如何将一个类应用于另一个类？

class Animal {
    public void eat() {
        System.out.println("Animal is eating");
    }
    
    public void sleep() {
        System.out.println("Animal is sleeping");
    }
}

class Dog extends Animal {
    public void bark() {
        System.out.println("Dog is barking");
    }
}

// 使用
Dog dog = new Dog();
dog.eat();  // 继承自Animal类
dog.sleep();  // 继承自Animal类
dog.bark();  // Dog类自己的方法

class Engine {
    public void start() {
        System.out.println("Engine is starting");
    }
    
    public void stop() {
        System.out.println("Engine is stopping");
    }
}

class Car {
    private Engine engine;
    
    public Car() {
        engine = new Engine();
    }
    
    public void startCar() {
        engine.start();
    }
    
    public void stopCar() {
        engine.stop();
    }
}

// 使用
Car car = new Car();
car.startCar();  // 调用Engine类的start()方法
car.stopCar();  // 调用Engine类的stop()方法