Using @Test in JUnit

In JUnit, @Test is an annotation that marks a method as a test method. When you run your tests, JUnit looks for methods annotated with @Test and executes them automatically.

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1. Add the Correct Import

For JUnit 5, use:

import org.junit.jupiter.api.Test;

You will usually also import assertions such as:

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

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2. Basic Example

import org.junit.jupiter.api.Test;

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

class CalculatorTest {

    @Test
    void addTwoNumbers() {
        int result = 10 + 15;

        assertEquals(25, result);
    }
}

Here:

• @Test tells JUnit this method should be run as a test.
• assertEquals(25, result) checks that the actual result is 25.
• If the assertion passes, the test passes.
• If the assertion fails, the test fails.

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3. Common JUnit 5 Assertions

import org.junit.jupiter.api.Test;

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

class ExampleTest {

    @Test
    void assertionsExample() {
        String message = "Hello JUnit";

        assertNotNull(message);
        assertTrue(message.contains("JUnit"));
        assertEquals("Hello JUnit", message);
    }
}

Common assertions include:

Assertion	Purpose
assertEquals(expected, actual)	Checks that two values are equal
assertTrue(condition)	Checks that a condition is true
assertFalse(condition)	Checks that a condition is false
assertNotNull(value)	Checks that a value is not null
assertThrows(...)	Checks that code throws an expected exception

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4. Testing Exceptions

Use assertThrows() when you expect code to throw an exception:

import org.junit.jupiter.api.Test;

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

class DivisionTest {

    @Test
    void divideByZeroThrowsException() {
        assertThrows(ArithmeticException.class, () -> {
            int result = 10 / 0;
        });
    }
}

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5. JUnit 5 Test Method Rules

In JUnit 5, test methods usually:

• Are annotated with @Test
• Return void
• Do not need to be public
• Should contain assertions
• Should have descriptive names

Example:

@Test
void shouldReturnSumWhenAddingTwoNumbers() {
    assertEquals(5, 2 + 3);
}

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6. JUnit 4 vs. JUnit 5 Import

Be careful: JUnit 4 and JUnit 5 use different @Test imports.

JUnit 5

import org.junit.jupiter.api.Test;

JUnit 4

import org.junit.Test;

For new projects, JUnit 5 is generally recommended.

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7. Running the Test

You can run JUnit tests from:

• Your IDE, such as IntelliJ IDEA or Eclipse
• Maven
• Gradle

With Maven:

mvn test

With Gradle:

gradle test

or:

./gradlew test

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Summary

Use @Test above a method to tell JUnit, “this is a test.”

import org.junit.jupiter.api.Test;

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

class MyTest {

    @Test
    void simpleTest() {
        assertEquals(4, 2 + 2);
    }
}

That method will be discovered and run by JUnit as part of your test suite.