When testing collection-based results, AssertJ provides dedicated assertion methods for Iterable, List, Set, and arrays. These go far beyond a simple size check — you can verify element presence, order, uniqueness, and even filter or extract elements before asserting.

Why These Assertions Matter

Collections are central to most Java applications. Verifying that a service returns the correct items, in the correct order, without unwanted entries, is a common test requirement. AssertJ's collection assertions let you express these checks concisely and fail with clear, descriptive messages.

Key Methods

hasSize, contains, containsExactly, containsOnly, doesNotContain, isEmpty, isNotEmpty, hasDuplicates, isSortedAccordingTo.

Example

package com.logicbig.example;

import org.junit.jupiter.api.Test;

import java.util.List;
import java.util.Set;
import static org.assertj.core.api.Assertions.*;

public class CollectionAssertionsExample {

    @Test
    void myTest() {
        List<String> fruits = List.of("apple", "banana", "cherry");

        // size and presence
        assertThat(fruits).hasSize(3)
                          .contains("banana")
                          .doesNotContain("mango");
        System.out.println("Size and presence passed");

        // exact order
        assertThat(fruits).containsExactly("apple", "banana", "cherry");
        System.out.println("Exact order check passed");

        // containsOnly: any order, no extras
        assertThat(fruits).containsOnly("cherry", "apple", "banana");
        System.out.println("containsOnly passed");

        // Set: no duplicates
        Set<Integer> nums = Set.of(1, 2, 3);
        assertThat(nums).isNotEmpty()
                        .hasSize(3);
        System.out.println("Set assertion passed");
    }
}

Output

$ mvn clean test -Dtest=*
[INFO] Scanning for projects...
[INFO] 
[INFO] ---------< com.logicbig.example:assertj-collection-assertions >---------
[INFO] Building assertj-collection-assertions 1.0-SNAPSHOT
[INFO]   from pom.xml
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- clean:3.2.0:clean (default-clean) @ assertj-collection-assertions ---
[INFO] 
[INFO] --- resources:3.3.1:resources (default-resources) @ assertj-collection-assertions ---
[INFO] skip non existing resourceDirectory D:\example-projects\assertj\assertj-collection-assertions\src\main\resources
[INFO] 
[INFO] --- compiler:3.11.0:compile (default-compile) @ assertj-collection-assertions ---
[INFO] No sources to compile
[INFO] 
[INFO] --- resources:3.3.1:testResources (default-testResources) @ assertj-collection-assertions ---
[INFO] skip non existing resourceDirectory D:\example-projects\assertj\assertj-collection-assertions\src\test\resources
[INFO] 
[INFO] --- compiler:3.11.0:testCompile (default-testCompile) @ assertj-collection-assertions ---
[INFO] Changes detected - recompiling the module! :source
[INFO] Compiling 1 source file with javac [debug target 17] to target\test-classes
[INFO] 
[INFO] --- surefire:3.2.5:test (default-test) @ assertj-collection-assertions ---
[INFO] Using auto detected provider org.apache.maven.surefire.junitplatform.JUnitPlatformProvider
[WARNING] file.encoding cannot be set as system property, use <argLine>-Dfile.encoding=...</argLine> instead
[INFO] 
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.logicbig.example.CollectionAssertionsExample
Size and presence passed
Exact order check passed
containsOnly passed
Set assertion passed
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.156 s -- in com.logicbig.example.CollectionAssertionsExample
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  5.665 s
[INFO] Finished at: 2026-03-03T11:06:47+08:00
[INFO] ------------------------------------------------------------------------

Conclusion

The output shows all collection assertions pass. The example illustrates how contains checks for element presence regardless of order, while containsExactly enforces strict ordering — choosing the right method makes intentions explicit in the test code.

Example Project

Dependencies and Technologies Used:

• assertj-core 3.27.7 (Rich and fluent assertions for testing in Java)
  
• junit-jupiter-engine 6.0.2 (Module "junit-jupiter-engine" of JUnit)
  
• JDK 17
• Maven 3.9.11