Abstract Classes vs Interfaces

Both abstract classes and interfaces are core abstraction mechanisms in Java, but they serve distinct architectural roles.

Abstract Classes: What Something IS

An abstract class is an incomplete blueprint that cannot be instantiated directly with new.

Can maintain instance variables (state).
Can contain both abstract methods (without bodies) and concrete methods (with implementations).
A concrete subclass must implement all inherited abstract methods.

public abstract class Shape {
    protected String color;

    public Shape(String color) {
        this.color = color;
    }

    public abstract double calculateArea();
}

Interfaces: What Something CAN DO

An interface establishes a behavioral contract or specification that any implementing class promises to satisfy.

A class can implement multiple interfaces (implements Drawable, Serializable).
Represents capabilities or roles rather than ontological taxonomy.

public interface Drawable {
    void draw();
}

Summary Comparison Table

Dimension	Abstract Class	Interface
Multiple Inheritance	No (`extends` only one)	Yes (`implements` many)
Instance State	Allowed (`private int x;`)	No instance state
Relationship	IS-A (`Dog IS-A Animal`)	CAN-DO (`Bird CAN Fly`)

Exercise 1

Why can an interface represent “CAN-DO” behavior across completely unrelated hierarchies?

Show solution ↓

Solution

Because interfaces define pure capabilities (Comparable, AutoCloseable, Flyable) without assuming common ancestry, allowing disparate classes (Plane, Duck) to share the same polymorphic contract.