Comparing different assertion methods and error messages

AgeVerificationTests.java

import org.assertj.core.api.SoftAssertions;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

import static org.assertj.core.api.Assertions.assertThat;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertTrue;

class Customer
{
	String name;
	Integer age;
}

public class AgeVerificationTests
{
	Customer customer;
	int expectedAge = 39;

	@BeforeEach
	public void setup()
	{
		customer = new Customer();
		customer.name = "John Doe";
		customer.age = 18;
	}

	/** Don't do this, assertTrue() loses data that helps us debug **/
	@Test
	public void testCustomerAge_usingAssertTrue()
	{
		assertTrue(customer.age >= 18);
		assertTrue(customer.age == expectedAge);
	}

	/** A custom message can show the expected results  **/
	@Test void testCustomerAge_usingAssertTrueWithMessage()
	{
		assertTrue(customer.age >= expectedAge, String.format("age %d was not same as expected %d", customer.age, expectedAge));
	}

	/** This is better because assertEquals() will show the comparison on failure **/
	@Test
	public void testCustomerAge_usingAssertEquals()
	{
		assertEquals(customer.age, expectedAge);
	}

	/** This is even better because message provides context **/
	@Test
	public void testCustomerAge_usingAssertEqualsWithMessage()
	{
		assertEquals(customer.age, expectedAge, "checking customer age");
	}

	/** using AssertJ **/
	@Test
	public void testCustomerAge_usingAssertjIsEqualTo()
	{
		assertThat(customer.age).isEqualTo(expectedAge);
	}

	/** This has the same problem as assertTrue() - We don't know what is being tested **/
	@Test
	public void testCustomerAge_usingAssertjIsTrue()
	{
		assertThat(customer.age == expectedAge).isTrue();
	}

	/** This gives us some context, but we have to enter it ourselves **/
	@Test
	public void testCustomerAge_usingAssertjIsTrueWithMessage()
	{
		assertThat(customer.age).overridingErrorMessage(String.format("age %d was not same as expected %d", customer.age, expectedAge)).isEqualTo(expectedAge);
	}

	/** This adds context and gives us the comparison **/
	@Test
	public void testCustomerAge_usingAssertjWithContext()
	{
		assertThat(customer.age).as("checking customer age").isEqualTo(expectedAge);
	}

	/** Soft Assertions allow you to perform multiple checks **/
	@Test
	public void testCustomerAge_withSoftAssertions()
	{
		var softly = new SoftAssertions();
		softly.assertThat(customer.age).as("check that customer age has been set").isNotNull();
		softly.assertThat(customer.age).as("check that customer age is correct").isEqualTo(expectedAge);
		softly.assertThat(customer.age).as("make sure customer is not a minor").isGreaterThanOrEqualTo(18);
		softly.assertAll();
	}
}

Run this set of tests to see what each assertion and error message looks like to compare the results.

test: AgeVerificationTests.testCustomerAge_usingAssertTrue

org.opentest4j.AssertionFailedError: 
Expected :true
Actual   :false
at AgeVerificationTests.testCustomerAge_usingAssertTrue(AgeVerificationTests.java:38)

---

test: AgeVerificationTests.testCustomerAge_usingAssertTrueWithMessage

org.opentest4j.AssertionFailedError: age 18 was not same as expected 39 ==> 
Expected :true
Actual   :false
at AgeVerificationTests.testCustomerAge_usingAssertTrueWithMessage(AgeVerificationTests.java:45)

---

test: AgeVerificationTests.testCustomerAge_usingAssertEquals

org.opentest4j.AssertionFailedError: 
Expected :18
Actual   :39
at AgeVerificationTests.testCustomerAge_usingAssertEquals(AgeVerificationTests.java:53)

---

test: AgeVerificationTests.testCustomerAge_usingAssertEqualsWithMessage

org.opentest4j.AssertionFailedError: checking customer age ==> 
Expected :18
Actual   :39
at AgeVerificationTests.testCustomerAge_usingAssertEqualsWithMessage(AgeVerificationTests.java:61)

---

test: AgeVerificationTests.testCustomerAge_usingAssertjIsEqualTo

org.opentest4j.AssertionFailedError: 
expected: 39
 but was: 18
Expected :39
Actual   :18
at AgeVerificationTests.testCustomerAge_usingAssertjIsEqualTo(AgeVerificationTests.java:69)

---

test: AgeVerificationTests.testCustomerAge_usingAssertjIsTrue

org.opentest4j.AssertionFailedError: 
Expecting value to be true but was false
Expected :true
Actual   :false
at AgeVerificationTests.testCustomerAge_usingAssertjIsTrue(AgeVerificationTests.java:77)

---

test: AgeVerificationTests.testCustomerAge_usingAssertjIsTrueWithMessage

java.lang.AssertionError: age 18 was not same as expected 39
at AgeVerificationTests.testCustomerAge_usingAssertjIsTrueWithMessage(AgeVerificationTests.java:85)

---

test: AgeVerificationTests.testCustomerAge_usingAssertjWithContext

org.opentest4j.AssertionFailedError: [checking customer age] 
expected: 39
 but was: 18
Expected :39
Actual   :18
at AgeVerificationTests.testCustomerAge_usingAssertjWithContext(AgeVerificationTests.java:93)

---

test: AgeVerificationTests.testCustomerAge_withSoftAssertions

[check that customer age is correct] 
expected: 39
 but was: 18
at AgeVerificationTests.testCustomerAge_withSoftAssertions(AgeVerificationTests.java:103)

Comparison Failure: 
Expected :39
Actual   :18

org.assertj.core.error.AssertJMultipleFailuresError: 
Multiple Failures (1 failure)
-- failure 1 --
[check that customer age is correct] 
expected: 39
 but was: 18
at AgeVerificationTests.testCustomerAge_withSoftAssertions(AgeVerificationTests.java:103)
at AgeVerificationTests.testCustomerAge_withSoftAssertions(AgeVerificationTests.java:105)