工厂 Pattern/ArrayList/Interface 问题

Question

我正在使用工厂模式通过 ElevatorMover 接口创建不同类型的电梯。工厂将电梯存储在 ArrayList 个 ElevatorMover 对象中。当我从 ArrayList 检索电梯 [ElevatorArrayList.get(i)] 时，我无法调用 PassengerElevator 方法。只有 ElevatorMover 方法（当然没有实现）。

我在这里做错了什么？

这是我试图在 main

中调用的代码

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

这是我的界面

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++);
    }
}

错误信息

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

Building.class构造函数

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
}

getElevator 方法

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

Answer 1

当你调用接口方法时——实际上是任何方法（静态方法除外），被调用的方法是实际class中的方法（或作为尽可能接近它）- 不是引用类型中的那个。

考虑这段代码：

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的引用，你调用了moveUp，那么无论是什么类型，它都会调用PassengerElevator中的moveUp方法引用是（只要引用的类型实际上有一个 moveUp 方法）。

Answer 2

我猜你没有创建 elevatorArrayList。示例

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

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

工厂 Pattern/ArrayList/Interface 问题

Factory Pattern/ArrayList/Interface Issue

java

interface

arraylist

factory-pattern