Java - 获取NullPointerException

Question

我在我的代码的第59行出现NullPointerException时出现问题。

该程序的目的是提示用户文件位置（其中有数字的PI）。然后程序应该通过Scanner类接受任意数量的数字（比如说k个数字）。然后，程序必须从文件中读取PI的k个数字。使用Scanner类，程序应该从用户那里获得一个0-9的数字，并打印它出现的第一个和最后一个位置，以及它出现的次数。只考虑小数点后的数字。该程序应该能够接受100,000位数的PI。

代码的样本输出低于：

Give the location of the file: 
C:\Users\Joe\Desktop\pi.txt 

Number of digits of PI to parse: 
10 

Give any number between 0-9: 
1 

1 appears 2 times 
First position in which appears: 1 
Last position in which appears: 3 

任何帮助，将不胜感激。

下面是我的代码：

import java.io.BufferedReader; 
import java.io.FileInputStream; 
import java.io.FileNotFoundException; 
import java.io.IOException; 
import java.io.InputStreamReader; 
import java.util.Arrays; 
import java.util.Scanner; 
import java.util.ArrayList; 


public class Problem2 { 
    @SuppressWarnings("null") 
    public static void main(String[] args) throws Exception { 
    FileInputStream inputstream = null; 
    BufferedReader reader = null; 

    @SuppressWarnings("resource") 
    Scanner input = new Scanner(System.in); 

    try { 
     System.out.println("Give the location of the file (example: C:\\Users\\Joe\\Desktop\\pi.txt):"); 
     String fileloc = input.nextLine(); 

     inputstream = new FileInputStream(fileloc); 
     reader = new BufferedReader(new InputStreamReader(inputstream)); 
     String stringinput; 

     System.out.println("Number of digits of PI to parse: "); 
     int parsenum = input.nextInt() + 2; 
     String[] stringarray = new String[parsenum]; 

     while((stringinput = reader.readLine()) != null) { 
      stringinput = stringinput.substring(2, parsenum); 

      for(int i = 0; i < stringinput.length(); i++) { 
       stringarray = stringinput.split(""); 
      } 
     } 

     System.out.println("Give any number between 0-9: "); 
     String searchnum = input.next(); 

     int count = 0; 

     for(int i = 1; i < parsenum - 1; i++) { 
      if(searchnum == stringarray[i]) { 
       count++; 
      } 
      else count++; 
     } 

     System.out.println(searchnum + " appears " + count + " time(s)"); 

     for(int i = 1; i < parsenum - 1; i++) { 
      System.out.print(stringarray[i]); 
     } 

     System.out.println(); 
     System.out.println("First position in which " + searchnum + " appears: " + stringinput.indexOf(searchnum)); 
     System.out.println("Second position in which " + searchnum + " appears: " + stringinput.lastIndexOf(searchnum)); 

    } 
    catch (FileNotFoundException exception) { 
     System.err.println("File not found, please try again"); 
     main(null); 
    } 
    catch (Exception e) { 
     System.err.println("Invalid input entered"); 
     e.printStackTrace(); 
     System.exit(0); 
    } 
    finally { 
     reader.close(); 
    } 

} 
} 

Answer 1

while((stringinput = reader.readLine()) != null) 

以上while循环运行，直到reader.readLine是null等都将成为stringinput。

现在用stringinput while循环的后：

stringinput.indexOf(searchnum) 
stringinput.lastIndexOf(searchnum) 

，从而得到NullPointerException。