Unit testing concept in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the JUnit test annotation.

JUnit

import org.junit.[1];

Drag options to blanks, or click blank then click option'

ATest

BBefore

CAfter

DIgnore

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Before or @After instead of @Test for test methods.

Forgetting to import the annotation.

The @Test annotation marks a method as a test method in JUnit.

2fill in blank

medium

Complete the code to assert that two values are equal in a test.

JUnit

assertEquals([1], actualValue);

Drag options to blanks, or click blank then click option'

AactualValue

Btrue

Cfalse

DexpectedValue

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping expected and actual values.

Using boolean values instead of expected results.

The assertEquals method compares the expected value to the actual value.

3fill in blank

hard

Fix the error in the test method declaration by completing the annotation.

JUnit

@[1]
public void testAddition() {
    assertEquals(5, Calculator.add(2, 3));
}

Drag options to blanks, or click blank then click option'

ATest

BBefore

CAfter

DIgnore

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Before or @After instead of @Test for test methods.

Forgetting the annotation causes the test not to run.

The @Test annotation is required to tell JUnit this method is a test.

4fill in blank

hard

Fill both blanks to create a test method that checks if a list is empty.

JUnit

@[1]
public void testListEmpty() {
    List<String> list = new ArrayList<>();
    assertTrue(list.[2]());
}

Drag options to blanks, or click blank then click option'

ATest

BBefore

CisEmpty

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Before instead of @Test annotation.

Using size() without comparing to zero.

The method is marked with @Test and uses isEmpty() to check if the list has no elements.

5fill in blank

hard

Fill all three blanks to write a test that checks if a string contains a substring.

JUnit

@[1]
public void testContains() {
    String text = "hello world";
    assertTrue(text.[2]("[3]"));
}

Drag options to blanks, or click blank then click option'

ATest

Bcontains

Cworld

DstartsWith

Attempts:

3 left

💡 Hint

Common Mistakes

Using startsWith instead of contains.

Forgetting to annotate the method with @Test.

The test method is annotated with @Test. It uses the contains method to check if the string includes the substring "world".

Practice

1. What is the main purpose of a unit test in JUnit?

easy

A. To check small parts of code for errors early

B. To test the entire application at once

C. To deploy the application automatically

D. To write documentation for the code

Solution

1. Step 1: Understand unit test scope

   Unit tests focus on small parts or units of code, not the whole app.

2. Step 2: Identify main goal

   The goal is to find errors early by testing these small parts individually.

3. Final Answer:

   To check small parts of code for errors early -> Option A

4. Quick Check:

   Unit test purpose = check small code parts early [OK]

Hint: Unit tests check small code parts, not whole app [OK]

Common Mistakes:

• Confusing unit tests with integration or system tests
• Thinking unit tests deploy or document code
• Assuming unit tests cover the entire application

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

easy

A. @Run

B. @Assert

C. @Test

D. @Check

Solution

1. Step 1: Recall JUnit test annotation

   JUnit uses @Test to mark methods as tests.

2. Step 2: Verify other options

   @Run, @Check, and @Assert are not valid JUnit test annotations.

3. Final Answer:

   @Test -> Option C

4. Quick Check:

   JUnit test method = @Test annotation [OK]

Hint: JUnit test methods use @Test annotation [OK]

Common Mistakes:

• Using @Run instead of @Test
• Confusing @Assert with test annotation
• Assuming @Check is a JUnit annotation

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

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

public class CalcTest {
    @Test
    void testSum() {
        int result = 2 + 3;
        assertEquals(5, result);
    }
}

medium

A. Test passes successfully

B. Test fails with assertion error

C. Compilation error due to missing import

D. Runtime exception occurs

Solution

1. Step 1: Analyze the test logic

   The test calculates 2 + 3 and stores 5 in result.

2. Step 2: Check assertion

   assertEquals(5, result) compares expected 5 with actual 5, so it matches.

3. Final Answer:

   Test passes successfully -> Option A

4. Quick Check:

   2+3=5 matches expected 5 [OK]

Hint: If expected equals actual, test passes [OK]

Common Mistakes:

• Assuming test fails when values match
• Thinking import is missing (it's present)
• Expecting runtime errors without cause

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 SampleTest {
    @Test
    public void testMultiply() {
        int result = 3 * 4
        assertEquals(12, result);
    }
}

medium

A. Incorrect import statement

B. Wrong assertion method used

C. Test method missing @Test annotation

D. Missing semicolon after multiplication statement

Solution

1. Step 1: Check syntax in testMultiply method

   The line 'int result = 3 * 4' is missing a semicolon at the end.

2. Step 2: Verify other parts

   @Test annotation is present, assertion method is correct, imports are correct.

3. Final Answer:

   Missing semicolon after multiplication statement -> Option D

4. Quick Check:

   Java statements need semicolons [OK]

Hint: Look for missing semicolons in Java test methods [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Thinking @Test annotation is missing
• Assuming wrong assertion method

5. You want to write a unit test that checks if a method throws an exception when given invalid input. Which JUnit 5 feature should you use?

hard

A. Use @BeforeEach to catch exceptions

B. assertThrows to verify the exception is thrown

C. assertEquals to compare exception messages

D. Use @Disabled to skip the test

Solution

1. Step 1: Understand exception testing in JUnit 5

   JUnit 5 provides assertThrows to check if a specific exception is thrown by code.

2. Step 2: Evaluate other options

   assertEquals compares values, not exceptions; @BeforeEach runs before tests; @Disabled skips tests.

3. Final Answer:

   assertThrows to verify the exception is thrown -> Option B

4. Quick Check:

   Use assertThrows for exception tests [OK]

Hint: Use assertThrows to test exceptions in JUnit 5 [OK]

Common Mistakes:

• Using assertEquals to check exceptions
• Misusing @BeforeEach for exception handling
• Disabling tests instead of testing exceptions