java - 为什么在使用文本输入而不是用户输入时会出现 NullPointerException？

Question

所以我必须编写一个程序，它是一个航类预订系统。如果我正常运行程序并在出现提示时输入值，没有任何问题，但是当我使用 .txt 文件输入时，我得到一个 NullPointerException。我得到 NullPointerException 的代码部分是

 public void instantiateAirplane() throws IOException{
        boolean creatingAirplane = true;
        String className= "";
        String seatFormat;
        int rowNums;
        while (creatingAirplane){
            input  = new BufferedReader(new InputStreamReader(System.in));
            System.out.println("Enter service class name or [ENTER] to finish: ");

            className = input.readLine();
            if (className.equals("")) {
                creatingAirplane = false;
            }
            else{   
                System.out.println("Enter seating pattern: ");
                seatFormat = input.readLine();
                System.out.println("Enter number of rows: ");
                rowNums = Integer.parseInt(input.readLine());
                plane.creatSeatingChart(className, rowNums, seatFormat);
            }
        }
    }

正常输入时，运行如下:

C:\Users\Owner\Desktop\Math&CIS\CS151\HW1>java UserInterface planes
Enter service class name or [ENTER] to finish:
First Class
Enter seating pattern:
WAAW
Enter number of rows:
2
Enter service class name or [ENTER] to finish:
Business Class
Enter seating pattern:
WAAW
Enter number of rows:
2
Enter service class name or [ENTER] to finish:

Add [P]assenger, Add [G]roup, [C]ancel Reservations, Print [S]eating chart, Prin
t [M]anifest, [Q]uit

但是当使用 .txt 输入时，会发生以下情况

C:\Users\Owner\Desktop\Math&CIS\CS151\HW1>java UserInterface planes < input.txt
Enter service class name or [ENTER] to finish:
Enter seating pattern:
Enter number of rows:
Enter service class name or [ENTER] to finish:
Exception in thread "main" java.lang.NullPointerException
        at UserInterface.instantiateAirplane(UserInterface.java:21)
        at UserInterface.main(UserInterface.java:123)

C:\Users\Owner\Desktop\Math&CIS\CS151\HW1>

异常发生在行 "if (className.equals("")) {"

输入文本如下:

First
WAAW
2
Economy
WCAACW
3

...

我不确定为什么它在使用输入文本时不起作用。

Answer 1

一旦迭代器到达所谓的 EOF 或文件末尾，该 readLine() 方法将返回 null。检查是否 ClassName == null || className.Equal("")...

您可能还想在循环之外声明 input = new BufferedReader(new InputStreamReader(System.in));，这样您就不会在每次循环迭代时都重新初始化读取器。

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