未知的 NullPointerException答案

【问题标题】：Unknown NullPointerException未知的 NullPointerException
【发布时间】：2013-12-06 21:33:42
【问题描述】：

我正在编写一个简单的类来模拟飞机预订系统。不幸的是，我的计划一直被多个空指针异常打断，我在任何地方都找不到它们。这是我完成的工作：

import java.util.*;
public class airplane
{
    boolean[] seats;
    Scanner scan = new Scanner(System.in);
    public airplane(int num_of_seats)
    {
        boolean[] seats = new boolean[num_of_seats];
        for (int counter = 0; counter < num_of_seats; counter++)
        {
            seats[counter] = true;
        }
    }
    public boolean seatFirstClass(int seat)
    {
        if ( seat <= seats.length / 4 )
            return true;
        else
            return false;
    }
    public void seatsReserveFirstClass()
    {
        for (int seat = 0; seat < seats.length; seat++)
        {
            if (seats[seat] == true && seat <= seats.length / 4)
            {
                seats[seat] = false;
                System.out.print("I have reserved one seat in first class. Please tell the passenger to enjoy their flight with these airlines! ");
                break;
            }
        }
        System.out.print("I could not find a seat in first class. Can the passenger switch to economy? (0 for no, 1 for yes) ");
        int choice = scan.nextInt();
        if (choice == 0)
            System.out.print("I could not add passenger due to lack of a seat.");
        else if (choice == 1)
        {
            this.seatsReserveEconomy();
        }
    }
    public void seatsReserveEconomy()
    {
        for (int seat = 0; seat < seats.length; seat++)
        {
            if (seats[seat] == true && seat > seats.length / 4)
            {
                seats[seat] = false;
                System.out.print("I have reserved a seat in economy. Please tell the passenger to enjoy their flight with these airlines! ");
                break;
            }
        }
        System.out.print("I could not find a seat in economy. Can the passenger switch to first class? (0 for no, 1 for yes) ");
        int choice = scan.nextInt();
        if (choice == 0)
            System.out.print("I could not add the passenger due to a lack of seats.");
        else if (choice == 1)
            this.seatsReserveFirstClass();
    }
    public void planeClear()
    {
        for (int seat = 0; seat < seats.length; seat++)
        {
            seats[seat] = true;
        }
    }
    public void seatUnReserve(int seat)
    {
        if (seat < seats.length)
        {
            if (seats[seat] == false)
            {
                seats[seat] = true;
                System.out.printf("Seat %d has been unreserved.\n", seat);
            }
            else
            {
                System.out.print("The seat you have entered is already unreserved.\n");
            }
        }
        else
        {
            System.out.printf("There is no seat number %d on this plane.\n", seat);
        }
    }
}

这是一个测试它的主类：

public class Test
{
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);
        boolean check = false;
        System.out.print("How many seats does the plane have? ");
        int num_of_seats = scan.nextInt();
        airplane Airplane = new airplane(num_of_seats);
        while (check == false)
        {
            System.out.print("Please enter 0 to reserve a first class seat, 1 to reserve an economy seat,\n2 to cancel a reservation, 3 to clear the plane,\nand 4 to quit the program. ");
            int choice = scan.nextInt();
            switch(choice)
            {
            case 0:
               Airplane.seatsReserveFirstClass();
               break;
            case 1:
                Airplane.seatsReserveEconomy();
                break;
            case 2:
                System.out.print("Which seat should I unreserve? ");
                int seat = scan.nextInt();
                Airplane.seatUnReserve(seat);
                break;
            case 3:
                Airplane.planeClear();
                break;
            case 4:
                check = true;
                break;
            }
        }
    }

}

如果运行，会出现可怕的 NullPointException 错误，我不知道什么叫这个野兽。

Exception in thread "main" java.lang.NullPointerException
at airplane.seatsReserveFirstClass(airplane.java:22)
at Test.main(Test.java:15)

【问题讨论】：

"...the dreaded NullPointException error shows up, and I'm not sure what calls this beast." -- 异常堆栈跟踪会告诉你！你的堆栈跟踪在哪里，它告诉你哪一行抛出了 NPE？不要忽略异常消息——研究它们！
test.main(test.java:58) 处飞机.seatsReserveFirstClass(airplane.java:63) 处的线程“main”java.lang.NullPointerException 中的异常
那么飞机.java 63 是哪一行？同样，这些消息会告诉您足够的信息来寻找解决方案，尽管您的答案可能低于 Hernan 的答案。关键是不要忽略异常堆栈跟踪消息，因为它们是关键。如果您想帮助其他人理解您的代码，请学习使用 Java 命名约定。
啊，对不起。我不知道（对java来说是全新的）。谢谢，很高兴知道。

标签： java arrays nullpointerexception

【解决方案1】：

这是一个字段成员：

boolean[] 席位；

Scanner scan = new Scanner(System.in);
public airplane(int num_of_seats)
{

在这里你用一个局部变量来隐藏字段成员，你没有触及另一个

    boolean[] seats = new boolean[num_of_seats];

    for (int counter = 0; counter < num_of_seats; counter++)
    {
        seats[counter] = true;
    }
}

应该是这样的：

public airplane(int num_of_seats)
{
    seats = new boolean[num_of_seats];
    for (int counter = 0; counter < num_of_seats; counter++)
    {
        seats[counter] = true;
    }
}

【讨论】：

【解决方案2】：

Herman 的快速眼睛已经为您找到答案了！！无论如何，我会为未来提供帮助......

你可以使用Java IDE，eclipse是不错的，开发程序。我相信其中一个非常重要的特性是调试，它使您能够完全控制您的应用程序并快速跟踪异常。

这里是如何从 Eclipse 调试应用程序的快速教程（请参阅 4.3 的异常断点）

http://www.vogella.com/articles/EclipseDebugging/article.html

【讨论】：