Tests and implementation -

Tests and implementation

Figure 301. Step 1 + 2: Specify method, write skeleton

/**
 * Dealing with prime numbers.
 */
public class Prime {
  /**
   * Check whether a given value is prime or not ❶
   * @param value A positive value
   * @return true if and only if value is a prime number.
   */
  public static boolean isPrime(int value) {
    return true ❷; //TODO: Dummy value to be implemented correctly 
  }
}

❶

An informal specification of the method's expected behaviour. This comprises the descriptions of all method parameters among with the expected outcome.

Note that boolean isPrime(int value) is being specified as a partial method: The value parameter must not contain negative values. Thus negative values likely lead to unexpected results.

❷

Since our current implementation is just a skeleton we simply return a constant value. Other choices:

return false;
return 231 < value;
...

In fact every syntactically correct expression will do since we defer our implementation to a later step. Thus the only requirement with respect to our code is its ability to get compiled. Returning a single value obviously is the most simple way to comply.

Figure 302. Execution yet being flawed

for (int i = 1; i < 20;i++) {
  System.out.println(i + " is " + (Prime.isPrime(i) ? " a " : " not a ")
    + " prime number");
}

1 is a prime number
2 is a prime number
3 is a prime number
4 is a prime number
5 is a prime number
...

Figure 303. Sample test data

Input	Expected output	Input	Expected output
`1`	`false`	`7`	`true`
`2`	`true`	`8`	`false`
`3`	`true`	`9`	`false`
`4`	`false`	`10`	`false`
`5`	`true`	`11`	`true`
`6`	`false`	`12`	`false`

Figure 304. Step 3: Junit based specification test

public class PrimeTest {

  @Test ❶ public void test_1_isNotPrime() {
    Assert.assertFalse(Prime.isPrime(1));
  }
  @Test ❶ public void test_2_isPrime() {
    Assert.assertTrue(Prime.isPrime(2));
  }

  void someOrdinaryMethod() ❷ {...}
...

❶	A `@Test` annotation triggers automatic method execution by the Junit framework.
❷	Any methods not being annotated by `@Test` will not be called by the Junit framework. They may however be called indirectly by test methods e.g. `test_1_isNotPrime()` thus acting as helper methods.

Figure 305. Junit skeleton test result (Maven CLI)

goik@goiki Prime_v01> mvn test
...
Running de.hdm_stuttgart.mi.sd1.PrimeTest
Tests run: 2, Failures: 1, Errors: 0, Skipped: 0, 
         Time elapsed: 0.065 sec <<< FAILURE!
...
test_1_isNotPrime(de.hdm_stuttgart.mi.sd1.PrimeTest)
         Time elapsed: 0.001 sec  <<< FAILURE!
java.lang.AssertionError
	at org.junit.Assert.fail(Assert.java:86)
	at org.junit.Assert.assertTrue(Assert.java:41)
	at org.junit.Assert.assertFalse(Assert.java:64)
	at org.junit.Assert.assertFalse(Assert.java:74)
...

Similar to a main(...) method execution we may execute Junit Tests using our IDE:

Figure 306. Junit skeleton test result (IDE)

❶	Test `test_1_isNotPrime()` accidentally failing due to dummy implementation Figure 301, “Step 1 + 2: Specify method, write skeleton ”.
❷	Test `test_2_isPrime()` accidentally succeeding due to dummy implementation Figure 301, “Step 1 + 2: Specify method, write skeleton ”.

Before replacing the skeleton implementation Figure 301, “Step 1 + 2: Specify method, write skeleton ” we supply additional tests:

Figure 307. Step 3: Providing more prime tests

@Test public void test_Primes() {
  Assert.assertTrue(Prime.isPrime(3));
  Assert.assertTrue(Prime.isPrime(5));
  Assert.assertTrue(Prime.isPrime(7));
  Assert.assertTrue(Prime.isPrime(11));
    ...  }
@Test public void testOddNonPrimes() {
  Assert.assertFalse(Prime.isPrime(9));
  Assert.assertFalse(Prime.isPrime(15));
  Assert.assertFalse(Prime.isPrime(21));...}

Since all even numbers greater than two are non-prime we add:

Figure 308. Step 3: Prime mass testing

@Test public void testEvenNonPrimes() {
  for (int i = 2; i < 100; i++) {
    Assert.assertFalse(Prime.isPrime(2 * i));
  }
}

Now its time actually implementation isPrime():

Figure 309. Step 4: Implement skeleton

public static boolean isPrime(int value) {
  for (int i = 2; i < value; i++) {
    if (0 == value % i) { // i divides value
      return false;
    }
  }
  return value != 1;
}

Figure 310. Step 5: Testing our first implementation

goik@goiki Prime_v01> mvn test
...
-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running de.hdm_stuttgart.mi.sd1.PrimeTest
Tests run: 5, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.055 sec

Results :

Tests run: 5, Failures: 0, Errors: 0, Skipped: 0

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