assertThrows for exceptions in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of assertThrows in JUnit?

assertThrows checks if a specific exception is thrown by a block of code during a test. It helps verify that error handling works as expected.

beginner

How do you use assertThrows to test that a method throws IllegalArgumentException?

assertThrows(IllegalArgumentException.class, () -> {
    methodThatShouldThrow();
});

This runs methodThatShouldThrow() and passes if it throws IllegalArgumentException.

intermediate

What does assertThrows return and how can it be used?

assertThrows returns the thrown exception object. You can use it to check the exception message or other properties.

Exception e = assertThrows(Exception.class, () -> { ... });
assertEquals("Error message", e.getMessage());

intermediate

Why is assertThrows better than using try-catch in tests?

assertThrows makes tests cleaner and clearer by directly expressing the expected exception. It avoids manual try-catch blocks and reduces boilerplate code.

beginner

Can assertThrows check for multiple exception types at once?

No, assertThrows checks for one specific exception type per call. To test multiple exceptions, write separate assertThrows calls for each.

What does assertThrows verify in a JUnit test?

A That no exceptions are thrown

B That a specific exception is thrown

C That a method returns true

D That a method completes within time

assertThrows checks if the expected exception is thrown by the tested code.

Which of the following is the correct syntax to test for NullPointerException using assertThrows?

A assertThrows(() -> method(), NullPointerException.class);

B assertThrows(method(), NullPointerException.class);

C assertThrows(NullPointerException.class, () -> method());

D assertThrows(NullPointerException, method());

The first argument is the exception class, second is a lambda with the code to test.

What can you do with the exception object returned by assertThrows?

A Check its message or properties

B Ignore it

C Change its type

D Throw it again automatically

You can inspect the exception details to verify error messages or other info.

If the tested code does NOT throw the expected exception, what happens to the test with assertThrows?

A The test fails

B The test passes

C The test is skipped

D The test throws a runtime error

The test fails because the expected exception was not thrown.

Is it possible to test multiple exceptions in one assertThrows call?

A Yes, by using a wildcard exception

B Yes, by listing exceptions separated by commas

C Yes, by using an array of exceptions

D No, test each exception separately

assertThrows accepts only one exception class per call.

Explain how to use assertThrows to verify that a method throws an exception and how to check the exception message.

Think about how you write the lambda and what assertThrows returns.

Describe why using assertThrows is preferred over try-catch blocks in JUnit tests.

Consider readability and test clarity.

Practice

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

easy

A. To check if a specific exception is thrown by a block of code

B. To verify the return value of a method

C. To measure the execution time of a method

D. To skip a test case conditionally

Solution

1. Step 1: Understand the role of assertThrows

   assertThrows is designed to check if a certain exception occurs when running some code.

2. Step 2: Compare with other options

   Options B, C, and D describe different test functions unrelated to exception checking.

3. Final Answer:

   To check if a specific exception is thrown by a block of code -> Option A

4. Quick Check:

   assertThrows checks exceptions = A [OK]

Hint: Remember: assertThrows tests exceptions thrown by code [OK]

Common Mistakes:

• Confusing assertThrows with assertEquals
• Thinking assertThrows measures performance
• Using assertThrows to skip tests

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

easy

A. assertThrows(Exception.class, () -> methodCall());

B. assertThrows(() -> methodCall(), Exception.class);

C. assertThrows(methodCall(), Exception.class);

D. assertThrows(Exception.class, methodCall());

Solution

1. Step 1: Recall the method signature

   The correct syntax is assertThrows(Exception.class, executable) where executable is a lambda or method reference.

2. Step 2: Check each option

   assertThrows(Exception.class, () -> methodCall()); matches the correct order and lambda usage. Options A, B, and D have wrong argument order or missing lambda.

3. Final Answer:

   assertThrows(Exception.class, () -> methodCall()); -> Option A

4. Quick Check:

   Exception class first, then lambda = C [OK]

Hint: Exception class first, then lambda with code [OK]

Common Mistakes:

• Swapping the order of arguments
• Passing method call instead of lambda
• Omitting the lambda expression

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

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

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

medium

A. Test passes but exception type is wrong

B. Test fails because no exception is thrown

C. Test fails due to syntax error

D. Test passes because ArithmeticException is thrown

Solution

1. Step 1: Analyze the code inside assertThrows

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

2. Step 2: Check if exception matches expected

   assertThrows expects ArithmeticException.class, which matches the thrown exception.

3. Final Answer:

   Test passes because ArithmeticException is thrown -> Option D

4. Quick Check:

   Exception thrown matches expected = A [OK]

Hint: Divide by zero throws ArithmeticException [OK]

Common Mistakes:

• Assuming no exception is thrown
• Confusing exception types
• Thinking test fails on exception

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

@Test
void testNullPointer() {
  assertThrows(NullPointerException, () -> {
    String s = null;
    s.length();
  });
}

medium

A. Lambda syntax is incorrect

B. NullPointerException is not thrown here

C. Missing .class after NullPointerException

D. assertThrows cannot test NullPointerException

Solution

1. Step 1: Check the exception argument

   The exception class must be passed as NullPointerException.class, not just the class name.

2. Step 2: Verify lambda and exception thrown

   The lambda syntax is correct and the code throws NullPointerException as expected.

3. Final Answer:

   Missing .class after NullPointerException -> Option C

4. Quick Check:

   Exception class needs .class suffix = B [OK]

Hint: Always add .class after exception name in assertThrows [OK]

Common Mistakes:

• Omitting .class after exception
• Miswriting lambda syntax
• Assuming exception not thrown

5. You want to test that a method parseNumber(String s) throws NumberFormatException only when the input is not a valid number. Which of these tests correctly uses assertThrows to check this behavior?

hard

A. assertThrows(NumberFormatException.class, () -> parseNumber(123));

B. assertThrows(NumberFormatException.class, () -> parseNumber("abc"));

C. assertThrows(Exception.class, () -> parseNumber(null));

D. assertThrows(NumberFormatException.class, () -> parseNumber("123"));

Solution

1. Step 1: Understand the method behavior

   parseNumber should throw NumberFormatException for invalid strings like "abc" but not for valid numbers like "123".

2. Step 2: Evaluate each test case

   assertThrows(NumberFormatException.class, () -> parseNumber("abc")); tests invalid input "abc" correctly. assertThrows(NumberFormatException.class, () -> parseNumber("123")); tests valid input, so no exception expected. assertThrows(Exception.class, () -> parseNumber(null)); uses a different exception and null input. assertThrows(NumberFormatException.class, () -> parseNumber(123)); passes an integer instead of string, causing compile error.

3. Final Answer:

   assertThrows(NumberFormatException.class, () -> parseNumber("abc")); -> Option B

4. Quick Check:

   Test invalid input throws NumberFormatException = D [OK]

Hint: Test invalid strings with assertThrows for NumberFormatException [OK]

Common Mistakes:

• Testing valid input expecting exception
• Passing wrong argument types
• Using wrong exception class