Thread: NullPointerException Error

Hi, I am new to programming. I have an assignment where I am to create a class that processes a sales slip that contains a salesperson number, product number, and the total dollar value of that product ssold for that month. I am to use a 2D Array. I created a arrays to find the product totals as well as the Salesperson total, but I get a NullPointerException Error when I run my tester. Can you see what is wrong with my code.

Class:

public class Sales
{
 public Sales()
 {
 }
 
 public Sales(double[][] sales)
 {
  this.sales=sales;
 }
 
 public void salesSlip(int salespersonNumber, int productNumber, double sum)
 {
  sales [salespersonNumber-1][productNumber-1]+=sum;
 }
 public void productTotal()
 {
  int cols=0;
  double sum=0;
  double[] productTotal= new double[sales[0].length];
  for(cols=0; cols<sales[0].length; cols++)
  {
   for(int rows=0; rows<sales.length; rows++)
   {
    sum+= sales[rows][cols];
   }
   productTotal[cols]=sum;
  }
 }
 
 public void salespersonTotal()
 {
  int rows=0;
  double sum=0;
  double[] salespersonTotal= new double[sales.length];
  for(rows=0; rows<sales.length; rows++)
  {
   for(int cols=0; cols<sales[0].length; cols++)
   {
    sum+= sales[rows][cols];
   }
   salespersonTotal[rows]= sum;
  }
 
 }
 
 public void calcTotal()
 {
  double total=0;
  for(int i=0; i<5; i++)
   total+= 0;
 
 }
 
 public String toString()
 {
 
  toString="\t\tProduct 1\tProduct 2\tProduct 3\tProduct 4\tProduct 5\tTOTAL\nSalesperson 1";
  for(int i=0; i<sales[0].length; i++)
      toString+="\t"+sales[0][i]+"0";
  toString+="\t"+(salespersonTotal[0]);
  toString+="\nSalesperson 2";
  for(int i=0; i<sales[0].length; i++)
      toString+="\t"+sales[1][i];
  toString+="\t"+(salespersonTotal[1]);
  toString+="\nSalesperson 3";
  for(int i=0; i<sales[0].length; i++)
      toString+="\t"+sales[2][i];
  toString+="\t"+(salespersonTotal[2]);
  toString+="\nTOTAL\t";
  for(int i=0; i<5; i++)
      toString+= "\t0.00";
  toString+="\t"+total+"0";
 
 
  return toString;
 
 
 }
 
 private double[][] sales;
 private double[] productTotal;
 private double[] salespersonTotal;
 private String toString;
 private double total;
 
}
 
Tester:
 
public class SalesTester
{
 public static void main(String[] args)
 {
  double[][] sales={{0.00,0.00,0.00,0.00,0.00},{0.00,0.00,0.00,0.00,0.00},{0.00,0.00,0.00,0.00,0.00}} ;
  Sales tester= new Sales(sales);
 
  tester.salesSlip(1,1,20.00);
  tester.salesSlip(1,2,30.00);
  tester.salesSlip(1,3,0.00);
  tester.salesSlip(1,4,10.00);
  tester.salesSlip(1,5,0.00);
  tester.salesSlip(2,1,0.00);
  tester.salesSlip(2,2,0.00);
  tester.salesSlip(2,3,10.00);
  tester.salesSlip(2,4,0.00);
  tester.salesSlip(2,5,30.00);
  tester.salesSlip(3,1,50.00);
  tester.salesSlip(3,2,60.00);
  tester.salesSlip(3,3,50.00);
  tester.salesSlip(3,4,30.00);
  tester.salesSlip(3,5,35.00);
 
  tester.productTotal();
  tester.salespersonTotal();
  tester.calcTotal();
  System.out.println(tester.toString());
 }
}

Error:

java.lang.NullPointerException
at Sales.toString(Sales.java:61)
at SalesTester.main(SalesTester.java:27)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Nativ e Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Native MethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(De legatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.ru nCommand(JavacCompiler.java:271)