获取 NullPointerException：“无法调用“ParkingSpace.getSpaceNum()”，因为“pl.parkingLot[5]”为空”？

Question

我正在用Java开发一个停车场项目，当我运行我的程序时，我收到以下错误消息：

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "ParkingSpace.getSpaceNum()" because "pl.parkingLot[5]" is null at ParkingLot.main(ParkingLot.java:67)

我不明白为什么

pl.parkingLot[5]

是

null

，因为我以为我已经用for循环在数组parkingLot的每个索引处初始化了

ParkingSpace

。

我认为当我在 ParkingLot 的类构造函数中使用 for 循环为数组

parkingLot

赋值时，调用

parkingLot[5]

不会导致

null

。

另外，我希望该程序运行并在控制台的一行上打印 5，并在下一行打印 4，但运行该程序会导致 NullPointerException。

这是我的 ParkingLot 类的代码：

public class ParkingLot {
    ParkingSpace[] parkingLot;
    int numSpaces;
    int numFloors;

    ParkingLot(int numSpaces, int numFloors){
        this.numSpaces = numSpaces;
        this.numFloors = numFloors;

        ParkingSpace[] array = new ParkingSpace[numSpaces];
        parkingLot = new ParkingSpace[numSpaces];
        char currentFloor = 'A';
        for(int i = 0; i < numFloors; i++){
            for(int j = 0; j < numSpaces; j++){
                parkingLot[j] = new ParkingSpace(j, currentFloor, false);
                if(j<4){
                   parkingLot[j].isCompact = true;
                }
                j++;
            }
            currentFloor++;
            i++;
        }
    }

    ParkingSpace enterLot(boolean com){
        ParkingSpace returnValue = new ParkingSpace(0,'A', false);
        for(int i = 0; i < numFloors; i++) {
            for (int j = 0; j < numSpaces; j++) {
                if (!parkingLot[j].isOccupied) {
                    if (com) {
                        returnValue = parkingLot[j];
                    } else if (i > 3) {
                        returnValue = parkingLot[j];
                    } else {
                        j++;
                    }
                } else {
                    j++;
                }

            }
            i++;
        }
        return returnValue;
    }

    void leaveLot(ParkingSpace space){
        for(int i = 0; i < numFloors; i++){
            if(i == space.floor){
                for(int j = 0; j < numSpaces; j++){
                    if(j == space.spaceNum){
                        space.isOccupied = false;
                    }else{
                        j++;
                    }
                }
            }else{
                i++;
            }
        }
    }

    public static void main(String[] args){
        ParkingLot pl = new ParkingLot(6, 6);

        System.out.println(pl.parkingLot[5].getSpaceNum());
        System.out.println(pl.enterLot(false));
    }
}

这是 ParkingSpace 类：

public class ParkingSpace {
    int spaceNum;
    char floor;
    boolean isCompact;
    boolean isOccupied;

    ParkingSpace(int spaceNum, char floor, boolean isCompact){
        this.spaceNum = spaceNum;
        this.floor = floor;
        this.isCompact = isCompact;
        isOccupied = false;
    }

    int getSpaceNum(){
        return spaceNum;
    }
    char getFloor(){
        return floor;
    }
    boolean getCompact(){
        return isCompact;
    }
    boolean getOccupied(){
        return isOccupied;
    }
    void setOccupied(boolean value){
        isOccupied = value;
    }
    String toAString(){
        char compact;
        char occupied;
        if(isCompact){
            compact = 'C';
        }else{
            compact = 'N';
        }

        if(isOccupied){
            occupied = 'X';
        }else{
            occupied = 'O';
        }

        return(floor + spaceNum + " (" + compact + " " + occupied + ")");
    }

Answer 1

为什么要在循环体中增加i和j？您初始化每个第二层和空间。