assertThrows for exceptions in JUnit - Build an Automation Script

Verify that dividing by zero throws ArithmeticException

Preconditions (2)

Step 1: Call divide(10, 0)

Step 2: Check that ArithmeticException is thrown

✅ Expected Result: The test passes only if ArithmeticException is thrown when dividing by zero

Automation Requirements - JUnit 5

Assertions Needed:

Verify that ArithmeticException is thrown when divide(10, 0) is called

Best Practices:

Use assertThrows method from JUnit 5

Use lambda expression to call the method that should throw

Keep test method names descriptive

Avoid catching exceptions manually in the test

Automated Solution

JUnit

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

public class CalculatorTest {

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

    @Test
    void testDivideByZeroThrows() {
        assertThrows(ArithmeticException.class, () -> divide(10, 0));
    }
}

The test class CalculatorTest contains a simple divide method that divides two integers.

The test method testDivideByZeroThrows uses assertThrows from JUnit 5 to check that calling divide(10, 0) throws an ArithmeticException.

The lambda () -> divide(10, 0) is passed to assertThrows so JUnit can run it and catch the exception.

This approach is clean and avoids try-catch blocks in the test, making the test easy to read and maintain.

Common Mistakes - 3 Pitfalls

Using try-catch block manually instead of assertThrows

Passing method call directly instead of lambda to assertThrows

Asserting wrong exception type

Bonus Challenge

Now add data-driven testing with 3 different inputs that throw ArithmeticException

Show Hint

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