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We all have been there: we spent so much time into a project, we make sure to run each and every possible scenario we might think off in order to make it as good as possible. But when you let someone else try it out they find small edge cases that can make your app behave in unexpected ways.

In order to prevent that from happening too often, we use different types of testing techniques: Unit Testing, Integration Testing and End-To-End Testing. Although the later (End-To-End Testing) can be made by either developers or Quality Assurance team members.

This is also known as the Testing Pyramid

Try not to be distracted about the other two types of test mentioned before, but rather focus on the idea that we can expect our unit test to be far more in quantity than the others, but keep in mind that they don't replace each other. They are all important.

Enough chitchat, let's start coding, shall we?

Requirements

Have the JDK installed on your machine (duh!).
An IDE (I would recommend IntelliJ IDEA Community Edition).
In terms of Java as a language, you need to be familiar with the concepts of a variables, constant, function, class and object in order to fully understand this post. (Which I might be able to help with this).
Add the Math class from this GitHub Gist to your project.

Getting Started

The very first thing we need to do is to add the TestNG framework to our project. This will provide us with a set of classes and annotations which will come in handy later.

Add TestNG to your project

This can be done 2 ways: manually or using Maven in your project. Feel free to skip the other depending on how you setup your project.

Manually

You can follow this guide in order to add it manually.

Via Maven

Open up your pom.xml, which should be on your project's root folder.
Inside the <project> tag, make a <dependencies> tag.
Add the following block of XML code:

<dependency>
    <groupId>org.testng</groupId>
    <artifactId>testng</artifactId>
    <version>6.14.2</version>
</dependency>

In case you haven't done it, make sure to select the Enable auto-import option that should appear on the bottom right of your screen. This will allow IntelliJ IDEA to automatically detect any changes made to your pom.xml and refresh it accordingly. Life saver :)

The test subject

For the sake of this post, we will not be applying TDD (Test Driven-Development) techniques, our focus will be on getting to know how to write test classes and methods.

So, in order for us to start, we will need to create a class inside our src/test/java/ directory, and name it MathTests.

The use of the suffix "Tests" at the end of a testing class name is a common naming convention that will allow other developers and yourself to quickly know, without opening the file, that it has testing logic inside of it.

Which should look like this:

public class MathTests {
    // ...
}

Can you spot any difference from a regular class and this one?

Nope.

And that's fine.

What makes a class a "testing class" is not it's signature, but actually it's methods and it's important for these classes to be inside a directory marked as a Test Source Root.

These methods should follow a particular naming convention, should have the public access modifier, should have void as their return type and the @Test annotation before their signature.

The @Test annotation indicates our IDE's test runner that this is not a regular method and will execute it accordingly.

So, our first test method would look like this:

@Test
public void add_TwoPlusTwo_ReturnsFour() {
    // ...
}

Adding testing logic

There's another convention that is really common among other programming langauges, known as the Tripple A:

Arrange: consists of a few lines of code that are used to declare and initialize the objects we need in our test.
Act: is usually a few lines of code where we perform the actions, whether it is some calculation or modify the state of our objects.
Assert: usually consists of a single line of code where we verify that the outcome of the Act part were made successfully. Comparing the actual value with an expected value that we plan to get.

In practice, this would be:

@Test
public void add_TwoPlusTwo_ReturnsFour() {
    // Arrange
    final int expected = 4;

    // Act
    final int actual = Math.add(2, 2);

    // Assert
    Assert.assertEquals(actual, expected);
}

The final keyword is used to prevent our value to be changed later, also known as a constant.

Here we can see how the whole arrange, act and assert come together.

If you take a look at the line numbers on the left of the add_TwoPlusTwo_ReturnsFour() method, there's a green play button, select it and then choose the first option from the context menu.

Wait a few moments and... the test runner panel should open up with the test results.

If you see that everything is green, it means that our test passed!

But, as a good tester, we should also try to make our test fail.

Let's change the act part so it adds 3 and 2 together, so our actual value becomes 5 and our test should fail.

...

Did it fail?

Great!

Now, some of you may be wondering why we use the assertEquals() method from the Assert class, we could manually try to use an if-else block that can simulate the same results, but the Assert class provides a handy set of methods to do various types of validations.

The most common ones are:

assertTrue(): evaluates a given condition or boolean, if it's value it's true, the test will be marked as PASSED, otherwise, it will be marked as FAILED.
assertFalse(): similar to the assertTrue(), but whenever the condition or boolean is false the test will be marked as PASSED, otherwise, it will be marked as FAILED.
asssertEquals(): commonly used to compare two given values that can be either primitive types (int, double, etc) or any objects.

If we were to implement our own logic using if-else, not only it would clutter our code, but also could lead to unwanted results. Since, if we forget to throw and exception in one of our if-else blocks, both of our code paths will be marked as PASSED.

Tip: most of the time we should only use 1 single Assert method per test, although there are exceptions to this rule. But this is normally recommended in order for our test to be really small and straight to the point. Each test should only verify 1 code path at a time. Also, if we had 3 assertions and the first one fails, the following ones will never be executed, so keep that in mind.

Now that we got that out of the way, let's continue with more tests!

How about you practice testing more scenarios for the add() method?

Add a positive number with a negative one.
Add two negative numbers.

If we take another look at our Math class, we can see there are 2 more methods.

I'll let you do the tests for the multiply() (hint: make sure to test when we multiply a number by zero) method and I'll focus on the divide() one for the rest of this article.

The `divide()` method

Let's take a closer look to this method:

public static double divide(int dividend, int divisor) {
    if (divisor == 0)
        throw new IllegalArgumentException("Cannot divide by zero (0).");

    return dividend / divisor;
}

As you can see, if the value of the divisor argument is 0, we will throw an IllegalArgumentException, otherwise, the division operation will be performed.

Note: the throw keyword not only throws a given exception, but also stops the code execution, so it works similar to the break keyword inside a loop or a switch block.

So, this method has 2 possible outcomes or "code paths". We need to make sure to test them.

The amount of tests per method, should be equal or more than the amount of code paths it has.

Which means, that we should at least have 2 tests.

Let's go ahead and make them!

Divide two numbers, where the divisor is any number but zero (0).
Divide two numbers, where the divisor is zero (0).

Our first test would be something like:

@Test
public void divide_TenDividedByFive_ReturnsTwo() {
    final double expected = 2.0;

    final double actual = Math.divide(10, 5);

    Assert.assertEquals(actual, expected);
}

And our second test would be:

@Test(expectedExceptions = IllegalArgumentException.class)
public void divide_TenDividedByZero_ThrowsIllegalArgumentException() {
    Math.divide(10, 0);
}

Wait wut!

Mr./Mrs Reader: "B-bu-but what happened with the arrange, act and the assert? what is the expectedExceptions part doing?"

Do not worry, I shall explain shortly!

I decided to skip the whole arrange, act and assert because the execution of our code will automatically be interrupted when the divide() method is ran. So the whole Tripple A can be omitted for this test in particular.
The expectedException part is needed in order to tell our test runner that the IllegalArgumentException is actually possible to happen in this test, if we were to change that to another exception, our test would fail.

Tip: remember to use the .class at the end of the exception name, otherwise, this code would not compile.

Testing objects

You have noticed that so far we have been testing static methods of our Math class, which means we don't have to create objects of it. Which is fine.

But what if we had a class that didn't have static methods?

For this, our testing framework (TestNG) provides a pair of annotations to make sure that each of our test use a fresh instance of our class.

Let's imagine we could create instances of the Math class.

In that case, our tests would look like this:

@Test
public void add_TwoPlusTwo_ReturnsFour() {
    final Math math = new Math();
    final int expected = 4;

    final int actual = Math.add(2, 2);

    Assert.assertEquals(actual, expected);
}

@Test
public void divide_TenDividedByFive_ReturnsTwo() {
    final Math math = new Math();
    final double expected = 2.0;

    final double actual = Math.divide(10, 5);

    Assert.assertEquals(actual, expected);
}

Which isn't that bad, but remember that we can make many more tests for this same class and having this Math objects initialized over and over will create more code noise.

If we have to ignore certain parts of our test, specially in the arrangement, it means we can use one of our testing framework's tools:

@BeforeMethod & @AfterMethod

These two annotations can be added to our test functions like we have been using the @Test one, but they work in a particular way.

@BeforeMethod: this code block will always be executed before any other @Test method.
@AfterMethod: this code block will always be executed after any other @Test method.

So, why would we use them?

In all of our @Test methods we would have to constantly initiate a new Math object, so with the help of the @BeforeMethod annotation we can get rid of this repetitive code.

First thing we need to do is to promote our Math object to a member variable or property.

public final class MathTests {
    private Math math;

    @Test
    public void add_TwoPlusTwo_ReturnsFour() {
        final int expected = 4;

        final int actual = math.add(2, 2);

        Assert.assertEquals(actual, expected);
    }

    @Test
    public void divide_TenDividedByFive_ReturnsTwo() {
        final double expected = 2.0;

        final double actual = math.divide(10, 5);

        Assert.assertEquals(actual, expected);
    }
}

Then add our @BeforeMethod function, which is commonly named as "setUp".

public final class MathTests {
    private Math math;

    @BeforeMethod
    public void setUp() {
        math = new Math();
    }

    @Test
    public void add_TwoPlusTwo_ReturnsFour() {
        final int expected = 4;

        final int actual = math.add(2, 2);

        Assert.assertEquals(actual, expected);
    }

    @Test
    public void divide_TenDividedByFive_ReturnsTwo() {
        final double expected = 2.0;

        final double actual = math.divide(10, 5);

        Assert.assertEquals(actual, expected);
    }
}

Now, in order to make sure we clear out our math object, we can set it's value to null inside our @AfterMethod function, which is usually called tearDown():

public final class MathTests {
    private Math math;

    @BeforeMethod
    public void setUp() {
        math = new Math();
    }

    @Test
    public void add_TwoPlusTwo_ReturnsFour() {
        final int expected = 4;

        final int actual = math.add(2, 2);

        Assert.assertEquals(actual, expected);
    }

    @Test
    public void divide_TenDividedByFive_ReturnsTwo() {
        final double expected = 2.0;

        final double actual = math.divide(10, 5);

        Assert.assertEquals(actual, expected);
    }

    @AfterMethod
    public void tearDown() {
        math = null;
    }
}

This means that the order of execution of our test would be:

The setup().
And add_TwoPlusTwo_ReturnsFour().
Then tearDown().
setup() again.
And divide_TenDividedByFive_ReturnsTwo().
Then tearDown() again.

Aaaaand that's it

With this you should be more familiar now with how Unit Testing works.

Although we didn't do any tests that required us using the assertTrue() and assertFalse(), I encourage you to do your own tests to play around with them for a little bit :)

Feel free to leave a comment if you have any questions and I'll do my best to clear them out!

If you would like to take a look at the entire project, head over to this repository on GitHub.

Introduction to Unit Testing with Java