Question

public class ParkedCar {
    private String make;
    private String model;
    private String color;
    private String licenseNum;
    public ParkedCar(String make, String model, String color, String licenseNum) {
        this.make = make;
        this.model = model;
        this.color = color;
        this.licenseNum = licenseNum;
    }
    public void setMake(String ma) {
        make = ma;
    }
    public void setModel(String mo) {
        model = mo;
    }
    public void setColor(String c) {
        color = c;
    }
    public void setLicenseNum(String ln) {
        licenseNum = ln;
    }
    public String getMake() {
        return make;
    }
    public String getModel() {
        return model;
    }
    public String getColor() {
        return color;
    }
    public String getLicenseNum() {
        return licenseNum;
    }
}

public class ParkingMeter {
    private ParkedCar parkedcar;
    private int timePurchased;
    private int timeParked;
    public ParkingMeter(ParkedCar parkedcar, int timePurchased, int timeParked) {
        this.parkedcar = parkedcar;
        this.timePurchased = timePurchased;
        this.timeParked = timeParked;
    }
    /*public ParkingMeter (ParkedCar parkedcar) {
        this.parkedcar = null;
    }*/
    public void setTimePurchased(int timePurchased) {
        this.timePurchased = timePurchased;
    }
    public int getTimePurchased() {
        return timePurchased;
    }
    public void setTimeParked(int timeParked) {
        this.timeParked = timeParked;
    }
    public int getTimeParked() {
        return timeParked;
    }
    public int TimeExpired() {
        if (timeParked > timePurchased) 
            return timeParked - timePurchased;
        else    
            return 0;
    }
    public String toString() {
        return "Make: " + parkedcar.getMake() + "\nModel: " + parkedcar.getModel() + "\nColor: " + parkedcar.getColor() + "\nLicense Number: " + parkedcar.getLicenseNum();
    }
}

public class ParkingTicket {
    private ParkingMeter parkingmeter;
    public ParkingTicket(ParkingMeter parkingmeter) {
        this.parkingmeter = parkingmeter;
    }
    public int TicketCost() {
        if (parkingmeter.getTimeParked() > parkingmeter.getTimePurchased()) {
            if (parkingmeter.getTimeParked() <= 60)
                return 25;
            else 
                return 25 + (10*(parkingmeter.TimeExpired())/60);
        }
        else 
            return 0;
    }                           
}

public class PoliceOfficer {
    private String OfficerName;
    private int OfficerNum;
    private ParkingMeter pm;
    private ParkingTicket pt;
    public PoliceOfficer(ParkingTicket pt, String OfficerName, int OfficerNum) {
        this.OfficerName = OfficerName;
        this.OfficerNum = OfficerNum;   
    }
    public void setOfficerName(String OfficerName) {
        this.OfficerName = OfficerName;
    }
    public void setOfficerNum(int OfficerNum) {
        this.OfficerNum = OfficerNum;
    }
    public String getOfficerName() {
        return OfficerName;
    }
    public int getOfficerNum() {
        return OfficerNum;
    }
    public boolean isExpired() {
        if (pm.getTimeParked() > pm.getTimePurchased())
            return true;
        else
            return false;
    }
    public String toString() {
        return "Officer Name: " + OfficerName + "\nOfficer Number: " + OfficerNum + "\n" + "\nFine: " + pt.TicketCost();    
    }
}

public class ParkingTicketDemo {
    public static void main(String[] args) {
        ParkedCar pc = new ParkedCar("Toyota", "Camry", "Blue", "BXZ 152");
        System.out.println(pc);
        ParkingMeter pm = new ParkingMeter(pc, 60, 120);
        ParkingTicket pt = new ParkingTicket(pm);
        PoliceOfficer po = new PoliceOfficer(pt, "Roger", 337);
        System.out.println(po);
    }
}

我一直在尝试创建一个程序来创建和发布停车票并遇到它编译的问题，但是当它运行时它会在线程“main”java.lang.NullPointerException中发出错误消息Exception。我是一个相当新的程序员，这是我第一次遇到这个问题，所以我还没有完全理解它，似乎无法修复它。我曾尝试在线阅读其他内容，但只是不明白我会喜欢对我的问题进行简单的解释。

Answer 1

NPE的发生是因为这两行：

PoliceOfficer po = new PoliceOfficer(pt, "Roger", 337);
System.out.println(po);

在PoliceOfficer的构造函数中，您不对ParkingTicket实例pt执行任何操作。

public PoliceOfficer(ParkingTicket pt /* not set anywhere */, String OfficerName, int OfficerNum) {
    this.OfficerName = OfficerName;
    this.OfficerNum = OfficerNum;   
}

字段ParkingMeter pm和ParkingTicket pt保持为空，因为您尚未初始化它们。

然后您尝试打印对象：System.out.println(po);这样做是在toString()上调用po，它等同于：

System.out.println(po.toString());

现在因为你的toString（）

public String toString() {
    return "Officer Name: " + OfficerName + "\nOfficer Number: " + OfficerNum + "\n" + "\nFine: " + pt.TicketCost();    
}

使用pt，它会创建一个NullPointerException，因为pt为null。

由于您已将ParkingTicket实例传递给PoliceOfficer的构造函数，因此请使用该实例分配其成员变量pt。

我得到“线程中的异常”主“java.lang.NullPointerException，即使没有值设置为null

1 个答案: