我正在编写一个简单的类来模拟飞机预订系统。不幸的是,我继续让我的计划让所有头等舱席位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要类:
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;
}
}
}
}
如果运行此命令,则显示可怕的NullPointException错误,并且我不确定是什么叫这个野兽。
Exception in thread "main" java.lang.NullPointerException
at airplane.seatsReserveFirstClass(airplane.java:22)
at Test.main(Test.java:15)
答案 0 :(得分:4)
这是一个字段成员:
布尔[]席位;
Scanner scan = new Scanner(System.in);
public airplane(int num_of_seats)
{
在这里,您使用局部变量遮蔽字段成员,而不是触及另一个
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;
}
}
答案 1 :(得分:0)
Herman快速的眼睛已经为你找到了答案!!无论如何,我延长了对未来的帮助......
你可以使用Java IDE,eclipse是好的,开发程序。我相信,其中一个非常重要的功能是调试,它使您能够完全控制应用程序并快速跟踪异常。
以下是如何从eclipse调试应用程序的快速教程(参见4.3 for Exception breakpoints)
http://www.vogella.com/articles/EclipseDebugging/article.html