CPSC 111 Assignment 1: Pets

In this part of the assignment you are given a Java program that contains both syntax and logic errors. Your task is to identify and correct each of these errors. You will learn useful tricks for helping to debug your programs.

Start by downloading the source file PetStore.java. This source file contains a Java program that helps the owners of the PetLand pet store figure out how much animal feed to order each week.

• Cats eat .33 cups of cat chow each day.
• Dogs eat .75 cups of dog chow each day.
• Snakes eat 1 mouse each week.
• The owner must pay 5% GST for every purchase.
• There is 7% PST charged on cat food and dog food, but the mice to feed the snakes are not taxed by the province.
• Dog and cat chow is $40 per bag, and there are 32 cups in a bag.
• Mice cost 75 cents each.

Your task in this part of the assignment is to hand in a corrected version of the source code.

Hints:

• How do I detect syntax errors?
  The Java compiler does a great job of detecting syntax errors! As it tries to translate the code, it will print an error message when it finds a statement that it cannot translate. The Java compiler will often suggest what might be wrong (e.g. ";" missing, or ")" missing). So, to detect syntax errors, simply try to compile your code. Take note of any errors and correct them. Each time you fix a syntax error, recompile your code. Keep doing this until there are no syntax errors.

  Usually, it's best to start with the first error Java reports after each compilation! Often the later errors are reported just because Java became confused by the first error.

  Also note that Java's error messages are not always entirely correct or helpful!

  If you're not sure of the syntax or semantics of a method, try looking it up its class in the Java API. For instance, the Math class has documentation for the round and ceil methods.

• How do I detect logic errors?
  Logic errors are much more difficult to find than syntax errors. The compiler is no use to you here. When testing a program, your goal is to detect logic errors. It's like reading a good murder mystery. Put your detective hat on, think critically and logically, and find the offending code!

  The best way to detect logic errors is to come up with a set of tests and check the output from the program. A test consists of a set of inputs, the expected set of outputs and the actual outputs from the program. If the actual output matches the expected output, then the test passes, otherwise the test fails and you must try to find the error in your code.

  When developing your tests, not only must you test "normal" or "usual" cases, you must also check "marginal" or "boundary" cases. For example, suppose you have a program that asks for a user's age and determines if the user is a senior citizen. Suppose that senior citizens are classified as people aged 65 or more. When you test the program, you want to test a value less than 65 and a value more than 65 and make sure that the program prints "Not senior citizen" and "Is senior citizen", respectively. These are "regular" cases. It is also critical to test the value 65. When the input is 65, the output should indicate that the user is a senior citizen. If the programmer uses "> 65" instead of ">= 65" in their code, the output will not be correct. It is a common error to write ">" when ">=" should be used - this error will be detected only if we test the value 65. We refer to 65 as a "boundary" value.

  Suppose you run a test that fails. How do you find the error? There are various techniques for doing this - you can simply "walk" through the code checking that each line is logically correct. With a small piece of code, this will often work quite well. However, for larger pieces of code you will want to try to narrow your search down to a smaller chunk of code. You can do this by inserting System.out.println(...); statements in your code. Specifically, you can print out the value of key variables and ensure that they are correct. Note that the only value printed by PetStore.java is the final amount and cost of food to buy. It might be helpful to print out some of the intermediate values computed along the way and check that each of them is correct. Try using this technique - you will find that you detect logic errors much more quickly than staring at the code!

import java.applet.Applet;
import java.awt.*;
public class PetLandFlag extends Applet
{
    /** 
     * method that is called automatically when the applet is to be painted
     * (i.e. when it is first displayed and when the window is resized)
     */
    public void paint( Graphics page )
    {
        super.paint(page);
    
        // get current width of the applet window
        int width = getWidth();
        int height = getHeight();

        // now draw the PetLand flag
		
    }

<html>
   <div align="center">
      <applet code="PetLandFlag.class" width=400 height=300>
      </applet>
   </div>
</html>

You can download this file PetLandFlag.html by right-clicking the link and choosing Save As... (or Save Link As...). Save the file in the same directory as your Java code. You can now view your applet using appletviewer:

appletviewer PetLandFlag.html

Note that appletviewer is part of the Java Development Kit (JDK), so if you've got the compiler and interpreter installed, you won't have to do anything else to run it.
 

Graphics Hints:

• Note that the paint method has a parameter of type Graphics. The Graphics parameter that is passed is already referencing a Graphics object so you do not have to instantiate this class yourself. In other words, do not attempt to do something like the following in your paint method:
      page = new Graphics();
   
• When the user resizes the window, the paint method is called automatically so that the applet window is repainted. Hence, the only code you need to supply is in the paint method but you must be sure to position your flag based on the current width and height of the window so that it is centred in the window.
   
• You will need to understand the coordinate system that is used by the Graphics class. The origin is at the top-left corner of the window with the x-coordinate increasing to the right and the y-coordinate increasing to the bottom of the window.
   
• You may find the following methods of the Graphics class useful:
  Graphics.setColor (y'all note the American spelling!)
  Graphics.fillPolygon
  Graphics.fillOval
  Graphics.fillRect
• To use the Graphics.setColor method, you'll also need the Color class which lets you easily specify several predefined ones. For example, to set the colour used for drawing subsequent shapes to black, you'd do:
   
  page.setColor( Color.BLACK );
   
  
• To use the Graphics.fillPolygon method, you'll also need the Polygon class and its Polygon.addPoint method. One way to draw the animal face is to create a series of Polygon objects, which you then draw with the fillPolygon method. For example, suppose you want to draw a triangle with vertices at the points (25, 25), (50, 75), (75, 25).
   
  

  We first create the Polygon: Polygon triangle = new Polygon(); triangle.addPoint( 25, 25 ); triangle.addPoint( 50, 75 ); triangle.addPoint( 75, 25 );   Then we draw the triangle: page.setColor( Color.GREEN ); page.fillPolygon( triangle );

  
  
   

Style Hints:

• Use constants (variables that are declared to be final) in your code to improve readability. Remember that programming is not just about "getting it going", but about creating code that is understandable by humans.  
   
  
• Be sure to use good programming style throughout including:
  • appropriate use of whitespace - indentation, spacing around operators, blank lines to separate sections of code that perform distinct tasks (e.g., leave a blank line between your variable declarations and the next piece of code), etc.
  • use of descriptive identifiers
  • comment statements - be sure to include a comment statement at the top of your code that describes what your program will do when it runs.
    

Problem Solving Hints:

Let's review some techniques for solving problems. If you're not careful, when trying to solve a problem like this, you can create more problems than you solve. A powerful approach to problem solving is to divide and conquer. One way to break down the current problem is:

1. draw just a yellow rectangle
   • make sure the rectangle still covers the whole window when you resize the applet window!
2. add a blue strip across the top of the flag
   • again, test to make sure the blue strip covers the right percentage of the window when you resize vertically
3. add a second blue strip across the bottom
4. add the animal face to the centre of the flag
   1. draw the face as a square black box
      • test see whether the face adapts correctly to resizing
   2. draw the ears as two more small black boxes
   3. draw the eyes as two small green boxes

Of course, there are many possible ways to break down a problem. For instance, you could draw the face with one big polygon that includes the ears as part of the face, instead of as separate boxes.

Don't be tempted to code the entire solution in one step! Code each part of the problem separately and test it before moving on to the next part. If you have problems with the first step, for example, you're not going to fix them by moving on to the next!
 

Testing:

Devise a series of tests that you will use to check that your program is working correctly. Each test could have:

• a name
• specified input from the user in the form of mouse actions (eg. user expands width of applet window by dragging right hand edge of window with the mouse)
• the expected output in the form of expected changes to the picture displayed in the applet window

Apply each of your tests to your program and record the result. Verify that the actual output matches the expected output. If any of your tests fails, you probably have some debugging to do!

Testing is very important in creating good programs, even though the tests aren't something that you hand in so that we grade them directly. We have created tests of our own to see if your code does the right thing!

Testing is also very useful during debugging - we strongly recommend creating tests for Part 1 as well!

Do you feel like doing more? Here are a few (completely optional!) suggestions:

• Make a fancier dog face: add more features like a mouth, a nose, a tongue hanging out, maybe some teeth. And/or make the face more realistic and less cartoony.
• Add a second face that looks like a cat.
• Add a snake across the bottom blue stripe, since PetLand sells snakes too!

/*
  Authors: 
  UNIX login IDs: 
  Student numbers: 
  Date:   
  
  By submitting this file, we acknowledge that the persons whose names
  appear above are the only authors of this code except as acknowledged in
  the code below.
*/

http://www.cs.ubc.ca/ugrad/facilities/windows/handin.shtml

You will have to install a security certificate on your machine (instructions will be provided on-screen) and then log in using your computer science undergraduate account ID and password. You will then see a page that looks like:

• Enter cs111 for the Course
• Enter assign1 for the Assignment name
• Click the Browse button and select the zip file assign1.zip that contains your assignment files.
• Click the Handin assignment button

Important: now check that your work has been submitted correctly by choosing the Check submissions button. Note that you have to specify the course and assignment name as you did when you first handed in your assignment. You will see a list of the files that were contained in your zip file. If any files are missing, repeat the process.

You can overwrite your submission any time before the due date by clicking the Overwrite previous box before you click the Handin assignment button.

If you want to check an assignment due date, select the List assignments button. Note that you must specify the Course (cs111).

Working in Teams:

Late Penalty:

• 10% deduction if your work is up to 24 hours late
• 30% deduction if your work is up to 48 hours late
• 100% deduction if your work is more than 48 hours late

Please note that weekends and holidays are counted!

A Word of Advice: