@NullSource and @EmptySource in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the @NullSource annotation do in JUnit?

It provides a single null argument to a parameterized test method, allowing you to test how your code handles null inputs.

beginner

What is the purpose of the @EmptySource annotation in JUnit?

It supplies an empty value (like an empty string, empty collection, or empty array) as an argument to a parameterized test method to test how code behaves with empty inputs.

intermediate

Can @NullSource and @EmptySource be used together in a single test method?

Yes, you can combine @NullSource and @EmptySource to run a parameterized test with both null and empty values as inputs, helping to cover edge cases.

beginner

Example usage: How do you write a JUnit test method that tests a String parameter with null and empty values?

Use @ParameterizedTest with @NullSource and @EmptySource annotations above the test method. For example:

@ParameterizedTest
@NullSource
@EmptySource
void testWithNullAndEmpty(String input) {
  // test logic here
}

beginner

Why is it important to test with @NullSource and @EmptySource?

Because null and empty inputs are common edge cases that can cause bugs if not handled properly. Testing these helps ensure your code is robust and error-free.

What input does @NullSource provide to a parameterized test?

A A zero integer

B An empty string

C A null value

D A random value

@NullSource provides a null value as input to test how the method handles null.

Which annotation supplies an empty collection or empty string to a test method?

A @NullSource

B @ValueSource

C @CsvSource

D @EmptySource

@EmptySource provides empty values like empty strings or collections to test empty input handling.

Can @NullSource be used without @ParameterizedTest?

A No, it must be used with @ParameterizedTest

B Only with @RepeatedTest

C Only with @Test

D Yes, always

@NullSource works only with @ParameterizedTest to supply null arguments.

What happens if you combine @NullSource and @EmptySource on a test method?

A The test runs only once with null

B The test runs twice: once with null, once with empty value

C The test runs only once with empty value

D The test fails to run

Combining both runs the test twice, once for each input, covering both edge cases.

Which of these is NOT a typical use of @EmptySource?

A Testing null values

B Testing empty strings

C Testing empty lists

D Testing empty arrays

@EmptySource does not provide null values; @NullSource is used for null.

Explain how @NullSource and @EmptySource help improve test coverage in JUnit parameterized tests.

Think about why testing null and empty inputs is important.

Describe how to write a JUnit test method that uses both @NullSource and @EmptySource annotations.

Remember the syntax and purpose of these annotations.

Practice

1. What is the main purpose of the @NullSource annotation in JUnit parameterized tests?

easy

A. To provide a null value as input to the test method parameter

B. To provide an empty string as input to the test method parameter

C. To provide multiple non-null values to the test method

D. To skip the test when the input is null

Solution

1. Step 1: Understand @NullSource role

   @NullSource is used to supply a null value as input to a parameterized test method.

2. Step 2: Differentiate from other sources

   @EmptySource provides empty values like empty strings or collections, not null. Other options do not match @NullSource behavior.

3. Final Answer:

   To provide a null value as input to the test method parameter -> Option A

4. Quick Check:

   @NullSource = null input [OK]

Hint: NullSource always means null input for tests [OK]

Common Mistakes:

• Confusing @NullSource with @EmptySource
• Thinking @NullSource provides empty strings
• Assuming @NullSource skips tests

2. Which of the following is the correct way to use both @NullSource and @EmptySource together in a JUnit 5 parameterized test?

easy

A. @ParameterizedTest
@NullSource
void testMethod(@EmptySource String input) { ... }

B. @ParameterizedTest
@EmptySource
@NullSource
void testMethod() { ... }

C. @ParameterizedTest
@NullSource
@EmptySource
void testMethod(String input) { ... }

D. @ParameterizedTest
@NullSource
@EmptySource
void testMethod() { ... }

Solution

1. Step 1: Check annotation placement

   Both @NullSource and @EmptySource must be placed above the test method along with @ParameterizedTest.

2. Step 2: Verify method signature

   The test method must accept a parameter matching the input type (e.g., String input) to receive values from sources.

3. Final Answer:

   @ParameterizedTest @NullSource @EmptySource void testMethod(String input) { ... } -> Option C

4. Quick Check:

   Annotations above method with parameter = correct [OK]

Hint: Put both sources above method with matching parameter [OK]

Common Mistakes:

• Placing @EmptySource inside method parameters
• Missing parameter in test method
• Wrong order or missing @ParameterizedTest

3. Given the following test method, what inputs will it receive during execution?

@ParameterizedTest
@NullSource
@EmptySource
void testInput(String input) {
    System.out.println(input == null ? "null" : "'" + input + "'");
}

medium

A. Only an empty string printed as: ''

B. null and an empty string printed as: null and ''

C. Only null printed as: null

D. null, empty string, and a string with spaces

Solution

1. Step 1: Identify inputs from annotations

   @NullSource provides a null input, and @EmptySource provides an empty string input.

2. Step 2: Understand output behavior

   The method prints "null" if input is null, otherwise prints the input in quotes. So outputs will be "null" and "''" for empty string.

3. Final Answer:

   null and an empty string printed as: null and '' -> Option B

4. Quick Check:

   NullSource + EmptySource = null + "" [OK]

Hint: NullSource gives null, EmptySource gives empty string [OK]

Common Mistakes:

• Assuming EmptySource provides null
• Expecting extra inputs like spaces
• Ignoring null input from NullSource

4. Consider this test code snippet:

@ParameterizedTest
@NullSource
@EmptySource
void test(String input) {
    assertNotNull(input);
}

What will happen when this test runs?

medium

A. Test fails with AssertionError due to null input

B. Test passes for all inputs

C. Compilation error because of missing imports

D. Test is skipped because of null input

Solution

1. Step 1: Analyze inputs from sources

   @NullSource provides a null input, and @EmptySource provides an empty string input.

2. Step 2: Check assertion behavior

   assertNotNull(input) will fail when input is null, causing an AssertionError.

3. Final Answer:

   Test fails with AssertionError due to null input -> Option A

4. Quick Check:

   assertNotNull fails on null input [OK]

Hint: assertNotNull fails if input is null from NullSource [OK]

Common Mistakes:

• Thinking test passes because empty string is valid
• Assuming test skips null input
• Confusing compilation errors with runtime failures

5. You want to test a method that should accept null, empty, and non-empty strings. Which combination of annotations correctly tests all these cases in one parameterized test?

hard

A. @ParameterizedTest
@NullSource
@ValueSource(strings = {"test"})
void testMethod(String input) { ... }

B. @ParameterizedTest
@EmptySource
@ValueSource(strings = {"test"})
void testMethod(String input) { ... }

C. @ParameterizedTest
@NullSource
@EmptySource
void testMethod() { ... }

D. @ParameterizedTest
@NullSource
@EmptySource
@ValueSource(strings = {"test"})
void testMethod(String input) { ... }

Solution

1. Step 1: Identify needed inputs

   You need null (from @NullSource), empty string (from @EmptySource), and a non-empty string (from @ValueSource).

2. Step 2: Verify method signature and annotations

   Method must accept a parameter to receive inputs. @ParameterizedTest @NullSource @EmptySource @ValueSource(strings = {"test"}) void testMethod(String input) { ... } includes all three annotations and a parameter.

3. Final Answer:

   @ParameterizedTest @NullSource @EmptySource @ValueSource(strings = {"test"}) void testMethod(String input) { ... } -> Option D

4. Quick Check:

   Combine NullSource, EmptySource, and ValueSource for all cases [OK]

Hint: Combine NullSource, EmptySource, and ValueSource for full coverage [OK]

Common Mistakes:

• Omitting NullSource or EmptySource
• Missing method parameter
• Using annotations without @ParameterizedTest