First JUnit test - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a JUnit test?

A JUnit test checks if a small part of your code works correctly by running it automatically and comparing the result to what you expect.

beginner

What annotation marks a method as a test in JUnit?

The @Test annotation tells JUnit that the method below it is a test method to run.

beginner

What does the assertion assertEquals(expected, actual) do in a JUnit test?

It checks if the actual result from the code matches the expected result. If they are equal, the test passes; if not, it fails.

beginner

How do you write a simple JUnit test method to check if 2 + 3 equals 5?

Use @Test annotation and inside the method call assertEquals(5, 2 + 3); to check the sum.

beginner

Why is it important to keep JUnit tests small and focused?

Small tests are easier to understand, faster to run, and help find problems quickly because they test only one thing at a time.

Which annotation is used to mark a method as a test in JUnit?

A @Check

B @Run

C @Verify

D @Test

The correct annotation to mark a test method in JUnit is @Test.

What does assertEquals(5, 2 + 3) check in a JUnit test?

A If 5 equals 2 + 3 plus 1

B If 2 + 3 is greater than 5

C If 2 + 3 equals 5

D If 5 is less than 2 + 3

It checks if the sum of 2 + 3 equals 5.

What happens if an assertion in a JUnit test fails?

A The test fails and reports an error

B The test passes silently

C The test skips the assertion

D The test restarts automatically

If an assertion fails, JUnit marks the test as failed and shows an error.

Which of these is a good practice for writing JUnit tests?

A Test one small thing per test method

B Test many things in one method

C Avoid using assertions

D Write tests only after the project is finished

Testing one small thing per method makes tests clear and easy to fix.

What is the minimum Java version required to run JUnit 5 tests?

A Java 17

B Java 8

C Java 11

D Java 6

JUnit 5 requires at least Java 8 to run.

Explain how to write and run your first JUnit test method.

Think about the steps from writing code to checking results.

Describe why assertions are important in JUnit tests.

Assertions are like checkpoints in your test.

Practice

1. What is the main purpose of a JUnit test method marked with @Test?

easy

A. To check if a piece of code works as expected

B. To create a new Java class

C. To print output to the console

D. To declare a variable

Solution

1. Step 1: Understand the role of @Test annotation

   The @Test annotation marks a method as a test method in JUnit, meaning it will be run to check code behavior.

2. Step 2: Identify the purpose of test methods

   Test methods verify if the code produces the expected results, helping find bugs or confirm correctness.

3. Final Answer:

   To check if a piece of code works as expected -> Option A

4. Quick Check:

   JUnit @Test marks test methods [OK]

Hint: JUnit tests verify code correctness using @Test [OK]

Common Mistakes:

• Thinking @Test creates classes
• Confusing test methods with print statements
• Using @Test to declare variables

2. Which of the following is the correct way to write a simple JUnit 5 test method?

easy

A. public void testMethod() { assertEquals(5, 2 + 3); }

B. void testMethod() { assertEquals(5, 2 + 3); }

C. @Test public void testMethod() { assertEquals(5, 2 + 3); }

D. @Test void testMethod() { System.out.println(5); }

Solution

1. Step 1: Check for @Test annotation

   JUnit 5 requires the @Test annotation to mark a method as a test.

2. Step 2: Verify method signature and assertion

   The method should be public void with an assertion like assertEquals(expected, actual). @Test public void testMethod() { assertEquals(5, 2 + 3); } matches this.

3. Final Answer:

   @Test public void testMethod() { assertEquals(5, 2 + 3); } -> Option C

4. Quick Check:

   JUnit test method needs @Test and assertion [OK]

Hint: Always use @Test before test methods [OK]

Common Mistakes:

• Missing @Test annotation
• Wrong method visibility or return type
• Using print statements instead of assertions

3. What will be the result of running this JUnit test?

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

public class SampleTest {
  @Test
  void additionTest() {
    assertEquals(10, 5 + 5);
  }
}

medium

A. Runtime exception occurs

B. Test passes successfully

C. Compilation error due to missing import

D. Test fails with assertion error

Solution

1. Step 1: Analyze the assertion

   The test asserts that 5 + 5 equals 10, which is true.

2. Step 2: Check for syntax and imports

   All necessary imports and syntax are correct, so the test will run without errors.

3. Final Answer:

   Test passes successfully -> Option B

4. Quick Check:

   Correct assertion means test passes [OK]

Hint: True assertions make tests pass [OK]

Common Mistakes:

• Assuming test fails when assertion is true
• Missing static import for assertions
• Confusing compilation errors with runtime errors

4. Identify the error in this JUnit test code:

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

public class CalcTest {
  @Test
  public void testSum() {
    int result = 2 + 2;
    assertEquals(5, result);
  }
}

medium

A. The assertion expects wrong value causing test failure

B. Missing @Test annotation

C. Method should return int, not void

D. No import for assertEquals

Solution

1. Step 1: Check the assertion values

   The test expects 5 but the actual result is 4 (2 + 2), so the assertion will fail.

2. Step 2: Verify annotations and imports

   The @Test annotation and imports are correct, so no syntax errors exist.

3. Final Answer:

   The assertion expects wrong value causing test failure -> Option A

4. Quick Check:

   Assertion mismatch causes test failure [OK]

Hint: Check expected vs actual values in assertions [OK]

Common Mistakes:

• Ignoring assertion expected value
• Thinking missing annotation causes failure here
• Confusing method return types in tests

5. You want to write a JUnit test that checks if a method multiply(int a, int b) returns 0 when one input is 0. Which test method is correct?

hard

A. @Test public void testMultiplyZero() { assertEquals(5, multiply(0, 5)); }

B. public void testMultiplyZero() { assertEquals(0, multiply(0, 5)); }

C. @Test public int testMultiplyZero() { return multiply(0, 5); }

D. @Test public void testMultiplyZero() { assertEquals(0, multiply(0, 5)); }

Solution

1. Step 1: Confirm @Test annotation and method signature

   The test method must be annotated with @Test and return void. @Test public void testMultiplyZero() { assertEquals(0, multiply(0, 5)); } meets this.

2. Step 2: Check assertion correctness

   The method should assert that multiplying by zero returns zero. @Test public void testMultiplyZero() { assertEquals(0, multiply(0, 5)); } asserts assertEquals(0, multiply(0, 5)), which is correct.

3. Final Answer:

   @Test public void testMultiplyZero() { assertEquals(0, multiply(0, 5)); } -> Option D

4. Quick Check:

   Use @Test, void method, correct assertion [OK]

Hint: Test methods need @Test, void, and correct assertions [OK]

Common Mistakes:

• Omitting @Test annotation
• Using wrong return type in test method
• Asserting wrong expected value