Replace Inheritance with Delegation

A subclass uses only a small part of its superclass or does not inherit its “is-a” relationship.

“A subclass uses only a small part of its superclass or does not inherit its ‘is-a’ relationship.”

Favor Composition over Inheritance.

graph LR
    A[Subclass] -.->|Inherits| B[Superclass]
    A -->|Refactoring| B
    A -->|Delegates| B

When to apply (Smells)

  • Refused Bequest: Inheriting unwanted methods.
  • Liskov Substitution Principle Violation: Subclass doesn’t behave like superclass.

Motivation

  • Clean interface: Expose only what is needed.

Mechanics (Steps)

  1. Create a field in the subclass that reference the superclass.
  2. Change inheritance to delegation (remove extends).
  3. Forward calls to the delegate.

Explanation

Problem

Stack extends Vector. Stack shouldn’t have insertAt(i).

Solution

Stack contains a Vector. Stack methods call Vector methods.

Pros and Cons

Pros Cons
  • Clarity
  • LSP compliance
    		•
  • New wrapper methods required
    		•

    Comparison

    • Inverse Refactoring: Replace Delegation with Inheritance.

    Code example

    Typescript

    Before

    After

    PHP

    Before

    After