@NullSource and @EmptySource in JUnit - Build an Automation Script

Test method behavior with null and empty inputs using @NullSource and @EmptySource

Preconditions (2)

Step 1: Create a parameterized test method using @NullSource and @EmptySource annotations

Step 2: Run the test method with null input from @NullSource

Step 3: Run the test method with empty string input from @EmptySource

Step 4: Verify that the method under test returns false for both null and empty string inputs

✅ Expected Result: The test method runs twice: once with null and once with empty string, and both times the assertion verifies the method returns false

Automation Requirements - JUnit 5

Assertions Needed:

Assert that the method returns false when input is null

Assert that the method returns false when input is empty string

Best Practices:

Use @ParameterizedTest with @NullSource and @EmptySource for concise input coverage

Use descriptive test method names

Keep assertions clear and focused

Automated Solution

JUnit

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

public class StringValidatorTest {

    // Method under test: returns true if input is not null and not empty
    public boolean isValid(String input) {
        return input != null && !input.isEmpty();
    }

    @ParameterizedTest
    @NullSource
    @EmptySource
    void testIsValidWithNullAndEmpty(String input) {
        assertFalse(isValid(input), "Expected isValid to return false for input: " + input);
    }
}

This test class defines a method isValid that returns true only if the input string is not null and not empty.

The test method testIsValidWithNullAndEmpty is annotated with @ParameterizedTest, @NullSource, and @EmptySource. This means the test will run twice: once with null and once with an empty string ("").

Inside the test, assertFalse checks that isValid returns false for both inputs, which matches the expected behavior.

This approach is clean and concise, covering two important edge cases without writing separate tests.

Common Mistakes - 3 Pitfalls

Not using @ParameterizedTest with @NullSource and @EmptySource together

Using @ValueSource with empty string instead of @EmptySource

Not asserting the expected result for null or empty inputs

Bonus Challenge

Now add data-driven testing with 3 different non-empty strings that should return true

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