assertDoesNotThrow in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of assertDoesNotThrow in JUnit?

assertDoesNotThrow checks that a block of code runs without throwing any exceptions. It helps confirm that the tested code executes smoothly.

beginner

How do you use assertDoesNotThrow in a test method?

You pass a lambda or method reference containing the code to test. For example:
assertDoesNotThrow(() -> someMethod());

beginner

What happens if the code inside assertDoesNotThrow throws an exception?

The test fails because assertDoesNotThrow expects no exceptions. The thrown exception causes the assertion to fail.

intermediate

Can assertDoesNotThrow return a value?

Yes, it returns the result of the executed code block if no exception is thrown. This allows further assertions on the returned value.

intermediate

Why is assertDoesNotThrow useful compared to a try-catch block in tests?

It makes tests cleaner and clearer by directly expressing the expectation that no exception should occur, avoiding manual try-catch and fail calls.

What does assertDoesNotThrow verify in a JUnit test?

A That the code does not throw any exception

B That the code throws an exception

C That the code returns null

D That the code runs slower than expected

assertDoesNotThrow checks that no exceptions are thrown during code execution.

How do you pass the code to test inside assertDoesNotThrow?

A As a string

B As a lambda or method reference

C As an integer

D As a boolean

You provide a lambda expression or method reference representing the code block to test.

What happens if the tested code throws an exception inside assertDoesNotThrow?

A The test passes

B The test retries automatically

C The test is skipped

D The test fails

The test fails because assertDoesNotThrow expects no exceptions.

Can assertDoesNotThrow return the result of the tested code?

A Only if the code returns void

B No

C Yes

D Only if the code throws an exception

It returns the result if no exception is thrown, allowing further checks.

Why might assertDoesNotThrow be preferred over try-catch in tests?

A It makes tests clearer and simpler

B It hides exceptions

C It makes tests longer

D It slows down test execution

It simplifies tests by clearly stating the expectation without manual try-catch blocks.

Explain how assertDoesNotThrow works and when you would use it in a test.

Think about testing code that should run smoothly without errors.

Describe the benefits of using assertDoesNotThrow instead of a try-catch block in JUnit tests.

Consider how test readability and simplicity improve.

Practice

1. What is the main purpose of assertDoesNotThrow in JUnit testing?

easy

A. To assert that a method returns a specific value

B. To check that a block of code runs without throwing any exceptions

C. To verify that a specific exception is thrown by the code

D. To measure the execution time of a code block

Solution

1. Step 1: Understand the purpose of assertDoesNotThrow

   This assertion is designed to confirm that no exceptions occur when running a code block.

2. Step 2: Compare with other assertions

   Unlike assertions that check for exceptions or return values, assertDoesNotThrow ensures smooth execution without errors.

3. Final Answer:

   To check that a block of code runs without throwing any exceptions -> Option B

4. Quick Check:

   assertDoesNotThrow means no exceptions thrown [OK]

Hint: Remember: assertDoesNotThrow means code runs without errors [OK]

Common Mistakes:

• Confusing assertDoesNotThrow with assertThrows
• Thinking it checks return values
• Assuming it measures performance

2. Which of the following is the correct syntax to use assertDoesNotThrow in JUnit 5?

easy

A. assertDoesNotThrow(methodToTest());

B. assertDoesNotThrow { methodToTest() };

C. assertDoesNotThrow(() -> methodToTest());

D. assertDoesNotThrow(methodToTest);

Solution

1. Step 1: Recall the lambda syntax for assertDoesNotThrow

   assertDoesNotThrow expects a lambda expression or executable block, written as () -> methodToTest().

2. Step 2: Identify incorrect options

   Options A, C, and D use incorrect syntax: A calls the method directly, C uses braces like other languages, D passes method reference without lambda.

3. Final Answer:

   assertDoesNotThrow(() -> methodToTest()); -> Option C

4. Quick Check:

   Use lambda () -> method() with assertDoesNotThrow [OK]

Hint: Use lambda syntax () -> method() inside assertDoesNotThrow [OK]

Common Mistakes:

• Calling the method directly inside assertDoesNotThrow
• Using curly braces instead of lambda arrows
• Passing method name without parentheses or lambda

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

assertDoesNotThrow(() -> { int x = 5 / 1; });

medium

A. Test fails due to syntax error

B. Test fails due to ArithmeticException

C. Test fails due to NullPointerException

D. Test passes because no exception is thrown

Solution

1. Step 1: Analyze the code inside assertDoesNotThrow

   The code divides 5 by 1, which is valid and does not throw any exception.

2. Step 2: Understand assertDoesNotThrow behavior

   Since no exception occurs, the assertion passes and the test succeeds.

3. Final Answer:

   Test passes because no exception is thrown -> Option D

4. Quick Check:

   Division by 1 is safe, so no exception [OK]

Hint: Check if code inside lambda throws exception or not [OK]

Common Mistakes:

• Assuming division always throws exception
• Confusing ArithmeticException with other exceptions
• Thinking assertDoesNotThrow checks return values

4. Identify the error in this JUnit test using assertDoesNotThrow:

assertDoesNotThrow(() -> { int y = 10 / 0; });

medium

A. ArithmeticException will cause test to fail

B. No error; test will pass

C. Syntax error due to missing semicolon

D. NullPointerException will cause test to fail

Solution

1. Step 1: Examine the code inside assertDoesNotThrow

   The code divides 10 by 0, which causes an ArithmeticException at runtime.

2. Step 2: Understand assertDoesNotThrow behavior on exceptions

   assertDoesNotThrow expects no exceptions; if one occurs, the test fails.

3. Final Answer:

   ArithmeticException will cause test to fail -> Option A

4. Quick Check:

   Division by zero throws ArithmeticException [OK]

Hint: Division by zero throws exception, so test fails [OK]

Common Mistakes:

• Thinking test passes despite exception
• Confusing exception types
• Ignoring runtime exceptions inside lambda

5. You want to test a method processData() that may throw a checked exception. How do you use assertDoesNotThrow to verify it runs without exceptions?

hard

A. assertDoesNotThrow(() -> processData());

B. assertDoesNotThrow(processData());

C. assertDoesNotThrow(() -> { try { processData(); } catch(Exception e) {} });

D. assertDoesNotThrow(() -> { processData() });

Solution

1. Step 1: Understand handling checked exceptions in lambdas

   assertDoesNotThrow accepts an Executable, whose execute() method throws Throwable, so checked exceptions are permitted in the lambda.

2. Step 2: Identify the correct lambda syntax

   The standard syntax is a concise lambda: () -> processData().

3. Step 3: Evaluate options

   A: correct single-expression lambda. B: direct method call without lambda. C: catches exceptions inside lambda, hiding potential failures. D: syntax error due to missing semicolon after processData() in the block lambda.

4. Final Answer:

   assertDoesNotThrow(() -> processData()); -> Option A

5. Quick Check:

   Lambda () -> processData() handles checked exceptions correctly [OK]

Hint: Use lambda syntax () -> processData() inside assertDoesNotThrow [OK]

Common Mistakes:

• Calling method directly without lambda
• Syntax errors like missing semicolon in block lambdas
• Catching exceptions inside lambda hiding test failures