如何从Java中的文本文件读取空集

Question

I have 3 String fields per line within my text file. There are 4 lines in total. The first 2 fields ( field[0] and field[1] ) are already filled in but field 3 ( field[2] ) is yet to be generated so it shall remain empty. Is there any way I can read in this text file line by line without getting a java.lang.ArrayIndexOutOfBoundsException: 1 error? I have included my code used for reading in the file.

import java.io.*;
public class PassGen {
   public static void main(String args[]) throws Exception{
      BufferedReader inKb = new BufferedReader(new InputStreamReader(System.in));
      BufferedReader inF = new BufferedReader(new FileReader(new File("students.txt")));

      String line = inF.readLine();
      int cnt = 0;
      Student pupil[] = new Student[6];

      while(line != null) {
          String field[] = line.split("//s");
          pupil[cnt] = new Student(field[0], field[1], field[2]);
          cnt++;
          inF.readLine();
      }       
   }
}

Answer 1

You can simply add a check on the number of fields:

if(field.length > 2) {
   pupil[cnt] = new Student(field[0], field[1], field[2]);
} else {
   pupil[cnt] = new Student(field[0], field[1], null);
}

Alternatively, you can use the overloaded split method that takes a limit parameter and set that to -1 to include the empty field. From the documentation of String#split(String regex, int limit) :

The limit parameter controls the number of times the pattern is applied and therefore affects the length of the resulting array. If the limit n is greater than zero then the pattern will be applied at most n - 1 times, the array's length will be no greater than n , and the array's last entry will contain all input beyond the last matched delimiter. If n is non-positive then the pattern will be applied as many times as possible and the array can have any length . If n is zero then the pattern will be applied as many times as possible, the array can have any length, and trailing empty strings will be discarded.

Note that you need to use \\\\s instead of //s for the whitespace regex (this needs to be corrected either way).

String field[] = line.split("\\s", -1);

Answer 2

I think you problem lies in the way you are managing your data, but you can have something like this to read from any array and not getting any exceptions:

  public static String getIfExists(final String[] values, final int position) {
    return (values != null) && (values.length > position) ? values[position] : null;
  }

Then you can fill every field like new Student(getIfExists(field, 0), getIfExists(field, 1), getIfExists(field, 2));

Of course you can optimize this a little bit more...but that would make the trick without having to think on how many fields you might get in the future or having a lot of if / case conditions.