java.lang.NullPointerException

[Lu Kim Hui]

What happen with my code? everytime i try to read or print the values from the arraylist , it shows java.lang.nullpointerexception... AH!!!

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i am trying to read the integer of the text file above and then store them into an Integer ArrayList

import java.util.ArrayList;
import java.util.Scanner;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.BufferedWriter;
import java.io.FileWriter;


public class SalesCounter{
private ArrayList<Integer> salesList;
private String filename;

public SalesCounter(){
}

public boolean scanFile(){
System.out.println("Please enter file name to read from (sales.txt) : ");
Scanner input = new Scanner(System.in);
filename = input.nextLine();
input.close();
if(filename.equals("sales.txt")){
return true;
}
else{
return false;
}
}

public void readFromFile()
{
try{
    BufferedReader read = new BufferedReader(new FileReader(filename));
    String line;
   while((line = read.readLine()) != null){ 
   salesList.add(new Integer(line));
    
}
  
} 
catch(Exception e) {
e.printStackTrace();
}
}


public void printSalesList()
{
for(Integer value: salesList){
System.out.println(value);
}
}

This is the second class to show the results

public class SalesCounterDemo{

public static void main(String[] args)throws Exception{
SalesCounter s = new SalesCounter();
s.scanFile();
s.readFromFile();
s.printSalesList();
}
}

[Maneesh Godbole]

Welcome to the Ranch!

It would help us help you better if you provide us with more information like which like does the NPE originate at. The simplest way to do it is copy past the stack trace you see on your console (Please UseCodeTags while posting it)

Typically a null pointer exception is thrown when you try to invoke any method on a null object.

This would be more suitable on the beginner forum. Moving...

[Lu Kim Hui]

sorry! edited... the full codes are up there

[Winston Gutkowski]

Lu Kim Hui wrote:sorry! edited... the full codes are up there

Could you also supply the NPE stack trace exactly as you received it? The first 10 lines or so should be sufficient.

Winston

[Jelle Klap]

Take a look at line 10 of the SalesCounter class. What anre't you doing there, or anywhere else in the code?

[Lu Kim Hui]

java.lang.NullPointerException
at SalesCounter.readFromFile(SalesCounter.java:35)
at SalesCounterDemo.main(SalesCounterDemo.java:6)
at __SHELL1.run(__SHELL1.java:6)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at bluej.runtime.ExecServer$3.run(ExecServer.java:724)
java.lang.NullPointerException
at SalesCounter.printSalesList(SalesCounter.java:48)
at SalesCounterDemo.main(SalesCounterDemo.java:7)

Sorry, this you mean?

[Campbell Ritchie]

Yes, that is what he means. Enter the following line before line 35 and after line 34:

System.out.printf("salesList != null %b%n", salesList != null); // test

[Lu Kim Hui]

Campbell Ritchie wrote:Yes, that is what he means. Enter the following line before line 35 and after line 34:

System.out.printf("salesList != null %b%n", salesList != null); // test

this is the result:

Please enter file name to read from (sales.txt) :
sales.txt
salesList != null false


but the NPE errors are still the same

[Winston Gutkowski]

Lu Kim Hui wrote:but the NPE errors are still the same

Yes, but ask yourself why. An NPE only has one cause: Something is null that wasn't expected to be.

Where do you ever set salesList to anything?

Winston

[Lu Kim Hui]

Erm... because this is an exercise given from my lecturer, and the requirement is reading from the file and store inside an INTEGER arraylist and lastly print them out ^^"

saleList was the Integer arraylist

[Winston Gutkowski]

Lu Kim Hui wrote:Erm... because this is an exercise given from my lecturer, and the requirement is reading from the file and store inside an INTEGER arraylist and lastly print them out ^^"

No. That's the what. You're trying to find out why you're getting an NPE. Have a look at my last question again.

Winston

[fred rosenberger]

This line:

private ArrayList<Integer> salesList;

says "Eventually, I will create an AraryList that can hold Integers, and I will refer to it by the name salesList".

That's like saying "Someday, i will have a dog and call him 'Rover'". That doesn't mean you HAVE a dog - it just means that someday you will, and you've already picked out the name.

You never create the ArrayList. So, when you further down say "add something into the ArrayList named 'salesList'", the JVM is saying "ummm....WHAT arraylist? There is no such thing!!!"

[Lu Kim Hui]

Winston Gutkowski wrote:

Lu Kim Hui wrote:Erm... because this is an exercise given from my lecturer, and the requirement is reading from the file and store inside an INTEGER arraylist and lastly print them out ^^"

No. That's the what. You're trying to find out why you're getting an NPE. Have a look at my last question again.

Winston

omg!! miss out the

ArrayList<Integer> salesList = new ArrayList<Integer>(); 

THANKS ALOT!!!

[Lu Kim Hui]

fred rosenberger wrote:This line:

private ArrayList<Integer> salesList;

says "Eventually, I will create an AraryList that can hold Integers, and I will refer to it by the name salesList".

That's like saying "Someday, i will have a dog and call him 'Rover'". That doesn't mean you HAVE a dog - it just means that someday you will, and you've already picked out the name.

You never create the ArrayList. So, when you further down say "add something into the ArrayList named 'salesList'", the JVM is saying "ummm....WHAT arraylist? There is no such thing!!!"

thanks!

[Winston Gutkowski]

Lu Kim Hui wrote:THANKS ALOT!!!

Another satisfied customer. And isn't it much better when you work it out for yourself?

Winston

[Lu Kim Hui]

Winston Gutkowski wrote:

Lu Kim Hui wrote:THANKS ALOT!!!

Another satisfied customer. And isn't it much better when you work it out for yourself?

Winston

Yup !

[Winston Gutkowski]

Another tip for you: Get used to reading your stacktraces thoroughly.

Go through them and work out exactly which statements are involved and the order they were invoked. Personally, I use a yellow marker pen; but I'm an old fart and I hate looking at large programs on screens (and I recycle my paper ).

Winston

[Lu Kim Hui]

Winston Gutkowski wrote:Another tip for you: Get used to reading your stacktraces thoroughly.

Go through them and work out exactly which statements are involved and the order they were invoked. Personally, I use a yellow marker pen; but I'm an old fart and I hate looking at large programs on screens (and I recycle my paper ).

Winston

ok thanks man!

[Campbell Ritchie]

Paper

There are two sorts of paper when I am around.

Type A paper: this is white and is a hazard to life and limb because I try to write on it and nobody can read it.

Type B paper: this is not all white, but has writing on already, and is a hazard to life and limb because the chances of my losing it are in proportion to the importance of the writing.

In my opinion, you should instantiate that List inside the constructor

private List<Integer> salesList; // List, not ArrayList
/**
 * Creates an object of SalesCounter class
 * This will produce no end of controversy from people who disagree with me ;-)
 */
public SalesCounter()
{
    salesList = new ArrayList<Integer>();
}  

You should also avoid if (...) b = true ; else b = false; Look here for a better example.
Are you closing a Scanner using System.in? You shouldn’t do that. To find out why, add this line after line 20

Scanner inScan = new Scanner(System.in);

[Winston Gutkowski]

Campbell Ritchie wrote:Type B paper: this is not all white...

Yup, know that stuff; except I call it type 'T' paper...

Winston

[Lu Kim Hui]

Campbell Ritchie wrote:Paper

There are two sorts of paper when I am around.

Type A paper: this is white and is a hazard to life and limb because I try to write on it and nobody can read it.

Type B paper: this is not all white, but has writing on already, and is a hazard to life and limb because the chances of my losing it are in proportion to the importance of the writing.

In my opinion, you should instantiate that List inside the constructor

private List<Integer> salesList; // List, not ArrayList
/**
 * Creates an object of SalesCounter class
 * This will produce no end of controversy from people who disagree with me ;-)
 */
public SalesCounter()
{
    salesList = new ArrayList<Integer>();
}  

You should also avoid if (...) b = true ; else b = false; Look here for a better example.
Are you closing a Scanner using System.in? You shouldn’t do that. To find out why, add this line after line 20

Scanner inScan = new Scanner(System.in);

Thanks for the advice..... i am still a newbie

[Campbell Ritchie]

Lu Kim Hui wrote: . . .

Thanks for the advice..... i am still a newbie

You’re welcome. Paper should be abolished, except in books and newspapers.