java.lang.nullpointer exception?

Question

import java.io.File;
import java.io.IOException;
import java.util.Scanner;

public class ReadCellPhones {
    public static void main(String Args[]) throws IOException {
        Scanner s = new Scanner(System.in);
        File Input = new File("Cellphone.txt");
        Scanner f = new Scanner(Input);

        String[] Cells = new String[20];
        Double[] Amounts = new Double[20];

        Double threshold = printmenu();
        int number = 0;

        while (f.hasNext()) {
            Cells[number] = f.next();
            Amounts[number] = f.nextDouble();
            number++;
        }

        System.out.println("NUMBER\tArmount");

        for (int i = 0; i < Amounts.length; i++) {
            if (Amounts[i] > threshold)// THIS IS WHERE THE NULLPOINTER
            // EXCEPTION OCCURS
            {
                System.out.print(Cells[i] + "\t" + Amounts[i]);
            }
        }
    }

    static Double printmenu() {
        Scanner s = new Scanner(System.in);

        System.out.print("Enter the filename: ");
        String Filename = s.nextLine();

        System.out.print("Cell Bill Threshold: ");
        Double threshold = s.nextDouble();

        return threshold;
    }
}

So what I'm trying to do is read in data from a file, store the data in 2 arrays and print out the arrays if the Amounts array values is greater then what is entered for the threshold variable. but when i try to run the program the nullpointer error pops up, any idea why?

please share your stack trace

sjr
– sjr

2012-12-04 01:00:27 +00:00
Commented Dec 4, 2012 at 1:00 — sjr
– sjr, Commented Dec 4, 2012 at 1:00

score 2 · Accepted Answer · 2012-12-04 01:08:40Z

2

The problem is that you have less than 20 records being read in.

Each Double in the Amounts array defaults with the value null. When java does unboxing in order to compare Amounts[i] with threshold, it tries to dereference this null value, thus creating the exception.

The solution is to mark how many values are successfully read in, and only compare that many values to the threshold.

edited Dec 4, 2012 at 1:08

answered Dec 4, 2012 at 1:02

user406009

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2 Comments

user1840435 Over a year ago

how would i do that?. Sorry to ask, im just new to programming and dont really know to much about it yet

user406009 Over a year ago

@user1840435 I would start by printing out Amounts[i] and i in your for loop before the comparison to verify that a null Amounts[i] is causing the null pointer exception(I could always be wrong). It seems you already have a variable for marking how many values were successfully read number, just change for(int i=0;i<Amounts.length;i++) to for(int i=0;i<number;i++).

awolfe91 · Accepted Answer · 2012-12-04 01:01:06Z

0

If the threshold is less than 20, your for loop will continue to the end of the Amounts array, which will include uninitialized doubles. Depending on what functionality you want, I would suggest looping until i

answered Dec 4, 2012 at 1:01

awolfe91

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Comments

namxee · Accepted Answer · 2012-12-04 01:11:55Z

0

there is no guarantee that count of file's data is 20.

so change your loop limit counter

//for(int i=0;i<Amounts.length;i++)
for(int i=0;i<number;i++)  //variable number has a count of file's data

good luck

answered Dec 4, 2012 at 1:11

namxee

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