@NullSource and @EmptySource in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of @NullSource in JUnit 5

What is the main purpose of using the @NullSource annotation in JUnit 5 parameterized tests?

A To provide a single null argument to the test method to check how it handles null inputs.

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

C To provide multiple null values to the test method.

D To skip the test if the input is null.

💡 Hint

Think about testing how your code behaves when it receives no object.

@NullSource adds a single null argument to the parameterized test, allowing you to verify how your code handles null inputs safely.

🧠 Conceptual

intermediate

Effect of @EmptySource in JUnit 5

Which of the following inputs does the @EmptySource annotation provide to a parameterized test method?

A Only an empty string regardless of parameter type.

B A null value only.

C Random non-empty values.

D Empty values such as empty string, empty collection, or empty array depending on the parameter type.

💡 Hint

Consider what 'empty' means for different data types.

@EmptySource provides empty values appropriate to the parameter type, such as "" for strings, empty lists for collections, or empty arrays.

❓ Predict Output

advanced

Output of Parameterized Test with @NullSource and @EmptySource

Consider this JUnit 5 parameterized test method. What will be the total number of test invocations executed?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.NullSource;
import org.junit.jupiter.params.provider.EmptySource;

class SampleTest {
    @ParameterizedTest
    @NullSource
    @EmptySource
    void testWithNullAndEmpty(String input) {
        System.out.println("Input: '" + input + "'");
    }
}

A 2 test invocations: one with null and one with empty string.

B 1 test invocation: only with null input.

C 3 test invocations: null, empty string, and one default value.

D 4 test invocations: null, empty string, empty array, and empty collection.

💡 Hint

Count how many inputs @NullSource and @EmptySource provide for a String parameter.

@NullSource provides one null input, and @EmptySource provides one empty string input for String parameters, so total 2 invocations.

❓ assertion

advanced

Assertion Behavior with @NullSource and @EmptySource

Given a parameterized test using @NullSource and @EmptySource on a String parameter, which assertion correctly verifies that the input is either null or empty?

JUnit

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.NullSource;
import org.junit.jupiter.params.provider.EmptySource;

class AssertionTest {
    @ParameterizedTest
    @NullSource
    @EmptySource
    void testInput(String input) {
        // Which assertion is correct here?
    }
}

A assertEquals("", input);

B assertTrue(input == null || input.isEmpty());

C assertNotNull(input);

D assertFalse(input.isEmpty());

💡 Hint

Remember that input can be null or empty string.

Since input can be null or empty string, the assertion must allow both conditions.

🔧 Debug

expert

Debugging a Failing Parameterized Test with @NullSource and @EmptySource

A parameterized test uses @NullSource and @EmptySource on a method with a non-nullable primitive int parameter. What error will occur when running the test?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.NullSource;
import org.junit.jupiter.params.provider.EmptySource;

class DebugTest {
    @ParameterizedTest
    @NullSource
    @EmptySource
    void testPrimitive(int number) {
        System.out.println(number);
    }
}

A Test runs successfully with 0 as default value for null input.

B Test fails with NumberFormatException due to empty input.

C Test fails with a ParameterResolutionException because null cannot be assigned to primitive int.

D Test runs only once with empty input converted to 0.

💡 Hint

Consider how Java primitives handle null values.

Primitive types like int cannot accept null values, so @NullSource causes a ParameterResolutionException.