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Thread: My encounter with Mrs. NullPointerException

  1. #1
    Junior Member BlackStones's Avatar
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    Default My encounter with Mrs. NullPointerException

    Hello companions,

    I've been working on a phone customer billing program for a few days now and I've been able to make considerable progress thanks to the aid of the members on this forum and the Oracle Java reference site. Yesterday, I made some minor changes to my Eclipse layout and basically from there I just started minimizing and maximizing and removing aspects of the interface unknowingly.

    Regardless, I have experienced a Null Pointer Exception and I've been doing the best debugging process I've ever done in my life. I still can't locate the resource or tip to help me get back on the right track.

    This is the program I am still working on;

    First Post (provides concept of program)

    This is my updated code;

    package spInputOutput;
     
    import java.io.FileNotFoundException;
    import java.io.FileReader;
    import java.io.BufferedReader;
    import java.io.IOException;
    import java.io.PrintWriter;
    import java.util.ArrayList;
     
    public class spInputOutput {
     
    	public static PrintWriter outputStream;
     
    	public static void main(String[] args) throws IOException
    	{
    		readData();
    	}
     
    	public static void readData() throws IOException
    	{
    		//Monthly Base Fee of $20
    		double monthlyBase = 20;
    		double startTime = 0.00;
    		double duration = 0.00;
    		//double amountDue = monthlyBase + (duration * startTime);
     
    		//ArrayList<Double> amtDue = new ArrayList<Double>();
    		ArrayList <String> accts = new ArrayList<String>();
    		//use 'contains' to check whether or not the account number has been accounted for, add or create new instance
     
    		double daytimeCall = 0.12; /*per minute for calls that started between 8:00 AM and 10:00PM, 
    		inclusive $0.05 per minute all other times*/
     
    		FileReader fr = new FileReader("input_data.txt");
    		BufferedReader br = new BufferedReader(fr);
     
    		String invoice = "invoice.txt";
            try {
    			outputStream = new PrintWriter(invoice);
    		}
            catch (FileNotFoundException e) {
    			e.printStackTrace();
    		}
     
    		//String[] a = line.split(":"); for times
     
            outputStream.println("Invoice");
            outputStream.println("**********");
            outputStream.println("");
            outputStream.println("Account");
            outputStream.println("");
            outputStream.println("Amount Due");
            //Should eventually use algorithm to calculate the amounts due for each account
     
            /*
             Created to simulate values for Amounts Due
     
             for(int i=1; i<4; i++)
            {
            	double z = ((10+(i*2))+(i*(Math.random())));
            	outputStream.printf("%.2f", z);  
            	outputStream.println("\n"); 
            }
            */
     
            outputStream.println("");
            outputStream.println("**********");
            outputStream.println("Total:");
     
            String text = "";
            String line = br.readLine();
     
    		while(line != null)
    		{
    			text += line;
    			line = br.readLine();
     
                            //THE LINE BELOW IS WHERE MY PROGRAM IDENTIFIES THE NULL POINTER EXCEPTION
    			String [] a = line.split(" ");
     
    			//Account numbers loop
    			for(int i=0; i<1; i++)
    			{
    				//If account number is already in array if()
    				accts.add(a[i]);
    				outputStream.println(a[i]);
    			}		
    		}
     
    		System.out.println(text);
     
    		System.out.println("Inovice File Incorrectly Created");
    		System.out.println(accts.get(1));
     
    		outputStream.close();
    		br.close();
     
    	}
    }

    This is the message displayed in the console after attempting to run;

    Exception in thread "main" java.lang.NullPointerException
    at spInputOutput.spInputOutput.readData(spInputOutput .java:79)
    at spInputOutput.spInputOutput.main(spInputOutput.jav a:18)


    As always, I am grateful for assistance. Any suggestions are welcome, thanks.


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: My encounter with Mrs. NullPointerException

    Exception in thread "main" java.lang.NullPointerException
    at spInputOutput.spInputOutput.readData(spInputOutput .java:79)
    Check the order in which a line is read and when the contents of line is checked for null. It looks like the code does it this way:
    read a line
    check if line is null
    read another line (what happened to first line read)
    use the contents of line (Without checking if null)
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: My encounter with Mrs. NullPointerException

    while(line != null)
    		{
    			text += line;
    			line = br.readLine();
     
                            //THE LINE BELOW IS WHERE MY PROGRAM IDENTIFIES THE NULL POINTER EXCEPTION
    			String [] a = line.split(" ");
    The line variable is null. This is because the br.readLine() method returned null. The while condition is ONLY checked at the beginning of each iteration of the loop. So, if the line variable becomes null during a loop iteration, the loop is not terminated automatically. In most cases, the dependent variables in the while condition (such as: line, in this case) should only be reevaluated as the last step in your loop, not in the beginning. There are several exceptions to this, but I'm just talking about generalization right now.
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  4. The Following User Says Thank You to aussiemcgr For This Useful Post:

    GregBrannon (March 11th, 2014)

  5. #4
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    Default Re: My encounter with Mrs. NullPointerException

    Very good explanation.

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