Interacting with files.

[apfroggy0408]

Currently writing a program that displays what's in a different file. What I need to do is process the data in the file so i can get three things. I need to count the number of records, and compute the mean age and GPA.

What's confusing me is how I would go about separating the file to do each thing individually.

Here's what I have so far.

/**
 * @(#)Lab17.java
 *
 *
 * @author 
 * @version 1.00 2008/3/17
 */

import java.io.*;
import java.text.DecimalFormat;

public class Lab17 {

    public static void main(String args[]) throws IOException
    {
    	DecimalFormat df = new DecimalFormat("00.000");
    	File f1 = new File("students17.dat");
    	File f2 = new File("passing.dat");
    	File f3 = new File("honors.dat");
    	f2.delete();
    	f3.delete();
    	
    	if (f1.exists())
    	{
    		FileReader inFile = new FileReader(f1);
    		BufferedReader inStream = new BufferedReader(inFile);
    		String inString;
    		while((inString = inStream.readLine()) != null)
    			System.out.println(inString);
    			int rec = Integer.parseInt(inString);
    			int sum = 0;
    			sum += rec;
    			inStream.close();
    			System.out.println(sum);
    		
    	}
    	else
    	{
    		System.out.println(f1.getName() + " does not exist");
    	}
    	System.out.println();
    	
   
    	
    }
    
    
}

I get this error with the current adding I have going. I'm betting because it's trying to add letters in as well.

Exception in thread "main" java.lang.NumberFormatException: null
    at java.lang.Integer.parseInt(Integer.java:415)
    at java.lang.Integer.parseInt(Integer.java:497)
    at Lab17.main(Lab17.java:30)

[hardwired]

File f1 = new File("students17.dat");
File f2 = new File("passing.dat");
File f3 = new File("honors.dat");

Files with a dat extension are often written and read with highly specialized DataOutput/InputStreams. We generally use a txt extension for reading with a FileReader. It may work; I haven't tried.
The error on line 30 suggests that the reader read something that came up as "null". If the data in the file is only a number on each line then your read method should work.
An impertinent suggestion is to move the sum variable up above the while loop. It gets reset/declared and assigned the value of zero in each trip throught the loop.

int sum = 0;
while((inString = inStream.readLine()) != null)
    System.out.println(inString);
    int rec = Integer.parseInt(inString);
//    int sum = 0;
    sum += rec;