我有點不清楚如何正確地制作這個類。我正在制作的數組找不到符號，我不確定如何更正。編譯錯誤是ArrayList<Vehicle> db = new ArrayList<Vehicle>();我想我只需要在某處初始化它才能正常工作。感謝您的幫助。class Vehicle {     int capacity;   String make;   void setCapacity(int setCapacity) {      this.capacity = setCapacity;      System.out.println("New Capacity = " + setCapacity);   }   Vehicle(int theCapacity, String theMake) {      capacity = theCapacity;      make = theMake;   }   void print() {      System.out.println("Vehicle Info:");      System.out.println("  capacity = " + capacity + "cc" );      System.out.println("  make = " + make );   }}class Car extends Vehicle {   public String type;   public String model;   public Car(int theCapacity, String theMake, String theType, String theModel) {      super(theCapacity, theMake);      type = theType;      model = theModel;   }   @Override   public void print() {      super.print();      System.out.println("  type = " + type);      System.out.println("  model = " + model);   }   @Override   public void setCapacity(int setCapacity) {      System.out.println("Cannot change capacity of a car");   }}class VehicleDB {   ArrayList<Vehicle> db = new ArrayList<Vehicle>();    void addVehicle(Vehicle c){      db.add(c);   }   void print(){      System.out.println("=== Vehicle Data Base ===");      for(Vehicle v: db){         v.print();      }   }}class Task4 {   public static void main (String[]args) {      VehicleDB db = new VehicleDB () ;      db.addVehicle (new Car (1200,"Holden","sedan","Barina"));      db.addVehicle(new Vehicle(1500,"Mazda"));      db.print();   }