Every object-oriented programming course teaches inheritance early. You learn to create class hierarchies, override methods, and extend functionality through parent-child relationships. It feels elegant—until your codebase grows and that elegant hierarchy becomes a tangled mess of dependencies you cannot safely modify.

The inheritance model promises code reuse and polymorphism, but it extracts a heavy toll. Changes to base classes ripple unpredictably through descendants. New requirements force awkward workarounds because your hierarchy assumed a structure the real world doesn't follow. Teams spend hours tracing through five levels of parent classes to understand what a single method actually does.

Composition offers an alternative that experienced developers increasingly prefer. Rather than building elaborate family trees of classes, you assemble objects from smaller, focused components. The result is code that adapts to change, communicates its intent clearly, and doesn't punish you for decisions made years ago.

The fragile base class problem represents inheritance's most insidious trap. When you modify a base class, you cannot predict how those changes affect all descendants. A seemingly innocent optimization in a parent method might break subclasses that depended on specific implementation details. The deeper your hierarchy, the more opportunities for these invisible breakages.

Deep hierarchies create cognitive overhead that compounds over time. To understand what a class does, you must mentally load its parents, grandparents, and beyond. Each level adds methods, state, and behavior that interact in subtle ways. When a bug appears, you trace through multiple files trying to locate which ancestor introduced the problematic behavior.

Inheritance also forces you to predict the future—badly. You design a hierarchy based on current requirements, but requirements evolve. When a new feature doesn't fit your carefully planned structure, you face unpleasant choices: force it into an ill-fitting class, create parallel hierarchies, or add special-case logic that violates your original design assumptions.

The diamond problem and multiple inheritance complications reveal a deeper issue. Inheritance couples classes by their identity rather than their behavior. A class inherits everything from its parent whether it needs it or not. This tight coupling means classes accumulate baggage, violate the Interface Segregation Principle, and resist the kind of selective composition that real-world software demands.

Takeaway

Before creating a subclass, ask whether you need the parent's identity or just some of its behavior. If it's behavior, inheritance is likely the wrong tool.

Composition replaces 'is-a' relationships with 'has-a' relationships. Instead of a SportsCar inheriting from Car inheriting from Vehicle, you create a Car that contains an Engine, a Transmission, and Wheels. Each component handles its responsibility independently. Swapping an electric engine for a gasoline engine requires no changes to the car's structure.

Delegation makes composition practical. When your object receives a request it doesn't handle directly, it forwards that request to an appropriate component. The calling code doesn't know or care whether the object does the work itself or delegates. This preserves encapsulation while allowing you to assemble complex behaviors from simple parts.

Runtime flexibility distinguishes composition from inheritance fundamentally. Inheritance relationships are fixed at compile time—a subclass cannot change its parent. Composed objects can swap components dynamically. A payment processor might use different validation strategies for different regions, switching implementations based on runtime conditions without conditional logic or type checking.

The Strategy, Decorator, and Composite patterns demonstrate composition's power in practice. Strategy encapsulates algorithms so clients can choose implementations. Decorator wraps objects to add responsibilities without subclassing. Composite treats individual objects and collections uniformly. Each pattern achieves flexibility that inheritance cannot match without contorting your class hierarchy beyond recognition.

Takeaway

Design objects around what they can do, not what they are. Compose behaviors from small, focused components that you can combine, swap, and test independently.

Migrating inheritance-heavy code requires patience and incremental progress. Start by identifying which inherited behavior subclasses actually use. Often, subclasses need only a fraction of their parent's functionality. Extract those specific behaviors into separate objects that can be composed rather than inherited.

The Extract Class refactoring moves groups of related methods and fields into new objects. When a subclass overrides methods primarily to modify inherited state, that state often belongs in its own object. Move it there, pass the new object to classes that need it, and watch your inheritance chain flatten naturally.

Introduce interfaces to decouple consumers from implementations. Code that depends on a specific class hierarchy cannot easily accept composed alternatives. When consumers depend on interfaces instead, you can provide either inherited or composed implementations interchangeably. This creates migration flexibility without requiring synchronized changes across your codebase.

Preserve behavior during migration by running existing tests continuously and adding characterization tests where coverage gaps exist. Replace inheritance relationships one at a time, verifying each change maintains expected behavior. The goal isn't eliminating all inheritance—it still serves legitimate purposes for true 'is-a' relationships—but reducing it to cases where the coupling cost is justified.

Takeaway

Refactor inheritance to composition gradually. Extract interfaces first, then migrate implementations one subclass at a time while maintaining comprehensive test coverage.

Inheritance remains a valid tool when modeling genuine type relationships, but decades of object-oriented experience have shown its costs. Most inheritance in real codebases exists for code reuse, not type relationships—and composition handles code reuse with far less coupling and greater flexibility.

The shift toward composition reflects a broader lesson in software design: favor explicit relationships over implicit ones. When you compose objects, dependencies are visible, testable, and changeable. When you inherit, dependencies hide in parent classes and constrain your evolution.

Build your objects from parts you can see, swap, and reason about independently. Your future self—and every developer who inherits your codebase—will thank you for the flexibility.