assertDoesNotThrow in JUnit - Build an Automation Script

Verify that a method does not throw any exception

Preconditions (2)

Step 1: Call the 'calculate' method with inputs 10 and 2

Step 2: Verify that no exception is thrown during the method call

✅ Expected Result: The 'calculate' method completes without throwing any exception

Automation Requirements - JUnit 5

Assertions Needed:

Use assertDoesNotThrow to verify no exception is thrown when calling calculate(10, 2)

Best Practices:

Use lambda expression inside assertDoesNotThrow for the method call

Keep test method names descriptive and clear

Avoid catching exceptions manually inside the test

Automated Solution

JUnit

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

public class CalculatorTest {

    // Method to test
    public int calculate(int a, int b) {
        return a / b;
    }

    @Test
    public void testCalculateDoesNotThrow() {
        assertDoesNotThrow(() -> {
            int result = calculate(10, 2);
        });
    }
}

This test class defines a method calculate that divides two integers.

The test method testCalculateDoesNotThrow uses assertDoesNotThrow from JUnit 5 to check that calling calculate(10, 2) does not throw any exception.

The method call is wrapped inside a lambda expression passed to assertDoesNotThrow. This is the recommended way to verify that no exception occurs during execution.

We do not catch exceptions manually because assertDoesNotThrow handles that and fails the test if an exception is thrown.

Common Mistakes - 3 Pitfalls

Catching exceptions manually inside the test instead of using assertDoesNotThrow

Passing the method call result directly to assertDoesNotThrow instead of a lambda

Using assertDoesNotThrow for methods that should throw exceptions

Bonus Challenge

Now add data-driven testing with 3 different input pairs to verify calculate does not throw exceptions

Show Hint

Practice

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

easy

A. To assert that a method returns a specific value

B. To check that a block of code runs without throwing any exceptions

C. To verify that a specific exception is thrown by the code

D. To measure the execution time of a code block

Solution

1. Step 1: Understand the purpose of assertDoesNotThrow

   This assertion is designed to confirm that no exceptions occur when running a code block.

2. Step 2: Compare with other assertions

   Unlike assertions that check for exceptions or return values, assertDoesNotThrow ensures smooth execution without errors.

3. Final Answer:

   To check that a block of code runs without throwing any exceptions -> Option B

4. Quick Check:

   assertDoesNotThrow means no exceptions thrown [OK]

Hint: Remember: assertDoesNotThrow means code runs without errors [OK]

Common Mistakes:

• Confusing assertDoesNotThrow with assertThrows
• Thinking it checks return values
• Assuming it measures performance

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

easy

A. assertDoesNotThrow(methodToTest());

B. assertDoesNotThrow { methodToTest() };

C. assertDoesNotThrow(() -> methodToTest());

D. assertDoesNotThrow(methodToTest);

Solution

1. Step 1: Recall the lambda syntax for assertDoesNotThrow

   assertDoesNotThrow expects a lambda expression or executable block, written as () -> methodToTest().

2. Step 2: Identify incorrect options

   Options A, C, and D use incorrect syntax: A calls the method directly, C uses braces like other languages, D passes method reference without lambda.

3. Final Answer:

   assertDoesNotThrow(() -> methodToTest()); -> Option C

4. Quick Check:

   Use lambda () -> method() with assertDoesNotThrow [OK]

Hint: Use lambda syntax () -> method() inside assertDoesNotThrow [OK]

Common Mistakes:

• Calling the method directly inside assertDoesNotThrow
• Using curly braces instead of lambda arrows
• Passing method name without parentheses or lambda

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

assertDoesNotThrow(() -> { int x = 5 / 1; });

medium

A. Test fails due to syntax error

B. Test fails due to ArithmeticException

C. Test fails due to NullPointerException

D. Test passes because no exception is thrown

Solution

1. Step 1: Analyze the code inside assertDoesNotThrow

   The code divides 5 by 1, which is valid and does not throw any exception.

2. Step 2: Understand assertDoesNotThrow behavior

   Since no exception occurs, the assertion passes and the test succeeds.

3. Final Answer:

   Test passes because no exception is thrown -> Option D

4. Quick Check:

   Division by 1 is safe, so no exception [OK]

Hint: Check if code inside lambda throws exception or not [OK]

Common Mistakes:

• Assuming division always throws exception
• Confusing ArithmeticException with other exceptions
• Thinking assertDoesNotThrow checks return values

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

assertDoesNotThrow(() -> { int y = 10 / 0; });

medium

A. ArithmeticException will cause test to fail

B. No error; test will pass

C. Syntax error due to missing semicolon

D. NullPointerException will cause test to fail

Solution

1. Step 1: Examine the code inside assertDoesNotThrow

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

2. Step 2: Understand assertDoesNotThrow behavior on exceptions

   assertDoesNotThrow expects no exceptions; if one occurs, the test fails.

3. Final Answer:

   ArithmeticException will cause test to fail -> Option A

4. Quick Check:

   Division by zero throws ArithmeticException [OK]

Hint: Division by zero throws exception, so test fails [OK]

Common Mistakes:

• Thinking test passes despite exception
• Confusing exception types
• Ignoring runtime exceptions inside lambda

5. You want to test a method processData() that may throw a checked exception. How do you use assertDoesNotThrow to verify it runs without exceptions?

hard

A. assertDoesNotThrow(() -> processData());

B. assertDoesNotThrow(processData());

C. assertDoesNotThrow(() -> { try { processData(); } catch(Exception e) {} });

D. assertDoesNotThrow(() -> { processData() });

Solution

1. Step 1: Understand handling checked exceptions in lambdas

   assertDoesNotThrow accepts an Executable, whose execute() method throws Throwable, so checked exceptions are permitted in the lambda.

2. Step 2: Identify the correct lambda syntax

   The standard syntax is a concise lambda: () -> processData().

3. Step 3: Evaluate options

   A: correct single-expression lambda. B: direct method call without lambda. C: catches exceptions inside lambda, hiding potential failures. D: syntax error due to missing semicolon after processData() in the block lambda.

4. Final Answer:

   assertDoesNotThrow(() -> processData()); -> Option A

5. Quick Check:

   Lambda () -> processData() handles checked exceptions correctly [OK]

Hint: Use lambda syntax () -> processData() inside assertDoesNotThrow [OK]

Common Mistakes:

• Calling method directly without lambda
• Syntax errors like missing semicolon in block lambdas
• Catching exceptions inside lambda hiding test failures