Introduction to JUnit

Introduction to Testing in Java

Maria Milusheva

Senior Software Engineer

Quick facts

JUnit is the most popular Java testing library
It is well-maintained and constantly updated

An image of the JUnit 5 logo.

Writing a JUnit test

Let's write a test for the addTwoNumbers() method:

public int addTwoNumbers(int a, int b) {
    return a + b;
}

JUnit @Test annotation

Install JUnit using an IDE or build tools (example in the citation)
Every test must have the @Test annotation:

import org.junit.jupiter.api.Test;

@Test 
// Test method will be here

Annotation - special metadata that indicates how the method should be handled by compilers and frameworks. Annotations begin with @

¹ https://www.jetbrains.com/help/idea/junit.html

JUnit test structure

import org.junit.jupiter.api.Test;

@Test 

void testAddTwoNumbers() {

    // Arrange - Given
    int num1 = 2;
    int num2 = 2;


    // Act - When  
    int actual = addTwoNumbers(num1, num2);


    // Assert - Then  
    assertEquals(4, actual); 
}

Assert statements

Quick way to compare actual outcomes to expected outcomes
If expected and actual outcomes don't match, test fails
Help quickly catch bugs and errors

Graphics representing assert statement

Test outcomes

Consider the assertion:

assertEquals(4, actual); // Will succeed if actual == 4

If successful, will produce a Test passed message

If the values are not equal, e.g. actual = 5:

org.opentest4j.AssertionFailedError: expected: <4> but was: <5>

Remember argument order! First argument is the expected one in JUnit.

Additional test for overflow

@Test 

void testAddTwoNumbers() { 
    // Given
    int num1 = 2147483647;
    int num2 = 1;  

    // When  
    int actual = addTwoNumbers(num1, num2); 

    // Then  
    assertEquals(-2147483648, actual);
}

Notes about JUnit and testing

There can be multiple assertions per test
All assertions need to succeed in order for the test to pass
We can import all assertions using:

import org.junit.jupiter.api.Assertions.*;

More notes

Following is a custom Datacamp tool, not needed in IDE:

import static com.datacamp.util.testing.CustomJUnitTestLauncher.launchTestsAndPrint;

import static package.Class.method allows using method as just method, without writing package.Class.method

import static java.lang.Math.max;

...

max(3,5); // Instead of Math.max(3, 5)

Let's practice!

Introduction to Testing in Java