assertThrows usage in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this JUnit test using assertThrows?

Consider the following JUnit 5 test method. What will be the result of running this test?

JUnit

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

class Calculator {
    int divide(int a, int b) {
        return a / b;
    }
}

public class CalculatorTest {
    @Test
    void testDivideByZero() {
        Calculator calc = new Calculator();
        assertThrows(ArithmeticException.class, () -> calc.divide(10, 0));
    }
}

A Test passes because ArithmeticException is thrown as expected.

B Test fails because no exception is thrown.

C Test fails with a compilation error due to wrong assertThrows usage.

D Test passes but throws NullPointerException instead.

💡 Hint

Remember assertThrows expects the exception class and a lambda that triggers the exception.

The divide method throws ArithmeticException when dividing by zero. The assertThrows checks that this exception is thrown, so the test passes.

❓ assertion

intermediate

Which assertThrows usage correctly verifies NullPointerException?

You want to test that calling method process(null) throws NullPointerException. Which option uses assertThrows correctly?

JUnit

class Processor {
    void process(String input) {
        if (input == null) throw new NullPointerException("Input is null");
    }
}

A assertThrows(() -> processor.process(null), NullPointerException.class);

B assertThrows(NullPointerException.class, () -> processor.process(null));

C assertThrows(NullPointerException.class, processor.process(null));

D assertThrows(NullPointerException.class, () -> processor.process());

💡 Hint

assertThrows takes the exception class first, then a lambda expression.

Option B correctly passes the exception class and a lambda that calls process with null. Others have wrong argument order or missing argument.

🔧 Debug

advanced

Why does this assertThrows test fail to catch the exception?

Examine the test below. Why does it fail even though divide(10, 0) throws ArithmeticException?

JUnit

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

class Calculator {
    int divide(int a, int b) {
        return a / b;
    }
}

public class CalculatorTest {
    @Test
    void testDivideByZero() {
        Calculator calc = new Calculator();
        assertThrows(ArithmeticException.class, calc.divide(10, 0));
    }
}

A The test fails because assertThrows requires a try-catch block inside.

B The test fails because ArithmeticException is not thrown by divide method.

C The test fails because the exception class is incorrect.

D The test fails because assertThrows expects a lambda, but a method call is passed instead.

💡 Hint

Check the second argument to assertThrows carefully.

assertThrows requires a lambda or executable that throws the exception when run. Passing calc.divide(10, 0) calls the method immediately, causing the exception before assertThrows runs.

🧠 Conceptual

advanced

What does assertThrows return and how can it be used?

In JUnit 5, assertThrows returns the caught exception. How can this feature be used in tests?

A To verify exception message or properties by inspecting the returned exception object.

B To suppress the exception so the test continues without failure.

C To automatically retry the test if the exception is thrown.

D To convert checked exceptions into unchecked exceptions.

💡 Hint

Think about why you might want to keep the exception object after catching it.

assertThrows returns the exception so you can check its message or other details to ensure it is the expected error.

❓ framework

expert

Which assertThrows usage correctly tests a checked exception in a method?

Given a method that throws IOException, which assertThrows usage correctly tests it?

JUnit

import java.io.IOException;

class FileReader {
    void readFile(String path) throws IOException {
        if (path == null) throw new IOException("Path is null");
    }
}

A assertThrows(IOException.class, () -> fileReader.readFile(null));

B assertThrows(IOException.class, () -> fileReader.readFile(null)); // with throws declaration on test method

C assertThrows(IOException.class, () -> { fileReader.readFile(null); });

D assertThrows(IOException.class, () -> fileReader.readFile(null)); // without throws declaration on test method

💡 Hint

Checked exceptions must be handled or declared. Lambdas in assertThrows must handle checked exceptions properly.

Option C uses a lambda block that can throw checked exceptions. Option C and C are similar but may cause compilation errors if the test method does not declare throws IOException. Option C is invalid if test method lacks throws declaration.

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