInches() method for your next checkpoint. Make sure to change the comments as well!

2 Show us your finished code for the centimetersToInches() method.

Examine the Demo class implementation. When you run this program, the main() method creates a MetricConverter object and tests the inchesToCentimeters() and centimetersToInches() methods.

1. Did your centimetersToInches() method perform as expected? (If not, fix it.)
2. Do you think the main() method thoroughly tests these two methods?

At this time, you should finish the implementation of the feetAndInchesToCentimeters() method. As you write it, follow these guidelines:

• Use the class constants defined in this class.
• Write the implementation using this strategy: first, calculate the total number of inches from the parameters feet and inches. (inches). Then, call another method in this class that is already defined to convert the total inches to centimeters.
• Make changes to your main() method in the Demo class to test your implementation. Keep these facts in mind:
  • feetAndInchesToCentimeters() takes two doubles as arguments. Use inputInches (declared as a double) and declare a new int local variable in your main() method called inputFeet to hold the input. (Do you think the int variable can be passed directly into the double argument that feetAndInchesToCentimeters() is expecting?)
  • Don't forget to ask the user for the number of feet to convert, and place that value in your new variable before you call feetAndInchesToCentimeters().
  • Make sure your output looks just as readable as those already shown in the main() method.

Close the files containing the Demo and MetricConverter classes.

Fun with Objects

A second constructor for a OurPoint object would be useful, so that we could set the x- and y- coordinate values at the time we construct an OurPoint object.

1. Implement a second constructor in the OurPoint class that allows the user to initialize a point with an x- and y- coordinate when it is created. For example, the following code should create aN OurPoint named testPoint that has an x-coordinate with value 2.3, and a y-coordinate with value -7.0.

                OurPoint testPoint = new OurPoint (2.3, -7);
        

2. Complete the implementation of the setY() stub.
3. Complete the implementation of the getY() stub.

Remove the comments that surround this main() method and compile and run the OurPoint class.

5 Show us how you have completed these methods, and the output from the execution of the main() method.

Visibility of Data Members and Methods

Add the following statement at the end of the main() method of OurPoint class

        System.out.println ("The x coordinate of q is: " + q.xCoord);

What if you placed the same System.out.println statement in a main() method that was not contained within the OurPoint class? Would this version compile? Why or why not? You can test out your answer. Open the Demo class in this subdirectory. This has a main() method that declares and constructs an OurPoint object q and that contains the statement above. Does this class compile?

6 Answer the questions listed above, and explain the difference between the two situations.

Local Variables and Instance Variables

Modify this program so that the output is consistent with values used to construct the OurPoint object q. You are not allowed to change anything in the main() method.

7 When you are ready, call us over and explain what happened before your modification and what you fixed.

Passing Objects

Compile and run the Demo program and observe the results. Notice, in particular, that the Rolls becomes dirty after Nigel drives it.

This directory has a file CarWash.java that contains most of the implemention of the CarWash class. Open this file and complete the implementation. In the Demo.java program, make a suitable instance of a CarWash object and finish the program by washing the Rolls and filling it with gas (at the appropriate place at the end of your Demo code indicated by comments).

Compile and run your Demo program and observe your results.

8 When you are ready, call us over and show us your completed code for Demo.java and the output of your program.

After the Lab

End of Lab