无法弄清楚如何解决 java 程序中的异常

Question

最终目标是创建一个读取文本文件的程序，然后确定文本文件中所有数字的最小值、最大值和平均值。我的程序没有错误，但在运行时抛出此错误：

Exception in thread "main" java.lang.NumberFormatException: null
    at java.lang.Integer.parseInt(Integer.java:417)
    at java.lang.Integer.parseInt(Integer.java:499)
    at PrintWriter.main(PrintWriter.java:40)

我的代码如下。如果有人能帮我解决这个问题，我将不胜感激！

import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;


public class PrintWriter 
{
    public static void main(String[] args)
{
    System.out.println("Enter the name of the file you wish to open");
    Scanner keyboard = new Scanner(System.in);
    String fileName = keyboard.nextLine();  
    Scanner inputStream = null;
    String[] numbers = new String[100];
    int index = 0, count = 0, average = 0, total = 0;
    String regex = "[0-9]+";

            //Open and read text file
     try
     {
         inputStream = new Scanner(new File(fileName));
     }
     catch(FileNotFoundException e)
     {
         System.out.println("Error opening the file " + fileName);
         System.exit(0);
     }
     while (inputStream.hasNextLine())
     {
         String line = inputStream.nextLine();
         numbers[index] = line;
         index++;
         count++;
     }
     int lowest = Integer.parseInt(numbers[0]);
     int highest = Integer.parseInt(numbers[0]);
     for (index = 0; index < numbers.length; index++)
     {

         if (Integer.parseInt(numbers[index]) <= lowest)
         {
             lowest = Integer.parseInt(numbers[index]);
         }
         else if (Integer.parseInt(numbers[index]) >= highest)
         {
             highest = Integer.parseInt(numbers[index]);
         }

         total = total + Integer.parseInt(numbers[index]);
         count++;
     }
     average = Math.round(total / count);

     System.out.println("\nThe average of all the numbers is " + average + ". The lowest number was " + lowest + ". The highest number was " + highest);
     System.out.println("\nThe numbers are listed below");
     for (index = 0; index < numbers.length; index ++)
     {
         if (numbers[index].matches(regex));
         {
             System.out.println(numbers[index]);
         }
     }
}
}

Answer 1

您的某些字符串可能不包含可解析的整数。可能，它们可能包含空格。所以使用 trim 方法来摆脱它。

来自javadoc：

public String trim()

Returns a copy of the string, with leading and trailing whitespace omitted.

更改这部分代码

     int lowest = Integer.parseInt(numbers[0].trim());
     int highest = Integer.parseInt(numbers[0].trim());
     for (index = 0; index < numbers.length; index++)
     {

         if (Integer.parseInt(numbers[index].trim()) <= lowest)
         {
             lowest = Integer.parseInt(numbers[index].trim());
         }
         else if (Integer.parseInt(numbers[index].trim()) >= highest)
         {
             highest = Integer.parseInt(numbers[index].trim());
         }

         total = total + Integer.parseInt(numbers[index].trim());
         count++;
     }

Answer 2

您的文本文件包含一行不是数字（包含空格或字母），或者文件中没有 100 个数字。

Answer 3

该文件没有指定它包含多少个数字。因此，只有数组的一部分填充了文件中的字符串。这些被正确解析。但在某些时候，代码到达了数组中没有行的部分。然后发生此调用：Integer.parseInt(someString)，其中 someString 是 null，导致 NullPointerException。要解决此问题，您可以为读取行添加一个计数器并使用此计数器代替 numbers.length，或者在从中解析 Integer 之前检查 String 是否为 null。

Answer 4

当您将数组初始化为 100 个元素并且循环遍历所有数组值时（for (index = 0; index < numbers.length; index++)），在某些时候，您将尝试解析导致异常的空值。所以循环应该迭代直到计数而不是数组长度。您不必在 for 循环内递增计数，因为您已经计算了前一个 while 循环中的所有元素。此外，您不必在循环中调用 Integer.parseInt 5 次，只需获取该整数一次并将其存储在一个 int 变量中，然后重用该变量：

for (index = 0; index < count; index++) {
  int value = Integer.parseInt(numbers[index]);
  if (value < lowest) {
       lowest = value;
  } else if (value > highest) {
       highest = value;
  }

  total = total + value;
}

Answer 5

Java代码

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileWriter;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.Random;
import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class TestProgram {

    public static void main(String[] args) throws FileNotFoundException {

        String line="";
        ArrayList<Integer> arl = new ArrayList<Integer>();
        Scanner inputStream = null;
        String[] numbers = new String[100];
        int index = 0, count = 0, average = 0, total = 0;
        String regex = "[0-9]+";

        // Open and read text file
        try {
            inputStream = new Scanner(new File("filename"));
        } catch (FileNotFoundException e) {
            System.out.println("Error opening the file "
                    + "filename");
            System.exit(0);
        }
        while (inputStream.hasNextLine()) {
            line = inputStream.nextLine();
            numbers[index] = line;
            index++;
            count++;
            Pattern pattern = Pattern.compile("[0-9]+");
            Matcher m = pattern.matcher(line);

            while (m.find()) { 
                int n =Integer.parseInt(m.group()); 
                System.out.println(n);
                arl.add(n);

            }
        }

        Collections.sort(arl);
        System.out.println(arl);
        int lowest = arl.get(0);
        int highest = arl.get(arl.size()-1);
        System.out.println("Lowest is: " + lowest );
        System.out.println("highest is: " + highest );
        total =  arl.size();

        Integer sum = 0;
        for (Integer mark : arl) {
            sum += mark;
        }
        int avg = sum /arl.size();
        average = Math.round(total / count);

        System.out.println("\nThe average of all the numbers is " + avg
                + ". The lowest number was " + lowest
                + ". The highest number was " + highest);
        System.out.println("\nThe numbers are listed below");

    }
}

无法弄清楚如何解决 java 程序中的异常

Cant figure out how to solve exception in java program

java

printwriter