assertNull and assertNotNull in JUnit - Build an Automation Script

Verify assertNull and assertNotNull behavior in JUnit

Preconditions (2)

Step 1: Call a method that returns null

Step 2: Use assertNull to verify the returned value is null

Step 3: Call a method that returns a non-null object

Step 4: Use assertNotNull to verify the returned value is not null

✅ Expected Result: assertNull passes when the value is null, assertNotNull passes when the value is not null

Automation Requirements - JUnit 5

Assertions Needed:

assertNull to check null values

assertNotNull to check non-null values

Best Practices:

Use descriptive test method names

Use @Test annotation for test methods

Keep tests independent and focused

Use clear assertion messages

Automated Solution

JUnit

import static org.junit.jupiter.api.Assertions.assertNull;
import static org.junit.jupiter.api.Assertions.assertNotNull;
import org.junit.jupiter.api.Test;

public class NullCheckTest {

    // Method that returns null
    public String getNullValue() {
        return null;
    }

    // Method that returns a non-null string
    public String getNonNullValue() {
        return "Hello";
    }

    @Test
    public void testAssertNull() {
        String result = getNullValue();
        assertNull(result, "Expected result to be null");
    }

    @Test
    public void testAssertNotNull() {
        String result = getNonNullValue();
        assertNotNull(result, "Expected result to be not null");
    }
}

This test class NullCheckTest has two methods: one returns null, the other returns a string.

The testAssertNull method calls the method returning null and uses assertNull to check the value is null. The assertion message helps understand the failure if it happens.

The testAssertNotNull method calls the method returning a string and uses assertNotNull to check the value is not null.

Each test is independent and clearly named to show what it verifies.

Common Mistakes - 4 Pitfalls

Using assertNull on a non-null value

Not providing assertion messages

Testing multiple unrelated things in one test method

Forgetting @Test annotation on test methods

Bonus Challenge

Now add data-driven testing with 3 different inputs: null, empty string, and a non-empty string

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