Unit testing with JUnit

Introduction to Testing in Java

Maria Milusheva

Senior Software Engineer

Unit testing - definition

Two key concepts:

Unit - the smallest testable part of an application, such as a method

Unit testing - A test that focuses on verifying the correctness of a single "unit" of code in isolation from other parts of the application

Introduction to Testing in Java

Unit testing is about details

Notes:

  • Unit testing is about small pieces of logic, not the big picture

  • Other types of testing focus on the big picture (e.g. integration)

  • JUnit gets its name because its primary use case is unit testing

A graphic intuitively illustrating the relationship between unit tests and production code

Introduction to Testing in Java

Types of testing - brief overview

  • In this course we will discuss mostly unit and integration tests
  • But there are many types of testing!

Diagram of software tests including functional testing such as unit testing, and non-functional testing such as usability testing

Introduction to Testing in Java

Types of testing - brief overview

  • In this course we will discuss mostly unit and integration tests
  • But there are many types of testing!

Diagram of software tests including functional testing such as unit testing, and non-functional testing such as usability testing. This version has unit and integration testing circled

Introduction to Testing in Java

JUnit syntax: assertTrue() and assertFalse()

  • assertTrue() checks that a statement is true: 
List<String> list = new ArrayList<>();

assertTrue(list.isEmpty()); // No error
  • assertFalse() checks the opposite:
List<String> list = new ArrayList<>();
list.add("A");

assertFalse(list.isEmpty()); // No error
Introduction to Testing in Java

assertNull() and assertNotNull()

null variables can lead to NullPointerException!

  • Assertions that check a variable is or isn't null - assertNull() and assertNotNull()

  • Common use case: checking a value retrieved from somewhere is not null

Map<String, Integer> catalogue = new HashMap<>();

catalogue.put("item1", 10);

// No errors
assertNotNull(catalogue.get("item1"));
assertNull(catalogue.get("item2"));
Introduction to Testing in Java

Asserting exceptions

Suppose we want to verify an ArrayIndexOutOfBoundsException is thrown:

public String getIndex(String[] array, int index) {
    return array[index];
}

JUnit provides multiple ways to do this!

Introduction to Testing in Java

assertInstanceOf()

  • JUnit provides assertThrows(), but it uses advanced Java syntax (lambda expressions)

  • Can use assertInstanceOf() like this:

try {
    getIndex(4);
} catch (Exception e) {
    // Pass the expected class of the exception and the exception itself  
    assertInstanceOf(ArrayIndexOutOfBoundsException.class, e); 
}
1 https://www.baeldung.com/junit-assert-exception
Introduction to Testing in Java

Naming unit tests

  • A typical project contains hundreds of unit tests

  • They should be lightweight and simple to understand

  • They should have informative names so that we can see at first glance what went wrong if one of them fails

For example: methodUnderTest_expectedBehavior_conditions()

1 https://dzone.com/articles/7-popular-unit-test-naming?fromrel=true
Introduction to Testing in Java

Let's practice!

Introduction to Testing in Java

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