README.md

FTest-driven Development (TDD) exercise

Let's take the code used to demonstrate TDD concepts. Start by creating the Euro class (src folder) with the following code:

public class Euro {
double euro;

public Euro(double e) {
    this.euro = e;
}
public String toString(){
    return String.format("EUR %.2f", this.euro);
}

@Override
public boolean equals(Object e2){
    return (e2 instanceof Euro) && this.euro == ((Euro) e2).euro;
}

public Euro minus(Euro oneEuro) {
    return new Euro(this.euro-oneEuro.euro);
}
}

Now, we need to create the EuroTest class, but before that, we create a folder that will contain all the tests. Afterward, we mark the folder as a Test Source Folder.

It's possible to use the quick-fix functionality in the Euro class to create the class EuroTest.

Add tests to the Test Case EuroTest.

class EuroTDD {

@BeforeEach
void setUp() {
}

@AfterEach
void tearDown() {
}

@Test
void testEuroObjectCreation(){
    Euro tenEuros = new Euro(10);
}

@Test
void testEuroToString(){
    Euro twoEuros = new Euro(2);
    assertEquals("EUR 2,00", twoEuros.toString());

    twoEuros = new Euro(2.5);
    assertEquals("EUR 2,50", twoEuros.toString());
}

@Test
void testEuroEquality() {
    Euro twoEuros = new Euro(2);
    Euro twoEuros2 = new Euro(2);

    assertTrue(twoEuros.equals(twoEuros2));
}

@Test
void testEuroInequality() {
    Euro twoEuros = new Euro(3);
    Euro sixEuros = new Euro(6);

    assertFalse(twoEuros.equals(sixEuros));
}

@Test
void testEuroEqualsDifferentObject() {
    Euro twoEuros = new Euro(2);
    Double twoEuros2 = new Double(2);

    assertFalse(twoEuros.equals(twoEuros2));
}

@Test
void testSubtraction() {
    Euro twoEuros = new Euro(2);
    Euro oneEuro = new Euro(1);

    System.out.print(twoEuros.minus(oneEuro));

    assertTrue(new Euro(1).equals(twoEuros.minus(oneEuro)));
}
}

Let's continue following the TDD process with the implementation of new features.

Exercise

As it's widely known, representation of floats and doubles can be inaccurate. To evaluate the numeric safety of class Euro, we can write a test that asserts if 0.61 euros is equals to 1.03 minus 0.42.

What happened? How to fix the problem?

One possible solution is to store the amount in Cents. That means that we have to refactor the amount from double to int. By selecting the attribute type directly in the class, it's possible by right-clicking the selection to access the Type Migration refactoring assistant. Alternatively, we can change the type in all dependencies.

//double euro;
int euro;

We may have to change the values in the test to represent cents instead of euros, without changing the constructor to not break dependencies.

Let's run the tests again. What happened? We broke the existing functionality, but our tests suite warned us. Good!

private static final double CENTS_PER_EURO = 100;

//...

return String.format("EUR %.2f", (double) this.euro/CENTS_PER_EURO);

Done!!!

Just create a fake object End() inside the @AfterAll method for the platform to understand when your test case stops running.

@AfterAll
static void tearDown() {
   new End();
}

Now, you can submit the JAR for testing. In case your test testEuroToString() has failed, try to determine the root cause of the problem.

Links

• PDF do exercício
• https://www.dropbox.com/s/6v1n4mn61iueq0z/TDD.pdf?dl=0