Java BufferedReader总是在第一次迭代时抛出NumberFormatException

Question

Why do I get the Not a number exception first and then get the correct output ?

import java.io.*;
import java.util.ArrayList;

public class readfile {
    public static void main(String args[]) {
        ArrayList<Integer> arr =new ArrayList<>();
        BufferedReader buff = null;
        FileInputStream fs = null;
        try {
            fs = new FileInputStream("/home/krishna/Documents/file/file");
            buff = new BufferedReader(new InputStreamReader(fs));

        String line = buff.readLine();
            while(line != null) {
                try {
                    arr.add(Integer.parseInt(line.trim()));
                }
                catch(NumberFormatException e) {
                    //System.out.println("Not a number");
                 e.printStackTrace();
                }
                line = buff.readLine();
            }
        }

        catch(FileNotFoundException e) {
            System.out.print(e);
        }
        catch(IOException e) {
            System.out.print(e);
         }
         sumOfArray(arr);
       }
     static void sumOfArray(ArrayList<Integer> arr) {
        int sum=0;
        for(Integer a:arr) {
            System.out.print(a+"\t");
            sum = sum+a;
        }
        System.out.println("Sum is : "+" "+sum);
        }
   }

The File contains numbers from 1 to 9 with each number in new line and there is no space or empty line in the beginning.

Stacktrace prints the following exception

output:
java.lang.NumberFormatException: For input string: ""
at   java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Integer.parseInt(Integer.java:592)
at java.lang.Integer.parseInt(Integer.java:615)
at com.mojang.readfile.main(readfile.java:18)
1   2   3   4   5   6   7   8   9   Sum is :  45

Answer 1

In your file , last line has a new line with \\n i think . Be aware of end of file there is no new line. Check with counter or open file and delete last new line. That means file must be like this;

// -->remove all if some char is here!!
1\n
2\n
3\n
4\n
.
.
.
9   //--> there is no new line !!!!!

Or change your code ;

if(line != null && !line.isEmpty()){
     arr.add(Integer.parseInt(line.trim()));
}

Answer 2

It appears you have a blank line in your input. I suggest using Scanner instead as it skips the whitespace for you.

public class ReadFile {
    public static void main(String[] args) throws IOException {
        String file = "/home/krishna/Documents/file/file";
        List<Integer> ints = new ArrayList<>();
        try (Scanner in = new Scanner(new File(file))) {
            while (in.hasNextInt())
                ints.add(in.nextInt());
        }
        sumOfArray(ints);
    }

    static void sumOfArray(List<Integer> ints) {
        long sum = 0;
        for (int a : ints) {
            System.out.print(a + "\t");
            sum += a;
        }
        System.out.println("\nSum is: " + sum);
    }
}