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INHERITANCE vs POLYMORPHISM

Distinguishing Inheritance and Polymorphism in UML Class Diagrams / OOP

1. CORE DIFFERENCE

ASPECT	INHERITANCE	POLYMORPHISM
Basic Meaning	A class inherits characteristics from another class.	The same operation/interface can behave differently depending on the object.
Main Question	"What does this class inherit from?"	"How can the same operation have different behaviors?"
Relationship	IS-A relationship.	ONE INTERFACE → MANY BEHAVIORS
UML Representation	Generalization arrow with a hollow triangle.	Usually represented through overridden operations in subclasses.
Primary Mechanism	Parent/Superclass → Child/Subclass.	Method overriding / dynamic dispatch.
Main Purpose	Reuse and specialization.	Flexibility and interchangeable objects.
Example	Car inherits from Vehicle.	Car.move() and Motorcycle.move() behave differently.

2. INHERITANCE

Inheritance is a structural relationship between classes. A subclass obtains attributes and/or operations from a superclass, and can add or specialize them.

┌─────────────────┐ │ Vehicle │ ├─────────────────┤ │ - brand │ │ - speed │ ├─────────────────┤ │ + move() │ └────────┬────────┘ △ │ ┌───────────┴───────────┐ │ │ ┌─────────────────┐ ┌─────────────────┐ │ Car │ │ Motorcycle │ ├─────────────────┤ ├─────────────────┤ │ - numberOfDoors │ │ - engineCC │ └─────────────────┘ └─────────────────┘

UML Generalization:
Car ─────────▷ Vehicle
Motorcycle ───▷ Vehicle

Conceptually: Car IS-A Vehicle.
Motorcycle IS-A Vehicle.

Therefore, inheritance answers:
"What class is this class derived from?"

3. POLYMORPHISM

Polymorphism means that the same operation/interface can produce different behavior depending on the actual object receiving the operation.

Vehicle │ ├── Car │ └── move() → moves on four wheels │ └── Motorcycle └── move() → moves on two wheels

The operation name remains the same:

move()

But its behavior changes according to the object's actual type.

Vehicle v v = Car v.move() → Car's implementation v = Motorcycle v.move() → Motorcycle's implementation

4. THE RELATIONSHIP BETWEEN THEM

INHERITANCE ↓ STRUCTURE / RELATIONSHIP ↓ "Dog is an Animal" ↓ common interface ↓ POLYMORPHISM ↓ BEHAVIOR / SUBSTITUTION ↓ "Animal reference can represent Dog or Cat" ↓ same operation → different implementation

Key Idea:

Inheritance creates the class relationship.
Polymorphism exploits a common interface so different classes can behave differently.

5. SIMPLE REAL-WORLD ANALOGY

ANIMAL │ ┌────────┴────────┐ ▼ ▼ DOG CAT

Inheritance

Dog IS-A Animal
Cat IS-A Animal

Polymorphism

makeSound() Dog.makeSound() → "Woof!" Cat.makeSound() → "Meow!"

The same message makeSound() produces different behavior.

6. UML CLASS-DIAGRAM VIEW

┌─────────────────────┐ │ Animal │ ├─────────────────────┤ │ + name │ ├─────────────────────┤ │ + makeSound() │ └──────────┬──────────┘ △ ┌──────────┴──────────┐ │ │ ┌─────────────────┐ ┌─────────────────┐ │ Dog │ │ Cat │ ├─────────────────┤ ├─────────────────┤ │ + makeSound() │ │ + makeSound() │ └─────────────────┘ └─────────────────┘ │ │ ▼ ▼ "Woof!" "Meow!"

Inheritance:
Dog ─────▷ Animal
Cat ─────▷ Animal

Polymorphism:
makeSound() → Dog behavior / Cat behavior

7. THE MOST IMPORTANT DISTINCTION

CONCEPT	FOCUS	QUESTION
Inheritance	Structure / Class Relationship	"What does this class inherit?"
Polymorphism	Behavior / Substitution	"What behavior occurs when this operation is invoked?"

FINAL SUMMARY

INHERITANCE = CLASS RELATIONSHIP

A subclass derives from a superclass and inherits/reuses its characteristics.

---

POLYMORPHISM = BEHAVIORAL VARIABILITY THROUGH A COMMON INTERFACE

Different objects can respond differently to the same operation/interface.

INHERITANCE → WHAT A CLASS IS / RECEIVES
POLYMORPHISM → HOW AN OBJECT CAN BEHAVE