To test null and non-null values in JUnit, use these assertions:

• assertNull(value) — passes if the value is null
• assertNotNull(value) — passes if the value is not null

In JUnit 5, these methods are available from org.junit.jupiter.api.Assertions.

Basic Example

import org.junit.jupiter.api.Test;

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

class NullCheckTest {

    @Test
    void valueShouldBeNull() {
        String name = null;

        assertNull(name);
    }

    @Test
    void valueShouldNotBeNull() {
        String name = "John";

        assertNotNull(name);
    }
}

Using Assertion Messages

You can add a message to make test failures easier to understand.

import org.junit.jupiter.api.Test;

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

class NullCheckTest {

    @Test
    void valueShouldBeNull() {
        String result = null;

        assertNull(result, "The result should be null");
    }

    @Test
    void valueShouldNotBeNull() {
        String result = "Hello JUnit";

        assertNotNull(result, "The result should not be null");
    }
}

Testing a Method That May Return Null

Suppose you have a method that returns a username:

class UserService {

    String findUsernameById(int id) {
        if (id == 1) {
            return "Alice";
        }
        return null;
    }
}

You can test both cases like this:

import org.junit.jupiter.api.Test;

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

class UserServiceTest {

    private final UserService userService = new UserService();

    @Test
    void findUsernameByIdReturnsNameWhenUserExists() {
        String username = userService.findUsernameById(1);

        assertNotNull(username);
        assertEquals("Alice", username);
    }

    @Test
    void findUsernameByIdReturnsNullWhenUserDoesNotExist() {
        String username = userService.findUsernameById(99);

        assertNull(username);
    }
}

Testing Null Arguments

If your code should reject null values, use assertThrows() to check that an exception is thrown.

import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;

class PersonTest {

    @Test
    void constructorThrowsExceptionWhenNameIsNull() {
        NullPointerException exception = assertThrows(
                NullPointerException.class,
                () -> new Person(null)
        );

        assertEquals("Name cannot be null", exception.getMessage());
    }
}

class Person {

    private final String name;

    Person(String name) {
        if (name == null) {
            throw new NullPointerException("Name cannot be null");
        }

        this.name = name;
    }
}

Summary

Use:

assertNull(value);
assertNotNull(value);

For null-related exceptions, use:

assertThrows(NullPointerException.class, () -> {
    // code that should throw NullPointerException
});

These assertions are the standard way to test null and non-null values in JUnit 5.