Assert list size

When testing collection sizes with JUnit Jupiter, it's easy to make mistakes with argument order or write verbose assertions. AssertJ provides more expressive and less error-prone alternatives.

Redundant null checks

Checking for null before checking collection size is redundant, as a null collection would fail the size check anyway.

Before

AssertListSize.java

import org.junit.jupiter.api.Test;

import java.util.List;

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

class AssertListSize {

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

    @Test
    void assertListSize() {
        assertNotNull(list);
        assertEquals(3, list.size());
    }
}

warning

The assertNotNull(list) check is redundant. If the list were null, assertEquals(3, list.size()) would throw a NullPointerException anyway.

After

AssertListSize.java

import org.junit.jupiter.api.Test;

import java.util.List;

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

class AssertListSize {

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

    @Test
    void assertListSize() {
        assertThat(list).hasSize(3);
    }
}

tip

AssertJ's hasSize() is concise, expressive, and will appropriately handle null collections with a clear error message.

Confusing failure messages

Using JUnit's assertEquals() with the actual and expected values swapped leads to confusing error messages.

Before

AssertListSize.java

import org.junit.jupiter.api.Test;

import java.util.List;

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

class AssertListSize {

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

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

danger

The arguments are backwards. JUnit 5's assertEquals() expects the expected value first, then the actual value. This produces a confusing error: expected: <3> but was: <4> when it should be expected: <4> but was: <3>.

After

AssertListSize.java

import org.junit.jupiter.api.Test;

import java.util.List;

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

class AssertListSize {

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

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

tip

AssertJ eliminates argument order confusion. The actual value is always the subject, and the assertion is always chained after it.