when().thenThrow() for exceptions in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to mock a method to throw an exception using when().thenThrow().

JUnit

when(mockedList.[1](0)).thenThrow(new IndexOutOfBoundsException());

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

Aremove

Bget

Cadd

Dclear

Attempts:

3 left

💡 Hint

Common Mistakes

Using methods like add or clear which do not take an index parameter.

Trying to mock a method that does not throw IndexOutOfBoundsException.

The get method is commonly mocked to throw exceptions like IndexOutOfBoundsException when accessing invalid indexes.

2fill in blank

medium

Complete the code to mock a service method to throw a custom exception.

JUnit

when(userService.[1](anyString())).thenThrow(new UserNotFoundException());

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

AfindUserById

BdeleteUser

CupdateUser

DcreateUser

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing methods that create or update users which usually do not throw not found exceptions.

Using methods that do not accept a string parameter.

The method findUserById is typically used to retrieve a user and can throw UserNotFoundException if the user does not exist.

3fill in blank

hard

Fix the error in the code to correctly mock throwing an exception with when().thenThrow().

JUnit

when(mockedMap.get([1])).thenThrow(new NullPointerException());

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

A"key"

Bkey

Cnull

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an unquoted string which causes compilation errors.

Passing null which is valid but not the intended key here.

The get method expects a key object. To pass a string key, it must be in quotes.

4fill in blank

hard

Fill both blanks to mock a method that throws an IllegalArgumentException when called with a negative number.

JUnit

when(calculator.[1]([2])).thenThrow(new IllegalArgumentException());

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

Adivide

B-5

Cadd

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using the add method which usually does not throw exceptions for negative inputs.

Passing positive numbers instead of negative.

The divide method can throw an exception when dividing by a negative number like -5.

5fill in blank

hard

Fill all three blanks to mock a repository method that throws SQLException when saving a null entity.

JUnit

when(repository.[1]([2])).thenThrow(new [3]());

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

Asave

Bnull

CSQLException

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete instead of save which may not throw SQLException here.

Passing a non-null argument instead of null.

The save method throws SQLException when called with a null entity.

Practice

1. What is the main purpose of using when().thenThrow() in JUnit tests?

easy

A. To verify that a method returns a specific value

B. To skip the execution of a test method

C. To create a new instance of a class

D. To simulate an exception being thrown by a mocked method

Solution

1. Step 1: Understand the role of when().thenThrow()

   This syntax is used in mocking frameworks like Mockito to simulate exceptions when a mocked method is called.

2. Step 2: Identify the purpose in testing

   It helps test how your code handles exceptions by forcing the mocked method to throw an exception.

3. Final Answer:

   To simulate an exception being thrown by a mocked method -> Option D

4. Quick Check:

   Exception simulation [OK]

Hint: Use when().thenThrow() to simulate errors in mocks [OK]

Common Mistakes:

• Confusing exception simulation with return value mocking
• Thinking it creates real exceptions instead of simulated ones
• Using it to skip tests instead of throwing exceptions

2. Which of the following is the correct syntax to make a mocked method throw an IllegalArgumentException using Mockito?

easy

A. when(mock.method()).throwException(IllegalArgumentException)

B. when(mock.method()).thenThrow(new IllegalArgumentException())

C. mock.method().thenThrow(IllegalArgumentException)

D. when(mock.method()).throws(IllegalArgumentException)

Solution

1. Step 1: Recall Mockito syntax for throwing exceptions

   The correct syntax uses when(mock.method()).thenThrow(new ExceptionType()).

2. Step 2: Match the options with correct syntax

   when(mock.method()).thenThrow(new IllegalArgumentException()) matches the correct syntax exactly, creating a new exception instance inside thenThrow().

3. Final Answer:

   when(mock.method()).thenThrow(new IllegalArgumentException()) -> Option B

4. Quick Check:

   Correct Mockito exception syntax [OK]

Hint: Use thenThrow(new Exception()) with when() for exceptions [OK]

Common Mistakes:

• Using throwException or throws which are not Mockito methods
• Not creating a new exception instance
• Calling thenThrow directly on mock.method()

3. Given the following code snippet, what will be the test result?

MyService mockService = mock(MyService.class);
when(mockService.getData()).thenThrow(new RuntimeException("Error"));

assertThrows(RuntimeException.class, () -> mockService.getData());

medium

A. Test passes because RuntimeException is thrown as expected

B. Test fails because no exception is thrown

C. Test fails due to syntax error in when().thenThrow()

D. Test passes but exception type is incorrect

Solution

1. Step 1: Analyze the mocking behavior

   The mocked method getData() is set to throw a RuntimeException when called.

2. Step 2: Understand the assertion

   assertThrows expects a RuntimeException from calling mockService.getData().

3. Final Answer:

   Test passes because RuntimeException is thrown as expected -> Option A

4. Quick Check:

   Exception thrown matches assertThrows [OK]

Hint: assertThrows expects the exception thrown by thenThrow() [OK]

Common Mistakes:

• Assuming no exception is thrown from mocked method
• Confusing exception types in assertThrows
• Thinking thenThrow causes syntax errors

4. Identify the error in the following test code using Mockito:

when(mockService.getData()).thenThrow(RuntimeException.class);
assertThrows(RuntimeException.class, () -> mockService.getData());

medium

A. Missing try-catch block around mockService.getData()

B. assertThrows should not be used with exceptions

C. Incorrect use of thenThrow with exception class instead of instance

D. No error; code is correct

Solution

1. Step 1: Check thenThrow argument type

   thenThrow requires an exception instance, not a class type.

2. Step 2: Identify correct usage

   The code should use thenThrow(new RuntimeException()) instead of passing the class.

3. Final Answer:

   Incorrect use of thenThrow with exception class instead of instance -> Option C

4. Quick Check:

   thenThrow needs exception instance [OK]

Hint: Use new Exception() inside thenThrow(), not exception class [OK]

Common Mistakes:

• Passing exception class instead of instance to thenThrow
• Expecting assertThrows to catch exceptions without mocking
• Adding unnecessary try-catch in test

5. You want to test that your service handles a NullPointerException thrown by a dependency's void call() method. Which of the following code snippets correctly mocks the dependency and verifies the exception handling using JUnit and Mockito?

hard

A. doThrow(new NullPointerException()).when(dependency).call();
assertThrows(NullPointerException.class, () -> service.callDependency());

B. when(dependency.call()).thenThrow(NullPointerException.class);
assertThrows(Exception.class, () -> service.callDependency());

C. when(dependency.call()).thenReturn(null);
assertThrows(NullPointerException.class, () -> service.callDependency());

D. when(dependency.call()).thenThrow(new NullPointerException());
assertThrows(NullPointerException.class, () -> service.callDependency());

Solution

1. Step 1: Understand mocking exceptions with void methods

   If dependency.call() is a void method, doThrow().when() is the correct way to mock exceptions.

2. Step 2: Verify correct assertion

   assertThrows(NullPointerException.class, () -> service.callDependency()) correctly checks that the service handles the exception.

3. Step 3: Analyze options

   when(dependency.call()).thenThrow(new NullPointerException()); assertThrows(NullPointerException.class, () -> service.callDependency()); uses when().thenThrow() which works only for non-void methods. when(dependency.call()).thenThrow(NullPointerException.class); assertThrows(Exception.class, () -> service.callDependency()); passes class instead of instance and wrong exception type in assertion. when(dependency.call()).thenReturn(null); assertThrows(NullPointerException.class, () -> service.callDependency()); returns null instead of throwing exception.

4. Final Answer:

   doThrow(new NullPointerException()).when(dependency).call(); assertThrows(NullPointerException.class, () -> service.callDependency()); -> Option A

5. Quick Check:

   Use doThrow for void methods + assertThrows [OK]

Hint: Use doThrow() for void methods, then assertThrows() to check [OK]

Common Mistakes:

• Using thenThrow() on void methods
• Passing exception class instead of instance
• Asserting wrong exception type
• Returning null instead of throwing exception