Extract Class Refactoring

TL;DR

Explanation

A class doing too much (God Class) should be split. Extract Class: identify a cohesive group of fields and methods (e.g., address-related fields in a User class), create a new class (Address), move the fields and methods, update the original class to hold a reference. Signs you need it: class has many unrelated fields, methods only use a subset of fields, natural groupings exist (User + Address, Order + PaymentDetails). In PHP: create the new class, inject it via constructor, update all references. Opposite of Inline Class.

Common Misconception

Why It Matters

Common Mistakes

• Creating classes with only getters/setters (anaemic) — extract behaviour too, not just data.
• Not updating all references after extraction — leaving the old fields in place as duplicates.
• Extracting too aggressively — small classes with one field and one method add unnecessary indirection.

Code Examples

class User {
    public string $name;
    public string $street;
    public string $city;
    public string $postcode;
    public string $country;

    public function formatAddress(): string {
        return "$this->street, $this->city";
    }
}

class Address {
    public function __construct(
        public readonly string $street,
        public readonly string $city,
        public readonly string $postcode,
        public readonly string $country,
    ) {}

    public function format(): string {
        return "$this->street, $this->city";
    }
}

class User {
    public function __construct(
        public readonly string $name,
        public readonly Address $address,
    ) {}
}

References

• ↗ https://refactoring.guru/extract-class

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