Null object

Defensive programming often clutters domain code with endless null checks: if (user != null && user.subscription != null && user.subscription.plan != null). A single missing check in production leads to fatal Cannot read properties of null or NullPointerException crashes.

The problem

1 // Defensive null checks littering every method:
2 class OrderService {
3   #logger;
4   constructor(logger = null) {
5     this.#logger = logger;
6   }
7   processOrder(order) {
8     // Boilerplate everywhere:
9     if (this.#logger) {
10       this.#logger.log(`Processing order: ${order.id}`);
11     }
12     // business logic...
13     if (this.#logger) {
14       this.#logger.log(`Order complete`);
15     }
16   }
17 }

The pattern

Null Object implements the target interface with safe, neutral "do-nothing" behavior. Instead of returning null or leaving references uninitialized, the system supplies a default NullObject (e.g. NullLogger, NullDiscount, GuestUser). Callers invoke methods polymorphically without checking for null.

The intent: Provide an object as a surrogate for the lack of an object of a given type. The Null Object provides intelligent do-nothing behavior, eliminating defensive null checks.

In code

When to use it, and when not to

Use Null Object when:

• An object requires a collaborator, but sometimes that collaborator does nothing (e.g. logging, telemetry, metrics, discounts).
• You want clients to treat real collaborators and absent collaborators uniformly without defensive if (x == null) branching.

Don't use Null Object when:

• null represents a critical error or an uninitialized state that should fail loudly and immediately.
• The absence of data requires branching logic in the user interface (e.g. showing a Login prompt when User is null).

Where you have already seen it

In Java 8+, Optional.empty() is a functional variant of the Null Object pattern. In Linux and Unix filesystems, /dev/null is an operating system-level Null Object that safely absorbs all written bytes. In game loops, a NullAudioDevice is used when running games in headless automated CI pipelines.

Related patterns

• Null Object vs State: A Null Object can be viewed as a special state in the State pattern that performs no actions.
• Null Object vs Strategy: Null Object often serves as the default, no-op strategy in a Strategy pattern setup.