Null Object Pattern

Also Known As

TL;DR

Explanation

The Null Object pattern (Bobby Woolf) provides a do-nothing implementation of an interface that can be used wherever a null check would otherwise be required. For example, a NullLogger that implements LoggerInterface but discards all messages replaces if ($logger !== null) $logger->log(...) checks everywhere. The Null Object makes the absence of something explicit and type-safe, simplifies calling code, and follows Tell Don't Ask. PHP 8+ Null Object are often combined with union types and nullsafe operators as complementary approaches.

Common Misconception

Why It Matters

Common Mistakes

• Not implementing the full interface — callers that use methods not on the null object still get null errors.
• Null objects that return null from methods — they should return safe defaults (empty string, 0, empty array).
• Using null objects where an Optional/Maybe type better communicates the possible absence.
• Applying null objects everywhere — sometimes null carries meaning that should be handled, not silently swallowed.

Code Examples

// Null checks spread throughout call sites
$discount = $user->getDiscount();
$price = $discount !== null ? $price * (1 - $discount->rate()) : $price;

interface Discount {
    public function apply(float $price): float;
}

class PercentDiscount implements Discount {
    public function __construct(private float $rate) {}
    public function apply(float $price): float { return $price * (1 - $this->rate); }
}

class NullDiscount implements Discount {
    public function apply(float $price): float { return $price; } // no-op
}

// Now: no null check needed at call sites
$price = $user->getDiscount()->apply($price);

References

• ↗ https://refactoring.guru/introduce-null-object

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