NumberFormatException while selecting random elements from a big file

问题

I have a very big file which contains user ids like this. Each line in that big file is an user id.

So in that big file, I will have around 30Million user id's. Now I am trying to select random user id's from that big file. Below is the program I have but at some point it always give me this exception like this- and I am not sure why this exception is happening.

Exception in thread "main" java.lang.NumberFormatException: For input string: ""
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:59)
    at java.lang.Integer.parseInt(Integer.java:481)
    at java.lang.Integer.parseInt(Integer.java:510)
    at com.host.bulls.service.lnp.RandomReadFromFile.main(RandomReadFromFile.java:65)

Below is the program I have-

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

    File f = new File("D:/abc.txt");
    RandomAccessFile file;

    try {

        file = new RandomAccessFile(f, "r");
        long file_size = file.length();

        // Let's start
        long chosen_byte = (long)(Math.random() * (file_size - 1));
        long cur_byte = chosen_byte;

        // Goto starting position
        file.seek(cur_byte);

        String s_LR = "";
        char a_char;

        // Get left hand chars
        for (;;)
        {
            a_char = (char)file.readByte();
            if (cur_byte < 0 || a_char == '\n' || a_char == '\r' || a_char == -1) break;
            else 
            {
                s_LR = a_char + s_LR;
                --cur_byte;
                if (cur_byte >= 0) file.seek(cur_byte);
                else break;
            }
        }

        // Get right hand chars
        cur_byte = chosen_byte + 1;
        file.seek(cur_byte);
        for (;;)
        {
            a_char = (char)file.readByte();
            if (cur_byte >= file_size || a_char == '\n' || a_char == '\r' || a_char == -1) break;
            else 
            {
                s_LR += a_char;
                ++cur_byte;
            }
        }

        // Parse ID
        if (cur_byte < file_size) 
        {
            int chosen_id = Integer.parseInt(s_LR);
            System.out.println("Chosen id : " + chosen_id);
        }
        else
        {
            throw new Exception("Ran out of bounds..");
        }

    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

Is there any problem in my above code?

回答1:

I tried to run your code and found one additional error - you have to check cur_byte before read as follows:

if (cur_byte < file_size) {
    a_char = (char) file.readByte();
}

Otherwise you will get EOFException.

With your sample abc.txt I don't get java.lang.NumberFormatException: For input string: "" exception.

But if I add empty lines in abc.txt I get this exception sooner or later. Thus the problem is with empty lines somewhere in abc.txt.

回答2:

Any unparsable String if you pass to parseInt method then it will raise NumberFormatException. Like empty String and also Integer can hold the maximum & minimum value an int can have, 2147483647 or -2147483648. And if value goes beyond of that then it raise NumberFormatException

If the string does not contain a parsable integer. ([Documentation][1])

回答3:

It seems that s_LR contains an empty String.

From what I'm figuring this could happen if you have windows style linebreaks (\r\n) and hit the '\r' with the random seek. Then the break-conditions in both loops would apply, before any char is added to s_LR.

Sidenote: you are using a very atypical coding style for java. While it has no influence on your programm, it is harder to read/understand for other java programmers and therefore you may not get an answer.

回答4:

Really that is look like you have empty string at the end of file or at the beginning of the file.

Or one of the numbers to long for Integer.

I see two solutions:

Add check for spaces and empty string for each element that you reads from file.
Change Integer to Long value.

来源：https://stackoverflow.com/questions/17206864/numberformatexception-while-selecting-random-elements-from-a-big-file

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java

random

numberformatexception