工厂模式/ ArrayList /接口问题

Question

我正在使用工厂模式创建使用ElevatorMover接口的不同类型的电梯。工厂将电梯存放在ElevatorMover对象的ArrayList中。当我从ArrayList检索电梯[ElevatorArrayList.get(i)]时，我无法拨打PassengerElevator方法。只有ElevatorMover方法（当然没有执行）。

我在这里没有做什么？

这里是我试图在主

// make elevator 1 go to the 11th floor 
testBuilding.getElevator(1).moveUp(11); 

调用代码这是我的接口

public interface ElevatorMover { 

public void moveUp(int i); 

public void moveDown(int i); 

public void openDoors(); 

这是PassengerElevator方法

@Override 
public void moveUp(int i) { 
    while (currentFloor != i) { 
     setCurrentFloor(currentFloor++); 
    } 
} 

错误消息

Exception in thread "main" java.lang.NullPointerException 
    at Elevator.Building.getElevator(Building.java:109) 
    at Elevator.ElevatorSimulatorMain.main(ElevatorSimulatorMain.java:34) 

个

Building.class构造

private ArrayList<Floor> floorArrayList; // the ArrayList of floors in the building 
private ArrayList<ElevatorMover> elevatorArrayList; // the ArrayList of elevators in the building 

public Building(int numFloors, int numElevators) { 

    this.numFloors = numFloors; 
    this.numElevators = numElevators; 

    // create each floor 
    if (numFloors > 0) // Must have 1 or more floors 
    { 
     for (int i = 1; i <= numFloors; i++) { 
      floorArrayList.add(new Floor(i)); 
     } 
    } 
    else 
    { 
     System.out.println("Building must have 1 or more floors."); 
    } 

    // create each elevator. 
    if (numElevators > 0) // Must have 1 or more elevators 
    { 
     for (int i = 1; i <= numElevators; i++) { 
     elevatorArrayList.add(ElevatorFactory.build("Passenger", i)); 
     } 
    } 
    else 
    { 
     System.out.println("Building must have 1 or more elevators."); 
    } 
} 

ElevatorFactory构建方法

public static ElevatorMover build(String type, int elevID) { 

    if (type.equals("Passenger")) { 
     return new PassengerElevator(elevID); 
    } 
    else return null; // don't know what this is 
} 

的getElevator方法

public ElevatorMover getElevator(int i) { 
    return elevatorArrayList.get(i); 
} 

Answer 1

我猜你没有创建elevatorArrayList。例如

List<ElevatorMover> elevatorArrayList = new ArrayList<ElevatorMover>();

private ArrayList<ElevatorMover> elevatorArrayList; // This is just declaring not creating elevatorArrayList 

Answer 2

当你调用的接口方法 - 实际上任何方法（除了静态方法），被调用的方法是实际类中的那个（或尽可能靠近它） - 不是引用类型中的那个。

考虑以下代码：

class A { 
    public void doStuff() { 
     System.out.println("In A"); 
    } 
} 
class B extends A { 
    public void doStuff() { 
     System.out.println("In B"); 
    } 
} 
class C extends B { 
    public void doStuff() { 
     System.out.println("In C"); 
    } 
} 

class Main { 
    public static void main(String[] args) { 
     A a = new C(); 
     a.doStuff(); // Prints "In C", NOT "In A", because the object is a C, even though the variable's type is "reference to A" 
    } 
} 

同样，如果你有一个PassengerElevator参考，并调用moveUp，那么它会调用moveUp方法PassengerElevator无论引用是什么类型的（只要该参考的类型实际上有一个moveUp方法）。