CS173: Intro to Computer Science - Testing

Activity Goals

The goals of this activity are:
  1. To be able to design test cases for unit testing a program
  2. To design code modules for unit testing

Warmup

We know that a signed 32-bit value, in seconds, will overflow in 68 years. How long would a 64-bit value last before overflowing?

The Activity

Directions

Consider the activity models and answer the questions provided. First reflect on these questions on your own briefly, before discussing and comparing your thoughts with your group. Appoint one member of your group to discuss your findings with the class, and the rest of the group should help that member prepare their response. Answer each question individually from the activity, and compare with your group to prepare for our whole-class discussion. After class, think about the questions in the reflective prompt and respond to those individually in your notebook. Report out on areas of disagreement or items for which you and your group identified alternative approaches. Write down and report out questions you encountered along the way for group discussion.

Model 1: Choosing Unit Tests

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public class Main {           
    public static int triangleArea(int base, int height) {
        // Broken Code!
        return 1 / 2 * base * height;
    }
     
    public static void main(String[] args) {
        System.out.println(triangleArea(5, 2));
        System.out.println(triangleArea(3, -2));
    }
}

Questions

  1. What's wrong with this code? (there is more than one answer!)
  2. How many calls would you make to triangleArea before you decide that it is "passing?" What parameter inputs would you supply to those calls?
  3. Visit this guide and design a unit test for triangleArea. You can just write the code in your notes: there is no need to compile or execute it now (we will do this in lab instead!).
  4. Recall that floating point types cannot always be compared directly for equality, due to rounding and precision limitations. Change this program to use double data types, and re-generate unit tests for it. Where do you think a floating point tolerance can be added with the assertEquals function?

Model 2: Thinking Critically about Code

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public class Main {           
    public static double calculateIncomeTax(double income) {
        double result = 0;
         
        double base10 = 9950 * 0.1;
        double base12 = base10 + (40525 - 9950) * 0.12;
        double base22 = base12 + (86375 - 40525) * 0.22;
        double base24 = base22 + (164925 - 86375) * 0.24;
        double base32 = base24 + (209425 - 164925) * 0.32;
        double base35 = base32 + (523600 - 209425) * 0.35;
         
        if(income < 9950) {
            result = income * 0.1;
        } else if(income < 40525) {
            result = base10 + (income - 9950) * 0.12;
        } else if(income < 86375) {
            result = base12 + (income - 40525) * 0.22;
        } else if(income < 164925) {
            result = base22 + (income - 86375) * 0.24;
        } else if(income < 209425) {
            result = base24 + (income - 164925) * 0.32;
        } else if(income < 523600) {
            result = base32 + (income - 209425) * 0.35;
        } else {
            result = base35 + (income - 523600) * 0.37;
        }
         
        return result;
    }
     
    public static void main(String[] args) {
        double taxOwed = calculateIncomeTax(25000);
         
        System.out.println("I owe: $" + taxOwed);
    }
}

Questions

  1. What kinds of inputs would make this function fail (or return values that don't make sense)? What can you do about this?
  2. What tests, at a minimum, would you propose to thoroughly exercise this function?

Model 3: Facilitating Unit Tests

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import java.util.Random;
 
public class Main {           
    public static boolean isHeads() {
        Random rng = new Random();
         
        // Use the random number generator (rng)
        // to generate a value >= 0.0 and < 1.0
        double randomValue = rng.nextDouble();
         
        boolean result = false;
        if(randomValue > 0.5) {
            result = true;
        }
         
        return result;
    }
     
    public static void main(String[] args) {
        System.out.println(isHeads());
        System.out.println(isHeads());
    }
}

Questions

  1. What makes this a difficult function to test?
  2. What could we do to better facilitate testing a function like this?
  3. Print the random number randomValue in the isHeads function when you compute it. Then, call rng.setSeed(100); right before the call to rng.nextDouble(), and try running the program again. What do you notice?

Embedded Code Environment

You can try out some code examples in this embedded development environment! To share this with someone else, first have one member of your group make a small change to the file, then click "Open in Repl.it". Log into your Repl.it account (or create one if needed), and click the "Share" button at the top right. Note that some embedded Repl.it projects have multiple source files; you can see those by clicking the file icon on the left navigation bar of the embedded code frame. Share the link that opens up with your group members. Remember only to do this for partner/group activities!

Submission

I encourage you to submit your answers to the questions (and ask your own questions!) using the Class Activity Questions discussion board. You may also respond to questions or comments made by others, or ask follow-up questions there. Answer any reflective prompt questions in the Reflective Journal section of your OneNote Classroom personal section. You can find the link to the class notebook on the syllabus.