15.4.2. JUnit Assertions

JUnit Assertions • assertEquals(expected, actual) • assertTrue(condition) • assertFalse(condition) • assertNull(value) • assertNotNull(value) • assertThrows(Exception.class, () -> method())

JUnit assertions are methods that allow you to verify whether a certain condition is true or false in your tests.

• assertEquals(expected, actual): This checks that the expected result is equal to the actual result. If they are not equal, the test fails.
• assertTrue(condition): This asserts that the provided condition is true. If it's false, the test fails.
• assertFalse(condition): It checks that the given condition is false, failing the test if it evaluates to true.
• assertNull(value): This checks that the given value is null; if the value is not null, the test fails.
• assertNotNull(value): This assertion verifies that a value is not null; failing if the value is indeed null.
• assertThrows(Exception.class, () -> method()): It checks that a specified exception is thrown when a method is executed. If no exception is thrown, or if a different exception is thrown, the test fails.

Think of JUnit assertions like a teacher grading exam papers. Just as a teacher checks if a student's answers match the correct ones (like checking expected vs. actual), JUnit checks if the results of your code match what you anticipate. For instance, if a student is meant to answer 5 + 3 = 8 correctly, the teacher marks it wrong if the answer is 9. Similarly, if our code supposed to return 8 but returns any other number, the test will flag a failure.

Example: Basic Unit Test Using JUnit

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class CalculatorTest {
    @Test
    void testAddition() {
        Calculator calc = new Calculator();
        assertEquals(5, calc.add(2, 3));
    }
}

This example shows a simple unit test using JUnit, specifically for testing an addition method in a Calculator class.

1. Import Statements: We import necessary classes from JUnit and static methods for assertions.
2. Test Class Definition: We define a public class named CalculatorTest. This class is where we write our tests.
3. @Test Annotation: The @Test annotation tells JUnit that the method that follows is a test case. This way, JUnit knows to execute this method when the tests are run.
4. Test Method: Inside the testAddition method, we create an instance of the Calculator class and then call its add method with 2 and 3 as parameters. We expect the result to be 5. By using assertEquals, we verify this expectation. If the result is anything other than 5, the test will fail.

Imagine you're a chef who has a recipe for a cake. You expect it to be perfectly sweet if you add 2 cups of sugar. Testing this using JUnit is like taking a small bite of the cake after baking it. If it tastes sweet as expected, you know your recipe (or code) works. If it tastes bland, you know there's a problem you need to fix.

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