Backwards

In both JUnit 4 and JUnit 5, assertEquals(expected, actual) expects the expected value first and the actual value second. However, the optional message parameter moved from the first position in JUnit 4 to the last in JUnit 5. This subtle change, combined with the common mistake of putting actual before expected, leads to confusing failure messages and even silently passing tests.

Confusing failure messages

This can lead to confusing error messages when tests fail, as the expected and actual values will be reported incorrectly.

Before

ArgumentOrderTest.java

import org.junit.jupiter.api.Test;

import java.util.List;

import static org.junit.jupiter.api.Assertions.assertEquals;

class ArgumentOrderTest {

    List<String> list = List.of("a", "b", "c");

    @Test
    void confusingFailureMessage() {
        assertEquals(list.size(), 4);
    }
}

warning

Warning: The arguments to assertEquals are in the wrong order. The expected value should be the first argument, and the actual value should be the second argument: assertEquals(4, list.size()).

After

ArgumentOrderTest.java

import org.junit.jupiter.api.Test;

import java.util.List;

import static org.assertj.core.api.Assertions.assertThat;

class ArgumentOrderTest {

    List<String> list = List.of("a", "b", "c");

    @Test
    void confusingFailureMessage() {
        assertThat(list).hasSize(4);
    }
}

tip

Using AssertJ's fluent assertions can help avoid issues with argument order, as the actual value is always the subject of the assertion.

Incorrectly passing tests

Worse still, if the values are of the same type, tests may pass when they should fail, or vice versa.

Before

ArgumentOrderTest.java

import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertNotNull;

class ArgumentOrderTest {
    @Test
    void incorrectlyPassing() {
        String actual = null;
        assertNotNull("message", actual);
    }
}

danger

The arguments to assertNotNull are in the wrong order. This test will incorrectly pass because the string "message" is not null.

After

ArgumentOrderTest.java

import org.junit.jupiter.api.Test;

import static org.assertj.core.api.Assertions.assertThat;

class ArgumentOrderTest {
    @Test
    void incorrectlyPassing() {
        String actual = null;
        assertThat(actual).as("message").isNotNull();
    }
}

tip

The message is now correctly associated with the assertion, and the test will fail as expected.