Implementing interfaces in classes in Typescript - Mini Project: Build & Apply

Implementing interfaces in classes

📖 Scenario: You are building a simple system to manage different types of vehicles. Each vehicle must have a method to display its details.

🎯 Goal: Create an interface called Vehicle with a method displayInfo(). Then create a class Car that implements this interface and shows the car's brand and model.

📋 What You'll Learn

Create an interface named Vehicle with a method displayInfo() that returns void

Create a class named Car that implements the Vehicle interface

Add a constructor to Car that takes brand and model as string parameters

Implement the displayInfo() method in Car to print the brand and model

Create an instance of Car with brand "Toyota" and model "Corolla"

Call the displayInfo() method on the Car instance

💡 Why This Matters

🌍 Real World

Interfaces help define clear contracts for classes in large applications, ensuring consistent behavior across different objects.

💼 Career

Understanding interfaces and class implementation is essential for TypeScript developers building scalable and maintainable codebases.

1

Create the Vehicle interface

Create an interface called Vehicle with a method displayInfo() that returns void.

Typescript

# Your code here
// Create interface Vehicle with method displayInfo()

Hint

Use the interface keyword and define displayInfo() with no parameters and return type void.

2

Create the Car class implementing Vehicle

Create a class called Car that implements the Vehicle interface. Add a constructor with parameters brand and model of type string, and store them as public properties.

Typescript

interface Vehicle {
    displayInfo(): void;
}

// Your code here
// Create class Car implementing Vehicle with constructor(brand: string, model: string)

Hint

Use implements Vehicle after the class name. Use public in constructor parameters to create properties automatically.

3

Implement the displayInfo() method

Inside the Car class, implement the displayInfo() method to print the message "Car: {brand} {model}" using console.log and template strings.

Typescript

interface Vehicle {
    displayInfo(): void;
}

class Car implements Vehicle {
    constructor(public brand: string, public model: string) {}
    
    // Your code here
    // Implement displayInfo() method
}

Hint

Use console.log with backticks ` and ${} to insert variables.

4

Create Car instance and call displayInfo()

Create a variable myCar as a new Car with brand "Toyota" and model "Corolla". Then call myCar.displayInfo() to print the car details.

Typescript

interface Vehicle {
    displayInfo(): void;
}

class Car implements Vehicle {
    constructor(public brand: string, public model: string) {}
    
    displayInfo(): void {
        console.log(`Car: ${this.brand} ${this.model}`);
    }
}

// Your code here
// Create myCar and call displayInfo()

Hint

Create the instance with new Car("Toyota", "Corolla") and call displayInfo().

Practice

1. What does the implements keyword do in a TypeScript class?

easy

A. It ensures the class follows the structure defined by an interface.

B. It creates a new interface inside the class.

C. It imports an interface from another file.

D. It automatically generates methods for the class.

Solution

1. Step 1: Understand the role of interfaces

   Interfaces define a contract of properties and methods that a class must have.

2. Step 2: Role of implements keyword

   The implements keyword makes sure the class follows that contract exactly.

3. Final Answer:

   It ensures the class follows the structure defined by an interface. -> Option A

4. Quick Check:

   implements enforces interface structure [OK]

Hint: Remember: implements means 'follow the interface rules' [OK]

Common Mistakes:

• Thinking implements creates or imports interfaces
• Believing implements auto-generates code
• Confusing implements with extends

2. Which of the following is the correct syntax to make a class Car implement an interface Vehicle?

easy

A. interface Car implements Vehicle {}

B. class Car implements Vehicle {}

C. class Car extends Vehicle {}

D. class Car uses Vehicle {}

Solution

1. Step 1: Identify correct keywords for class and interface

   Classes use the keyword implements to follow interfaces.

2. Step 2: Check syntax correctness

   class Car implements Vehicle {} is the correct way to declare this.

3. Final Answer:

   class Car implements Vehicle {} -> Option B

4. Quick Check:

   Class uses implements for interfaces [OK]

Hint: Use 'implements' keyword after class name for interfaces [OK]

Common Mistakes:

• Using extends instead of implements for interfaces
• Trying to implement interface in interface declaration
• Using 'uses' keyword which doesn't exist

3. What will be the output of this code?

interface Animal {
  name: string;
  speak(): string;
}

class Dog implements Animal {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
  speak() {
    return `${this.name} says woof`;
  }
}

const dog = new Dog('Buddy');
console.log(dog.speak());

medium

A. undefined

B. "Dog says woof"

C. "Buddy says woof"

D. Error: Dog does not implement speak()

Solution

1. Step 1: Check class implements interface correctly

   Dog class has property name and method speak() as required.

2. Step 2: Understand method output

   speak() returns a string with the dog's name and 'says woof'.

3. Final Answer:

   "Buddy says woof" -> Option C

4. Quick Check:

   Method returns correct string with name [OK]

Hint: Look for method return values matching console.log output [OK]

Common Mistakes:

• Assuming missing method error when method exists
• Confusing property values with method output
• Ignoring string template usage

4. Identify the error in this code:

interface Shape {
  area(): number;
}

class Circle implements Shape {
  radius: number;
  constructor(radius: number) {
    this.radius = radius;
  }
  area(): string {
    return (Math.PI * this.radius * this.radius).toFixed(2);
  }
}

medium

A. Method area() returns string instead of number.

B. Missing constructor in Circle class.

C. radius property is not declared.

D. Circle should extend Shape, not implement.

Solution

1. Step 1: Check interface method signature

   Interface Shape requires area() to return a number.

2. Step 2: Compare with class method return type

   Circle's area() returns a string because toFixed() returns string.

3. Final Answer:

   Method area() returns string instead of number. -> Option A

4. Quick Check:

   Return type must match interface [OK]

Hint: Match method return types exactly with interface [OK]

Common Mistakes:

• Returning string when interface expects number
• Forgetting to declare properties
• Using extends instead of implements for interfaces

5. You have two interfaces:

interface Printer {
  print(): void;
}
interface Scanner {
  scan(): void;
}

How can you create a class AllInOneMachine that implements both interfaces correctly?

hard

A. interface AllInOneMachine implements Printer, Scanner {}

B. class AllInOneMachine implements Printer {
print() { console.log('Printing'); }
scan() { console.log('Scanning'); }
}

C. class AllInOneMachine extends Printer, Scanner {
print() { console.log('Printing'); }
scan() { console.log('Scanning'); }
}

D. class AllInOneMachine implements Printer, Scanner {
print() { console.log('Printing'); }
scan() { console.log('Scanning'); }
}

Solution

1. Step 1: Understand multiple interface implementation

   TypeScript allows a class to implement multiple interfaces separated by commas.

2. Step 2: Check class methods match all interfaces

   Class must have both print() and scan() methods.

3. Final Answer:

   class AllInOneMachine implements Printer, Scanner { print() { console.log('Printing'); } scan() { console.log('Scanning'); } } -> Option D

4. Quick Check:

   Multiple interfaces implemented with comma [OK]

Hint: Use comma to implement multiple interfaces in one class [OK]

Common Mistakes:

• Using extends instead of implements for interfaces
• Implementing only one interface but adding all methods
• Trying to implement interfaces in interface declaration