java - 未知的 NullPointerException

Question

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，我无法在任何地方找到它们。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;
            }
        }
    }

}

如果运行它，就会出现可怕的 NullPointException 错误，而且我不确定这是什么怪物。

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

Answer 1

这是一个字段成员:

boolean[] 席位；

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

这里你用一个局部变量隐藏了字段成员，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;
    }
}

它应该是这样的:

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