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Module 3: Interfaces & Abstract Classes (The Contract)

📚 Module 3: Interfaces & Abstract Classes

Focus: Moving from “What it is” to “What it can do.”

In professional Java development, we don’t just build objects; we build Contracts. These ensure that our system works correctly even when we swap parts.

🏗️ Step 1: The Interface (The “Contract”)

An Interface is a list of methods that a class MUST have, but it doesn’t say how to do them.

🧩 The Analogy: The Electrical Outlet

  • The wall outlet is an Interface. It says: “If you want power, you must have two prongs of this exact size.”
  • The outlet doesn’t care if you plug in a TV, a Toaster, or a Lamp.
  • As long as you follow the Contract (the prongs), you get power.

In Java:

public interface Playable {
    void play(); // No body, just a definition!
}

public class Mp3Player implements Playable {
    public void play() {
        System.out.println("Playing audio file...");
    }
}

🏗️ Step 2: Abstract Classes (The “Half-Built House”)

An Abstract Class is a class that cannot be built on its own. It has some finished parts and some “To-Be-Finished” parts.

🧩 The Analogy: The Cake Mix

  • You can’t eat a Cake Mix (Abstract Class). It’s not a cake yet.
  • It has some things ready (Flour, Sugar).
  • But it says: “You must add eggs and milk to finish this.”

🏗️ Step 3: Why do we use them?

  1. Decoupling: You can write code that works with Playable objects without knowing if it’s an MP3, a Video, or a CD.
  2. Safety: The compiler forces you to finish the “Cake Mix” before you can use it.

🥅 Module 3 Review

  1. Interface: A strict contract of what a class MUST do.
  2. Implements: The keyword for following an interface.
  3. Abstract Class: A partially finished blueprint.
  4. Extends: The keyword for finishing an abstract class.