Day 12: Implementing Interfaces in Classes

The Contract

Welcome to Day 12! This is the final day of Phase 2. We’ve seen Interfaces for objects, and Classes for logic. Now let’s combine them.

implements

You can enforce that a Class meets a specific contract (Interface) using the implements keyword.

interface ClockInterface {
  currentTime: Date;
  setTime(d: Date): void;
}

class Clock implements ClockInterface {
  currentTime: Date = new Date(); // Must be public

  setTime(d: Date) {
      this.currentTime = d;
  }

  constructor(h: number, m: number) { }
}

If you fail to implement a property or method from the interface, TypeScript will show an error.

Interface for Public Side Only

Remember, an interface only describes the public side of the class. You cannot use it to check for private properties.

Multiple Interfaces

A class can implement multiple interfaces.

class SmartPhone implements Phone, Computer, Camera {
   // ... needs to implement everything from all 3
}

Phase 2 Wrap-Up

Congratulations! You’ve mastered Object-Oriented TypeScript. You now know:

• Classes and Instances.
• Public, Private, Protected modifiers.
• Inheritance and Abstract Classes.
• Statics, Getters, and Setters.
• Using Interfaces with Classes.

Coming up in Phase 3 (Day 13-18): Advanced Types (Unions, Generics, Type Guards). This is where TypeScript gets really powerful.

Challenge for Today

1. Define an interface Playable with a method play().
2. Define an interface Recordable with a method record().
3. Create a class MediaPlayer that implements both.
4. Try to remove one of the methods and observe the error.

See you in Phase 3!