Test Driven Development

Testing State Versus Testing Identity

Defining the class to be Tested

• Consider a Person POJO whose definition is as follows:

public class Person {
    private String fName, lName, email;

    public Person(String fName, String lName, String email) {
        this.fName = fName;
        this.lName = lName;
        this.email = email;
    }
    // getters and setters omitted for brevity
}

Comparing Instances with Identical State

• Consider our intent is return true if two objects have identical state.
  • State is determined by the aggregation of field-values.
• The test below expresses our intent. However, its assertion can never be true.

public class PersonTest {
    @Test
    public void testState() {
        // Given
        String firstName = "Leon";
        String lastName = "Hunter";
        String email = "leon@perscholas";
        
        // when
        Person person1 = new Person(firstName, lastName, email);
        Person person2 = new Person(firstName, lastName, email);
        
        //then
        Assert.assertEquals(person1, person2);
    }
}

Understanding Assert.assertEquals

• Assert.assertEquals takes an argument of two Object, say obj1, and obj2.
• Assert.assertEquals output is determined by the invokation of the obj1.equals(obj2)
• By default .equals returns true if the object passed into the parameter is the same object that is invoking the method.
• To change this behavior, we can override the .equals to implement a custom comparison.

Undserstanding .equals to Compare by Object-State

• The .equals definition below expresses our intent. However, it has faulty implementation.

  public class Person {
    private String fName, lName, email;
    // constructor omitted for brevity
  
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
  
        Person person = (Person) obj;
        return  fName.equals(person.fName) &&
                lName.equals(person.lName) &&
                email.equals(person.email);
    }
    // getters and setters omitted for brevity
  }
  

Understanding .equals to Compare by Object-State

• Consider that the implicit parameter (invoker of .equals) has a null field.
  • The test will throw a NullPointerException upon invokation of .equals.

public class PersonTest {
    @Test
    public void testState() {
        // Given
        String firstName = null;
        String lastName = "Hunter";
        String email = "leon@perscholas";
        
        // when
        Person person1 = new Person(firstName, lastName, email);
        Person person2 = new Person(firstName, lastName, email);
        
        //then
        Assert.assertEquals(person1, person2); // NullPointerException
    }
}

Undserstanding Objects.equals to Compare by Object-State

• The Objects.equals method ensures operands are not null before comparison, avoiding potential NullPointerExceptions.

  public class Person {
    private String fName, lName, email;
    // constructor omitted for brevity
  
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
  
        Person person = (Person) obj;
        return Objects.equals(fName, person.fName) &&
                Objects.equals(lName, person.lName) &&
                Objects.equals(email, person.email);
    }
    // getters and setters omitted for brevity
  }
  

Understanding Assert.assertArrayEquals

• it is impossible to override the .equals method in an array.
  • because of this, a custom assertArrayEquals method has been defined by the junit framework.
• This method sorts each array to be compared, then compares each element in the array.
  • If any of the comparisons in the array evaluate to false, then the test asserts a failure.

Comparing Arrays with Identical Contents

Example 1

• Here, we compare an array of identical values, but differing order.
• The test passes as a success; the junit considers the arrays equal.

@Test
public void testArrayEquivalence() {
    Integer[] array1 = new Integer[]{7,1,5,2};
    Integer[] array2 = new Integer[]{1,5,2,7};

    Assert.assertArrayEquals(array1, array2);
}

Comparing Arrays with Identical Contents

Example 2

• Here, we compare an array of identical values, but differing order.
• The test passes as a success; the junit considers the arrays equal.

@Test
public void testArrayEquivalence() {
    Object object1 = new Object();
    Object object2 = new Object();
    Object object3 = new Object();
    Object object4 = new Object();

    Objcet[] array1 = new Object[]{object1, object2, object3, object4};
    Objcet[] array2 = new Object[]{object3, object4, object2, object1};

    Assert.assertArrayEquals(array1, array2);
}

Comparing Arrays with Non-Identical Contents

Example 1

• Here, we compare an array of differing content.
• The test does not pass; the junit does not consider the arrays equal.

@Test
public void testArrayEquivalence() {
    Object object1 = new Object();
    Object object2 = new Object();
    Object object3 = new Object();
    Object object4 = new Object();

    Objcet[] array1 = new Object[]{object1, object2, object3, object4};
    Objcet[] array2 = new Object[]{object1, object2, object3};

    Assert.assertArrayEquals(array1, array2);
}

Comparing Arrays with Non-Identical Contents

Example 2

• Here, we compare an array of differing content.
• The test does not pass; the junit does not consider the arrays equal.

@Test
public void testArrayEquivalence() {
    Object object1 = new Object();
    Object object2 = new Object();
    Object object3 = new Object();
    Object object4 = new Object();

    Objcet[] array1 = new Object[]{object1, object2, object3, object4};
    Objcet[] array2 = new Object[]{object1, object2, object3, object4, null};

    Assert.assertArrayEquals(array1, array2);
}