Why do I get a NullPointerException?

Question

I keep getting this NullPointer Exception when i run the code below with the following data file(input.dat):
5
1-2 1-3 2-4 3-4 4-5
I've spent over 2 hours on this and cant figure it out. Here is my codeimport java.io.*;
import java.util.*;
import java.lang.*;
class ClassOb {
    String classNumber;
    String classRelation;
public class Test {
        ClassOb[] co;
        public Test () throws Exception {
                Read("input.dat");
        public void Read(String fileName) {
                String num; //number of cells
                String cells = " "; //cell data
                int number;
                int x = 0;
                try {
                        FileReader file = new FileReader(fileName);
                        BufferedReader in = new BufferedReader (file);
                        num = in.readLine();
                        number = Integer.valueOf(num).intValue();
                        co = new ClassOb[number];
                        System.out.println("There are " + number + " total classes");
                        while ((cells = in.readLine()) != null) {
                                StringTokenizer st = new StringTokenizer(cells, "- ");
                                while (st.hasMoreTokens()) {
                                   co[x].classNumber = st.nextToken();
                                   co[x].classRelation = st.nextToken();
                                       x++;
                catch (IOException e) {
                        System.out.println("cannot read file");
                        System.exit(0);
                for (int k = 0; k < x; k++) {
                     System.out.println(co[k].classNumber + "-" + co[k].classRelation);
        public static void main (String[] args) throws Exception {
                new Test ();
} thanks for your help!

Answer 1

try this:
At the top, make your ClassOb declaration like this:
ClassOb[] co = new ClassOb[10];//Or some number more than high enough
Then, loop like this:
while ((cells = in.readLine()) != null) {
System.out.println("cells : " + cells);
StringTokenizer st1 = new StringTokenizer(cells, " ");
while(st1.hasMoreTokens()){
String ST1 = st1.nextToken();
System.out.println("ST1 : " + ST1);
StringTokenizer st2 = new StringTokenizer(ST1, "-");
String ST2 = st2.nextToken();
System.out.println("ST2 : " + ST2);
co[x] = new ClassOb();
co[x].classNumber = ST2;
ST2 = st2.nextToken();
System.out.println("ST2 : " + ST2);
co[x].classRelation = ST2;
x++;
Not sure if this is the desired output, but I got this at the end:
1-2
1-3
2-4
3-4
4-5
Polarbear