assertThrows usage in JUnit

Introduction

We use assertThrows to check if a piece of code correctly throws an error when something goes wrong. This helps us catch mistakes early.

When you want to test that a method throws an exception for invalid input.

When you want to make sure your code handles errors properly.

When you want to confirm that a specific error type is thrown in a certain situation.

Syntax

JUnit

assertThrows(ExpectedException.class, () -> {
    // code that should throw the exception
});

The first argument is the type of exception you expect.

The second argument is a lambda with the code that should throw the exception.

Examples

This checks that parsing a non-number string throws an IllegalArgumentException.

JUnit

assertThrows(IllegalArgumentException.class, () -> {
    Integer.parseInt("abc");
});

This checks that calling a method on null throws NullPointerException.

JUnit

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

Sample Program

This test checks that dividing by zero throws an ArithmeticException.

JUnit

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

public class ExceptionTest {

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

Important Notes

Always specify the exact exception class you expect to catch.

Use lambda expressions to keep the test code clean and focused.

Summary

assertThrows helps verify that errors happen when they should.

It improves test reliability by checking for specific exceptions.

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