Question

I was trying to solve this problem in UVa, but I get a NullPointerException at line 16 (at the for loop). I’m fairly new in Java and I don’t know why is it happening. The output looks okay, but I would like to know why it is returning a NPE. Here’s my code:

import java.io.*;

class Crypt {
    public static void main(String[] args) throws IOException {
    String line = "";
    String buffer = "";
    char[][] hash = {{'a', 'p'}, {'b', 'l'}, {'c', 'o'}, {'d', 'k'}, {'e', 'm'},
             {'f', 'i'}, {'g', 'j'}, {'h', 'n'}, {'i', 'u'}, {'j', 'h'},
             {'k', 'b'}, {'l', 'y'}, {'m', 'g'}, {'n', 'v'}, {'o', 't'},
             {'p', 'f'}, {'q', 'c'}, {'r', 'r'}, {'s', 'd'}, {'t', 'x'}, 
             {'u', 'e'}, {'v', 's'}, {'w', 'z'}, {'x', 'a'}, {'y', 'q'},
             {'z', 'w'}};

    BufferedReader f = new BufferedReader(new FileReader("crypt.in"));
    while(f != null) {
        line = f.readLine();
        buffer = "";
        for(int i = 0; i < line.length(); i++) {
        if(line.charAt(i) == ' ') {
            buffer += ' ';
        } else {
            for(int j = 0; j < hash.length; j++) {
            if(line.charAt(i) == hash[j][1]) {
                buffer += hash[j][0];
            }
            }
        }
        } 
        System.out.println(buffer);
    } 
    }
}

I’m looking forward for a good explanation. Thanks.

Answer 1

Your line variable seems to be null. Use f.ready() to check if there are any more lines remaining in the file. f != null is incorrect because it merely tests if the BufferedReader object f exists, so it will never be false in this case.