refactor-moving-features

Refactor: Moving Features Between Objects

Overview

These 8 techniques redistribute responsibilities between classes. They address "which class should own this behavior?" — reducing coupling, increasing cohesion, and focusing each class on a single responsibility.

When to Use

• A method uses more data from another class than its own (Feature Envy)
• A class has grown too large with unrelated responsibilities (Large Class, Divergent Change)
• A class is too thin to justify its existence (Lazy Class)
• One change requires edits in many classes (Shotgun Surgery)
• Client code navigates through object chains (Message Chains)

Quick Reference

Technique	Problem	Solution
Move Method	Method uses another class more than its own	Move to the class it envies
Move Field	Field used more by another class	Move to the class that uses it most
Extract Class	One class doing the work of two	Split into two focused classes
Inline Class	Class does too little	Merge into another class
Hide Delegate	Client calls through chain to reach another object	Create delegating methods on the first object
Remove Middle Man	Too many delegating methods	Let client call delegate directly
Introduce Foreign Method	Need a method on a library class you can't modify	Utility method with the class as first param
Introduce Local Extension	Need several methods on unmodifiable class	Create a wrapper or subclass

Techniques in Detail

1. Move Method

The most common fix for Feature Envy.

Before:

class Account {
  overdraftCharge(daysOverdrawn: number): number {
    if (this.type.isPremium()) {
      const baseCharge = 10;
      if (daysOverdrawn <= 7) return baseCharge;
      return baseCharge + (daysOverdrawn - 7) * 0.85;
    }
    return daysOverdrawn * 1.75;
  }
}

After:

class AccountType {
  overdraftCharge(daysOverdrawn: number): number {
    if (this.isPremium()) {
      const baseCharge = 10;
      if (daysOverdrawn <= 7) return baseCharge;
      return baseCharge + (daysOverdrawn - 7) * 0.85;
    }
    return daysOverdrawn * 1.75;
  }
}

class Account {
  overdraftCharge(daysOverdrawn: number): number {
    return this.type.overdraftCharge(daysOverdrawn);
  }
}