NullPointerException for LinkedList queue array

Question

I am getting a weird NullPointerExcpetion in line 20:

regs[Integer.parseInt(str.split(" ")[1]) - 1].add(line.poll());

I do not know what has caused this. Can someone please help me fix this?

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

public class shoppay
{
public static void main (String[] args) throws IOException
{
    BufferedReader f = new BufferedReader(new FileReader("shoppay.in"));
    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("shoppay.out")));
    Queue<Integer> line = new LinkedList <Integer>();
    int num = Integer.parseInt(f.readLine());
    String str;
    LinkedList<Integer>[] regs = (LinkedList<Integer>[]) new LinkedList[num];

    while ((str = f.readLine()) != null)
    {
        if (str.charAt(0) == 'C')
            line.add(Integer.parseInt(str.split(" ")[1]));
        else
            regs[Integer.parseInt(str.split(" ")[1]) - 1].add(line.poll());
    }

    out.close();
    System.exit(0);
}
}

Also, I am getting a warning:

Type safety: Unchecked cast from java.util.LinkedList[] to java.util.LinkedList[]

Does this have something to do with the error?

Edit: Input is just a string of lines. First line is a number, and the rest are either "C" or "R" followed by a number. Also, I need a queue for regs.

Answer 1

Not knowing what your input looks like, I can only guess at the cause of the error. I would guess that when you split the String, you are splitting something that results in array of size 1. Are you aware that this will be zero-indexed? This means that the first position in the array is 0 and the second is 1 and so forth. If you did intend to select the second item in the list, then ensure that your input will always split into at least 2 items.

Answer 2

Don't create an array of generic lists. For technical reasons, it doesn't really work cleanly. It's much better to use a list of lists:

BufferedReader f = new BufferedReader(new FileReader("shoppay.in"));
PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("shoppay.out")));
Queue<Integer> line = new LinkedList <Integer>();
int num = Integer.parseInt(f.readLine()); // not needed
String str;
List<List<Integer>> regs = new ArrayList<List<Integer>>(num);

for (int i = 0; i < num; ++i) {
    regs.add(new LinkedList<Integer>());
}

while ((str = f.readLine()) != null)
{
    if (str.charAt(0) == 'C')
        line.add(Integer.parseInt(str.split(" ")[1]));
    else
        regs.get(Integer.parseInt(str.split(" ")[1]) - 1).add(line.poll());
}

As a side issue, is there any reason you're using LinkedList for regs instead of ArrayList ?

Answer 3

Whoops. I changed my mind and decided to use an array. (int[]) I guess it worked fine.