CSCI 241 Labs: Lab 7
Static Methods with Triangles

Review these instructions before beginning the exercises.

There are 4 checkpoints , plus the clean-up checkpoint, in this lab. You and your partner should work together using just one of your accounts. If you need help with any exercise, raise your hand.

Getting Started

Copy the directory /home/student/Classes/Cs241/Labs/Lab07 to your account.

Change directories into Lab07 and start BlueJ by entering bluej&.

Online Documentation Review

As you have seen, Java programmers need to find details about the pre-built Java classes they use. We have used classes that are both standard (included with the Java language, such as String and Scanner) and non-standard (added for specific purposes, such as the ACM package of classes we have used for drawing circles in previous lab exercises).

When you have an operating system such as Microsoft Windows installed on a machine, the main screen you see is called a 'desktop'. Using Gnome, you have the potential to work with four 'virtual' desktops (in other words, there are four desktops available, and you can see one at a time). Gnome calls these workspaces.

An easy way to keep documentation handy is to have Firefox running in one workspace while you run BlueJ in a different workspace. In case you don't know how to do this:

Return to the desktop in which you are running BlueJ. Click on one of the three other workspaces that are empty and start running Firefox.

In Firefox, go to your course web page: www.cs.uwp.edu/Classes/Cs241. In the Course Related Documents and Websites section, we have included links to documentation for both the standard Java classes and also for the ACM library.

Click on "Java Platform Standard Edition & Java Development Kit Version 11 API Specification". This document is the API Application Programming Interface) specification for the version of the Java language that we are using.

Find the java.base module in the table and click it.
The java.base module holds the most commonly used classes including all that we will need this semester.
Find the java.lang package in the table and click it.

A general summary of the java.lang package appears in the introductory paragraphs. Scroll down to the Class Summary section. You now see all the classes contained in the java.lang package.  Examine the list; what looks familiar?  We'll be asking you about which classes and methods you have used in the next checkpoint.

Click the Integer class to see its details.  Review its documentation.  What have you used from it?

Now, look at the details about the Math class.  What do you notice about it?  How many overloaded methods do you see?

1 Show us that you found this package and its classes, and answer these questions:

  1. Which classes in java.lang have you already used this semester?  Which methods inside those classes?
  2. Remember how you learned to extract an int value from a String? You wrote a line that looked like:
    int x = Integer.parseInt(xString);
    To calculate a square root, you write a line that looks like:
    double y = Math.sqrt(number);
    How are the formats of right-hand sides of these two lines similar to each other?
  3. Which methods in the Math class are overloaded?  How does Java tell them apart?
  4. There are three methods that "round" numbers in the Math class. They are: round(), ceil() and floor(). How do they differ?

Triangle sides

Move back to the desktop containing BlueJ and open the Lab07 project. In this project, open the Triangle class in the editor window. This partially-written class contains various calculations about a triangle, such as that seen in this diagram:

Triangle diagram

When the program runs completely, the main() method will ask the user for three sets of (x,y) coordinates. It will then be able to calculate the length of each side.  Once you have the lengths of the sides, all kinds of things can be calculated.

The first set of changes you need to make fall within the main() method. For this checkpoint, complete the following:

  1. Comments in the code tell you which variables are missing in the declarations.  Complete those declarations, thinking carefully about their data types.  Initialize each variable to zero.
  2. Add code to calculate the height of the triangle.  The height of this program's triangle will be the absolute value of the difference in y-coordinates between point 1 and point 2.  Use the appropriate Math class method to help you with this calculation.
Not much more can be done until you calculate the length of each side. 

The formula for the distance between two points (x1, y1) and (x2, y2) can be written as:

distance formula

Here are 2 sample triangles and their points as would match up in the diagram:

Because this code will be used 3 times, it makes sense to ask a method to do the calculation for us:

  1. Complete the method named calcSideLength(int, int, int, int), which is already in the code.  It is written as a stub, which means that the signature is complete but the body of the code needs work.  Use the formula above to calculate the value to be returned to the caller, which is the main() method.  Which method in the Math class helps you this time?
  2. Right-click on the class in BlueJ's project window to execute the method with the points from triangles A and B, listed above.

    For triangle A, you should get: side1 = 2.24, side2 = 3.61, side3 = 4.00
    For triangle B, you should get: side1 = 2.00, side2 = 2.83, side3 = 2.00

     What would you expect to get for a side length if the x-values are the same, and the y-values are different?
  3. Once you have tested this method by itself, uncomment the lines in the main() method that fill in the distances for each side.  Does it run correctly?
  4. Add lines to your main() method to print all your newly calculated values.  Don't forget to include text next to each number so we know what it is.
2 Show us the code you have written and run it for us with triangles A and B.

Lengths, Perimeter and Area

More calculations!  Now that we have determined the sides of the triangle, we can write other code for calculations that depend on that information.  Specifically, we want you to:

  1. Calculate the perimeter of the triangle by adding all of the side lengths together.  This code should be done inside the main() method.
  2. Set the base length of the triangle.  It is defined as the same as the length of side 3.
  3. Write a new static method named calcArea(double, double) which takes the base length and height as parameters, and returns the area.  The formula for the area of a triangle is:
area formula
  1. Test your calcArea() method in BlueJ's project window by right-clicking on the Triangle class. Use the points from triangles A and B above.

    For triangle A, you should get: area = 4.00
    For triangle B, you should get: area = 2.00

  2. Add a method call to your main() method that will place the value this method returns into your area variable.
  3. Add your perimeter, base length and area to the part of your program that prints your triangle's information.  Again, be sure to include descriptive text next to each number.

3 Show us the code you have written and run it for us with triangles A and B. What happens if you ask Java to run the calcArea() method but you don't save the value it returns?


Boolean Methods

We can compare sides of triangle and use that information to classify the triangle into one of these categories:
We don't expect you to write all these methods for this checkpoint, but you can do one of them.  

  1. Write a method that will determine if the triangle is an isosceles triangle.  Its signature looks like:
        isIsosceles(double, double, double)

            Each of the parameters is a side length.  The method should return true if 2 of the sides are the same length, otherwise false.  As you know, comparing two double values to each other using == or != is a bad idea. Your method should return true if 2 of the lengths are within 0.01 of each other.  (Hint:  find the differences, then test the value of the difference in your condition.)

  1. Triangle B is not only a right triangle, it is also isosceles. Triangle A is not. Try both sets of points when you test this method directly.
  2. Once again, add code to invoke this method from main().  Use an if -statement to print either "is isosceles" or "is not isosceles".
4 Show us the code you have written and run the program for us. Would we be able to tell if the triangle was equilateral by using this method, or would we need to write a separate method to check all 3 sides?

Extra for Experts

Write methods to determine if the triangle is equilateral or scalene. Hint: you can call any method from any other method; use already-written methods if that helps!

After the Lab

Return to Firefox, log onto Canvas and get your next programming assignment. Don't forget to log out from Canvas and close Firefox before you log out.

5 Show us that you have logged out, cleaned up, turned off your monitor and pushed in your chairs for this last checkpoint.

End of Lab