assertNull and assertNotNull in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does assertNull check in a JUnit test?

assertNull checks that the object being tested is null. If the object is not null, the test fails.

beginner

What is the purpose of assertNotNull in JUnit?

assertNotNull verifies that the object is not null. If the object is null, the test fails.

beginner

Write a simple example of assertNull usage in JUnit.

Example:<br>

String result = null;
assertNull(result);

This test passes because result is null.

beginner

Write a simple example of assertNotNull usage in JUnit.

Example:<br>

String result = "Hello";
assertNotNull(result);

This test passes because result is not null.

intermediate

Why is it important to use assertNull and assertNotNull in tests?

They help confirm that objects are in the expected state (null or not null), which prevents errors like NullPointerException in real use.

What happens if assertNull(object) is used but object is not null?

A The test fails

B The test passes

C The test is skipped

D The test throws an exception

assertNull expects the object to be null. If it is not, the test fails.

Which assertion would you use to check that a method returns a valid object and not null?

A assertTrue

B assertEquals

C assertNull

D assertNotNull

assertNotNull confirms the object is not null, meaning the method returned a valid object.

In JUnit, what is the return type of assertNull and assertNotNull methods?

A boolean

B Object

C void

D int

Both assertNull and assertNotNull are void methods that throw exceptions if the assertion fails.

Which of these is a correct way to use assertNotNull?

A assertNotNull("text");

B assertNotNull(null);

C assertNotNull();

D assertNotNull(5);

You must pass a non-null object to assertNotNull. "text" is a non-null String.

What message can you add to assertNull to help understand test failures?

A assertNull(object, "Object should be null");

B assertNull("Object should be null", object);

C assertNull(object); // Object should be null

D assertNull(object, null);

In JUnit, the message comes first: assertNull(String message, Object object).

Explain in your own words when and why you would use assertNull and assertNotNull in a test.

Think about checking if something exists or not in your code.

Describe a simple test scenario where assertNull would be useful.

Imagine a method that returns null if no user is found.