Rational: ex05

Addressing bugs with test-driven development (expose bug with test first, then make test pass to squash bug)

Part of a series of tutorial articles about a Rational class.

Code examples referred to on this page can be found here: https://github.com/UCSB-CS56-pconrad/cs56-rational-ex05

cs56-rational-example/ex05

A bug?

Consider this code:

  Rational r = new Rational(3,6);
  Rational s = new Rational(15,5);
  Rational t = new Rational(0,7);
  System.out.println("r=" + r + " s=" + s + " t=" + t);

The output will be:

r=3/6 s=15/5 t=0/7

This is probably not what we want. We probably want this:

r=1/2 s=3 r=0

That is, we want:

• fractions to be printed in reduced form
• the denominator to be ommitted when:
  • denominator is 1, or
  • numerator is 0

To be able to reduce fractions, we’ll need a gcd method (greatest common divisor). That’s a good candidate to be a static method of the Rational class.

A static method of a class is used when we need a function that isn’t necessarily attached to any particular object. It instead, belongs to the class itself. This is similar to the way that static methods are used in C++.

Start with unit tests

We’ll start by adding some unit tests for a static gcd method to our RationalTest class.

These methods will fail to compile until we add a stub method to the Rational class.

That’s ok. That’s the TDD way of doing things.

Here are some tests:

    @Test
    public void test_gcd_5_15() {
	assertEquals(5, Rational.gcd(5,15));
    }

        @Test
    public void test_gcd_15_5() {
	assertEquals(5, Rational.gcd(15,5));
    }

    @Test
    public void test_gcd_24_6() {
	assertEquals(6, Rational.gcd(24,6));
    }

    @Test
    public void test_gcd_17_5() {
	assertEquals(1, Rational.gcd(17,5));
    }
    
    @Test
    public void test_gcd_1_1() {
	assertEquals(1, Rational.gcd(1,1));
    }

    @Test
    public void test_gcd_20_25() {
	assertEquals(5, Rational.gcd(20,25));
    }

As expected, when we ant compile, we get:

compile:
    [javac] Compiling 3 source files to ...ex05/build
    [javac] .../ex05/src/RationalTest.java:33: error: cannot find symbol
    [javac]    assertEquals(5, Rational.gcd(5,15));
    ...

So, we add a stub to src/Rational.java along with some reasonable javadoc comments:

    /** 
	greatest common divisor of a and b
	@param a first number
	@param b second number
	@return gcd of a and b
    */
    public static int gcd(int a, int b) {
	return -42;
    }

Now we can compile, but our tests don’t pass:

compile:
    [javac] Compiling 2 source files to ...ex05/build

test:
    [junit] Testsuite: RationalTest
    [junit] Tests run: 10, Failures: 6, Errors: 0, Skipped: 0, Time elapsed: 0.041 sec
    [junit]
    [junit] Testcase: test_gcd_5_15(RationalTest):	FAILED
    [junit] expected:<5> but was:<-42>
    [junit] junit.framework.AssertionFailedError: expected:<5> but was:<-42>
    [junit] at RationalTest.test_gcd_5_15(Unknown Source)
    [junit]
...				

By the way, that (Unknown Source) thing is annoying. We can take care of that with this adjustment to our build.xml: on the javac line, include the debug="true" attribute:

    <javac srcdir="src"
	   destdir="build"
	   includeantruntime="false"
	   debug="true" >

Then after running ant clean and ant test again, we get the proper line numbers in our error messages:

compile:
    [javac] Compiling 3 source files to ...ex05/build

test:
    [junit] Testsuite: RationalTest
    [junit] Tests run: 10, Failures: 6, Errors: 0, Skipped: 0, Time elapsed: 0.026 sec
    [junit]
    [junit] Testcase: test_gcd_5_15(RationalTest):	FAILED
    [junit] expected:<5> but was:<-42>
    [junit] junit.framework.AssertionFailedError: expected:<5> but was:<-42>
    [junit]   at RationalTest.test_gcd_5_15(RationalTest.java:33)

Now its time to actually implement a gcd algorithm.

Euclid’s gcd algorithm should do the trick.

    public static int gcd(int a, int b) {
	if (a==0)
	    return b;
	else if (b==0)
	    return a;
	else
	    return gcd(b%a, a);
    }

Now it passes the tests:

test:
    [junit] Testsuite: RationalTest
    [junit] Tests run: 10, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.021 sec
    [junit]

So, next we defined some unit tests that test for the behavior we want.

Here, it may be helpful to make use of a setUp method that sets up some instances of the Rational class that we can reuse in our tests:

Take a look at these features of the code:

• setUp method in src/Rational.test
• additional unit tests for toString, and modified tests for getters
• modifications to the two argument constructor, and the toString method

That’s all we are going to do in ex05.

In the next article, Rational: ex06, we’ll look at:

• Adding links to the Java Standard Libraries into our javadoc
• The situation with negative rational numbers, and negative parameters to our constructor