assertThrows usage in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assert that a NullPointerException is thrown.

JUnit

assertThrows([1].class, () -> { throw new NullPointerException(); });

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

AIllegalArgumentException

BIndexOutOfBoundsException

CArithmeticException

DNullPointerException

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong exception class.

Not using the .class suffix.

The assertThrows method expects the exception class that should be thrown. Here, NullPointerException.class is correct.

2fill in blank

medium

Complete the code to assert that an ArithmeticException is thrown when dividing by zero.

JUnit

assertThrows([1].class, () -> { int result = 10 / 0; });

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

ANullPointerException

BNumberFormatException

CArithmeticException

DIllegalStateException

Attempts:

3 left

💡 Hint

Common Mistakes

Using NullPointerException instead of ArithmeticException.

Forgetting to use .class after the exception name.

Dividing by zero throws an ArithmeticException, so that is the correct exception to assert.

3fill in blank

hard

Fix the error in the assertThrows usage to correctly check for IndexOutOfBoundsException.

JUnit

assertThrows([1], () -> { List<String> list = List.of(); list.get(1); });

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AIndexOutOfBoundsException()

BIndexOutOfBoundsException.class

Cnew IndexOutOfBoundsException()

DIndexOutOfBoundsException

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the exception class name without .class.

Passing a new exception instance instead of the class.

The assertThrows method requires the exception class literal, which is IndexOutOfBoundsException.class. Passing the class name without .class or an instance causes errors.

4fill in blank

hard

Fill both blanks to assert that a NumberFormatException is thrown when parsing an invalid number.

JUnit

assertThrows([1].class, () -> { Integer.parseInt([2]); });

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ANumberFormatException

B"abc"

C"123"

DIllegalArgumentException

Attempts:

3 left

💡 Hint

Common Mistakes

Using a valid number string like "123" which does not throw.

Using the wrong exception class.

The code should assert NumberFormatException.class and parse the invalid string "abc" which causes the exception.

5fill in blank

hard

Fill all three blanks to assert that an IllegalArgumentException is thrown with a specific message.

JUnit

IllegalArgumentException exception = assertThrows([1].class, () -> { throw new IllegalArgumentException([2]); });
assertEquals([3], exception.getMessage());

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

AIllegalArgumentException

B"Invalid input"

DNullPointerException

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different exception class.

Mismatching the exception message string.

The test asserts that IllegalArgumentException.class is thrown with the message "Invalid input". The message is passed to the exception and checked with assertEquals.

Practice

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

easy

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

B. To verify the return value of a method

C. To measure the execution time of a method

D. To assert that no exceptions are thrown

Solution

1. Step 1: Understand the role of assertThrows

   assertThrows is used to test if a particular exception occurs when running a piece of code.

2. Step 2: Compare with other assertions

   Other assertions check values or states, but assertThrows specifically checks for exceptions.

3. Final Answer:

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

4. Quick Check:

   assertThrows = Exception check [OK]

Hint: assertThrows checks exceptions thrown by code [OK]

Common Mistakes:

• Confusing assertThrows with assertEquals
• Using assertThrows to check return values
• Expecting assertThrows to measure performance

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 assertThrows method signature

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

2. Step 2: Analyze options

   assertThrows(Exception.class, () -> methodCall()); matches the correct syntax: exception class first, then lambda expression.

3. Final Answer:

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

4. Quick Check:

   assertThrows(Exception.class, lambda) = correct syntax [OK]

Hint: Exception class first, then lambda in assertThrows [OK]

Common Mistakes:

• Swapping parameters order
• Passing method call instead of lambda
• Omitting lambda expression

3. What will be the result of this JUnit 5 test code?

assertThrows(IllegalArgumentException.class, () -> {
  throw new IllegalArgumentException("Invalid argument");
});

medium

A. Test passes but ignores the exception message

B. Test fails because no exception is thrown

C. Test fails due to a syntax error

D. Test passes because the expected exception is thrown

Solution

1. Step 1: Check the exception thrown

   The lambda throws IllegalArgumentException, which matches the expected exception class.

2. Step 2: Understand assertThrows behavior

   assertThrows passes if the expected exception is thrown during execution.

3. Final Answer:

   Test passes because the expected exception is thrown -> Option D

4. Quick Check:

   Thrown exception matches expected = pass [OK]

Hint: assertThrows passes if expected exception is thrown [OK]

Common Mistakes:

• Thinking test fails if exception is thrown
• Confusing exception message with test result
• Assuming syntax error without checking code

4. Identify the error in this JUnit 5 test snippet:

assertThrows(NullPointerException.class, () -> {
  String s = null;
  s.length();
});

medium

A. Exception class should be RuntimeException.class

B. Lambda syntax is incorrect

C. No error; test will pass as NullPointerException is thrown

D. s.length() does not throw NullPointerException

Solution

1. Step 1: Analyze the code inside lambda

   Calling s.length() on a null string throws NullPointerException.

2. Step 2: Check assertThrows parameters

   Expected exception is NullPointerException.class, matching the thrown exception.

3. Final Answer:

   No error; test will pass as NullPointerException is thrown -> Option C

4. Quick Check:

   NullPointerException thrown and expected = pass [OK]

Hint: NullPointerException thrown matches expected exception [OK]

Common Mistakes:

• Assuming s.length() throws different exception
• Thinking lambda syntax is wrong
• Confusing exception class with superclass

5. Consider this test method:

void testDivideByZero() {
  ArithmeticException thrown = assertThrows(ArithmeticException.class, () -> {
    int result = 10 / 0;
  });
  assertEquals("/ by zero", thrown.getMessage());
}

What does this test verify beyond just exception throwing?

hard

A. It verifies the method returns zero

B. It verifies the exception message is exactly "/ by zero"

C. It verifies no exception is thrown

D. It verifies the exception type is RuntimeException

Solution

1. Step 1: Understand assertThrows return value

   assertThrows returns the thrown exception object, here stored in thrown.

2. Step 2: Analyze the assertEquals check

   The test checks that the exception message equals "/ by zero", verifying the exact error message.

3. Final Answer:

   It verifies the exception message is exactly "/ by zero" -> Option B

4. Quick Check:

   Exception message checked = It verifies the exception message is exactly "/ by zero" [OK]

Hint: assertThrows returns exception to check message [OK]

Common Mistakes:

• Thinking it checks return value of division
• Assuming exception type is RuntimeException
• Ignoring the message assertion