L - Liskov Substitution

❌ Vi phạm LSP - Square phá kỳ vọng của Rectangle

class Rectangle {
  protected width = 0;
  protected height = 0;
  setWidth(w: number)  { this.width = w; }
  setHeight(h: number) { this.height = h; }
  area() { return this.width * this.height; }
}

class Square extends Rectangle {
  // Ep hai canh luon bang nhau -> pha tinh "doc lap" cua width/height
  setWidth(w: number)  { this.width = w; this.height = w; }
  setHeight(h: number) { this.width = h; this.height = h; }
}

function printArea(r: Rectangle) {
  r.setWidth(5);
  r.setHeight(10);
  // Ky vong: 5 * 10 = 50
  console.log(r.area());
}

printArea(new Rectangle()); // 50  ✅
printArea(new Square());     // 100 ❌  -> thay the lam SAI ket qua

✅ Tuân thủ LSP - abstraction đúng mức

abstract class Shape {
  abstract area(): number;   // hop dong: moi Shape deu tinh duoc dien tich
}

class Rectangle extends Shape {
  constructor(private w: number, private h: number) { super(); }
  area() { return this.w * this.h; }
}

class Square extends Shape {
  constructor(private side: number) { super(); }
  area() { return this.side * this.side; }
}

// Thay the thoai mai: moi Shape giu dung hop dong area()
function totalArea(shapes: Shape[]): number {
  return shapes.reduce((sum, s) => sum + s.area(), 0);
}

totalArea([new Rectangle(5, 10), new Square(4)]); // 50 + 16 = 66 ✅

Tách hành vi thay vì ném lỗi

interface Bird { eat(): void; }
interface Flyable { fly(): void; }

class Sparrow implements Bird, Flyable {
  eat() {}
  fly() {}
}
class Penguin implements Bird {   // khong implement Flyable
  eat() {}
}