1. Introduction

In this article, we're going to explore in details the assertions available within JUnit.

Following the migrating from JUnit 4 to JUnit 5 and A Guide to JUnit 5 articles, we're now going into details about the different assertions available in JUnit 4 and JUnit 5.

We'll also highlight the enhancements made on the assertions with JUnit 5.

2. Assertions

Assertions are utility methods to support asserting conditions in tests; these methods are accessible through the Assert class, in JUnit 4, and the Assertions one, in JUnit 5.

In order to increase the readability of the test and of the assertions itself, it's always recommended to import statically the respective class. In this way, we can refer directly to the assertion method itself without the representing class as a prefix.

Let's start exploring the assertions available with JUnit 4.

3. Assertions in JUnit 4

In this version of the library, assertions are available for all primitive types, Objects, and arrays (either of primitives or Objects).

The parameters order, within the assertion, is the expected value followed by the actual value; optionally the first parameter can be a String message that represents the message output of the evaluated condition.

There's only one slightly different in how is defined the assertThat assertions, but we'll cover it later on.

Let's start with the assertEquals one.

3.1. assertEquals

The assertEquals assertion verifies that the expected and the actual values are equal:

public void whenAssertingEquality_thenEqual() {
    String expected = "Baeldung";
    String actual = "Baeldung";

    assertEquals(expected, actual);

It's also possible to specify a message to display when the assertion fails:

assertEquals("failure - strings are not equal", expected, actual);

3.2. assertArrayEquals

If we want to assert that two arrays are equals, we can use the assertArrayEquals:

public void whenAssertingArraysEquality_thenEqual() {
    char[] expected = {'J','u','n','i','t'};
    char[] actual = "Junit".toCharArray();
    assertArrayEquals(expected, actual);

If both arrays are null, the assertion will consider them equal:

public void givenNullArrays_whenAssertingArraysEquality_thenEqual() {
    int[] expected = null;
    int[] actual = null;

    assertArrayEquals(expected, actual);

3.3. assertNotNull and assertNull

When we want to test if an object is null we can use the assertNull assertion:

public void whenAssertingNull_thenTrue() {
    Object car = null;
    assertNull("The car should be null", car);

In the opposite way, if we want to assert that an object should not be null we can use the assertNotNull assertion.

3.4. assertNotSame and assertSame

With assertNotSame, it's possible to verify if two variables don't refer to the same object:

public void whenAssertingNotSameObject_thenDifferent() {
    Object cat = new Object();
    Object dog = new Object();

    assertNotSame(cat, dog);

Otherwise, when we want to verify that two variables refer to the same object, we can use the assertSame assertion.

3.5. assertTrue and assertFalse

In case we want to verify that a certain condition is true or false, we can respectively use the assertTrue assertion or the assertFalse one:

public void whenAssertingConditions_thenVerified() {
    assertTrue("5 is greater then 4", 5 > 4);
    assertFalse("5 is not greater then 6", 5 > 6);

3.6. fail

The fail assertion fails a test throwing an AssertionFailedError. It can be used to verify that an actual exception is thrown or when we want to make a test failing during its development.

Let's see how we can use it in the first scenario:

public void whenCheckingExceptionMessage_thenEqual() {
    try {
        fail("Exception not thrown");
    } catch (UnsupportedOperationException e) {
        assertEquals("Operation Not Supported", e.getMessage());

3.7. assertThat

The assertThat assertion is the only one in JUnit 4 that has a reverse order of the parameters compared to the other assertions.

In this case, the assertion has an optional failure message, the actual value, and a Matcher object.

Let's see how we can use this assertion to check if an array contains particular values:

public void testAssertThatHasItems() {
      Arrays.asList("Java", "Kotlin", "Scala"), 
      hasItems("Java", "Kotlin"));

Additional information, on the powerful use of the assertThat assertion with Matcher object, is available at Testing with Hamcrest.

4. JUnit 5 Assertions

JUnit 5 kept many of the assertion methods of JUnit 4 while adding few new ones that take advantage of the Java 8 support.

Also in this version of the library, assertions are available for all primitive types, Objects, and arrays (either of primitives or Objects).

The order of the parameters of the assertions changed, moving the output message parameter as the last parameter. Thanks to the support of Java 8, the output message can be a Supplier, allowing lazy evaluation of it.

Let's start reviewing the assertions available also in JUnit 4.

4.1. assertArrayEquals

The assertArrayEquals assertion verifies that the expected and the actual arrays are equals:

public void whenAssertingArraysEquality_thenEqual() {
    char[] expected = { 'J', 'u', 'p', 'i', 't', 'e', 'r' };
    char[] actual = "Jupiter".toCharArray();

    assertArrayEquals(expected, actual, "Arrays should be equal");

If the arrays aren't equal, the message “Arrays should be equal” will be displayed as output.

4.2. assertEquals

In case we want to assert that two floats are equals, we can use the simple assertEquals assertion:

public void whenAssertingEquality_thenEqual() {
    float square = 2 * 2;
    float rectangle = 2 * 2;

    assertEquals(square, rectangle);

However, if we want to assert that the actual value differs by a predefined delta from the expected value, we can still use the assertEquals but we have to pass the delta value as the third parameter:

public void whenAssertingEqualityWithDelta_thenEqual() {
    float square = 2 * 2;
    float rectangle = 3 * 2;
    float delta = 2;

    assertEquals(square, rectangle, delta);

4.3. assertTrue and assertFalse

With the assertTrue assertion, it's possible to verify the supplied conditions are true:

public void whenAssertingConditions_thenVerified() {
    assertTrue(5 > 4, "5 is greater the 4");
    assertTrue(null == null, "null is equal to null");

Thanks to the support of the lambda expression, it's possible to supply a BooleanSupplier to the assertion instead of a boolean condition.

Let's see how we can assert the correctness of a BooleanSupplier using the assertFalse assertion:

public void givenBooleanSupplier_whenAssertingCondition_thenVerified() {
    BooleanSupplier condition = () -> 5 > 6;

    assertFalse(condition, "5 is not greater then 6");

4.4. assertNull and assertNotNull

When we want to assert that an object is not null we can use the assertNotNull assertion:

public void whenAssertingNotNull_thenTrue() {
    Object dog = new Object();

    assertNotNull(dog, () -> "The dog should not be null");

In the opposite way, we can use the assertNull assertion to check if the actual is null:

public void whenAssertingNull_thenTrue() {
    Object cat = null;

    assertNull(cat, () -> "The cat should be null");

In both cases, the failure message will be retrieved in a lazy way since it's a Supplier.

4.5. assertSame and assertNotSame

When we want to assert that the expected and the actual refer to the same Object, we must use the assertSame assertion:

public void whenAssertingSameObject_thenSuccessfull() {
    String language = "Java";
    Optional<String> optional = Optional.of(language);

    assertSame(language, optional.get());

In the opposite way, we can use the assertNotSame one.

4.6. fail

The fail assertion fails a test with the provided failure message as well as the underlying cause. This can be useful to mark a test when it's development it's not completed:

public void whenFailingATest_thenFailed() {
    // Test not completed
    fail("FAIL - test not completed");

4.7. assertAll

One of the new assertion introduced in JUnit 5 is assertAll.

This assertion allows the creation of grouped assertions, where all the assertions are executed and their failures are reported together. In details, this assertion accepts a heading, that will be included in the message string for the MultipleFailureError, and a Stream of Executable.

Let's define a grouped assertion:

public void givenMultipleAssertion_whenAssertingAll_thenOK() {
      () -> assertEquals(4, 2 * 2, "4 is 2 times 2"),
      () -> assertEquals("java", "JAVA".toLowerCase()),
      () -> assertEquals(null, null, "null is equal to null")

The execution of a grouped assertion is interrupted only when one of the executables throws a blacklisted exception (OutOfMemoryError for example).

4.8. assertIterableEquals

The assertIterableEquals asserts that the expected and the actual iterables are deeply equal.

In order to be equal, both iterable must return equal elements in the same order and it isn't required that the two iterables are of the same type in order to be equal.

With this consideration, let's see how we can assert that two lists of different types (LinkedList and ArrayList for example) are equal:

public void givenTwoLists_whenAssertingIterables_thenEquals() {
    Iterable<String> al = new ArrayList<>(asList("Java", "Junit", "Test"));
    Iterable<String> ll = new LinkedList<>(asList("Java", "Junit", "Test"));

    assertIterableEquals(al, ll);

In the same way of the assertArrayEquals, if both iterables are null, they are considered equal.

4.9. assertLinesMatch

The assertLinesMatch asserts that the expected list of String matches the actual list.

This method differs from the assertEquals and assertIterableEquals since, for each pair of expected and actual lines, it performs this algorithm:

  1. check if the expected line is equal to the actual one. If yes it continues with the next pair
  2. treat the expected line as a regular expression and performs a check with the String.matches() method. If yes it continues with the next pair
  3. check if the expected line is a fast-forward marker. If yes apply fast-forward and repeat the algorithm from the step 1

Let's see how we can use this assertion to assert that two lists of String have matching lines:

public void whenAssertingEqualityListOfStrings_thenEqual() {
    List<String> expected = asList("Java", "\\d+", "JUnit");
    List<String> actual = asList("Java", "11", "JUnit");

    assertLinesMatch(expected, actual);

4.10. assertNotEquals

Complementary to the assertEquals, the assertNotEquals assertion asserts that the expected and the actual values aren't equal:

public void whenAssertingEquality_thenNotEqual() {
    Integer value = 5; // result of an algorithm
    assertNotEquals(0, value, "The result cannot be 0");

If both are null, the assertion fails.

4.11. assertThrows

In order to increase simplicity and readability, the new assertThrows assertion allows us a clear and a simple way to assert if an executable throws the specified exception type.

Let's see how we can assert a thrown exception:

void whenAssertingException_thenThrown() {
    Throwable exception = assertThrows(
      () -> {
          throw new IllegalArgumentException("Exception message");
    assertEquals("Exception message", exception.getMessage());

The assertion will fail if no exception is thrown, or if an exception of a different type is thrown.

4.12. assertTimeout and assertTimeoutPreemptively

In case we want to assert that the execution of a supplied Executable ends before a given Timeout, we can use the assertTimeout assertion:

public void whenAssertingTimeout_thenNotExceeded() {
      () -> {
        // code that requires less then 2 minutes to execute

However, with the assertTimeout assertion, the supplied executable will be executed in the same thread of the calling code. Consequently, execution of the supplier won't be preemptively aborted if the timeout is exceeded.

In case we want to be sure that execution of the executable will be aborted once it exceeds the timeout, we can use the assertTimeoutPreemptively assertion.

Both assertions can accept, instead of an Executable, a ThrowingSupplier, representing any generic block of code that returns an object and that can potentially throw a Throwable.

5. Conclusion

In this tutorial, we covered all the assertions available in both JUnit 4 and JUnit 5.

We highlighted briefly the improvements made in JUnit 5, with the introductions of new assertions and the support of lambdas.

As always, the complete source code for this article is available over on GitHub.