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Abstract Classes

Intermediate~18 min

Abstract classes provide a blueprint for other classes, defining methods that must be implemented by derived classes.

TypeScript Abstract Classes

Abstract Classes

Output 
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Abstract Class Rules:
  • Cannot be instantiated directly
  • Can have abstract methods (no implementation)
  • Can have concrete methods (with implementation)
  • Child classes MUST implement abstract methods

Abstract vs Interface

Feature Abstract Class Interface
Can have implementation ✓ Yes ✗ No
Can be instantiated ✗ No ✗ No
Multiple inheritance ✗ No ✓ Yes
Constructor ✓ Yes ✗ No
Use case Shared behavior + contract Pure contract

Common Mistakes

1. Trying to Instantiate Abstract Class

abstract class Shape {
    abstract getArea(): number;
}

let shape = new Shape();  // ✗ Error!

// ✓ Create concrete child class
class Circle extends Shape {
    getArea() { return 0; }
}
let circle = new Circle();  // ✓ OK

2. Not Implementing Abstract Methods

abstract class Animal {
    abstract makeSound(): void;
}

class Dog extends Animal {
    // ✗ Error - must implement makeSound()
}

// ✓ Correct
class Dog extends Animal {
    makeSound() { console.log("Woof"); }
}
When to Use: Use abstract classes when you have shared implementation AND want to enforce a contract.

Summary

  • Abstract classes combine interfaces and base classes
  • Cannot instantiate abstract classes
  • Child classes must implement abstract methods
  • Can have both abstract and concrete members
Previous ← Inheritance
Next Static Members →