I have a question about handling binary files. I am trying to create a method called read in a user defined class called Record that will handle the data from the file.

Each record has 84bytes and the first 20bytes is a String, and the rest, 64bytes is for 16 int values. My code so far looks like this:

import java.io.*;

class Record
{
    private String country;
    private int[] popData = new int[16];
    
    public void read(DataInputStream is) throws IOException
    {
        byte[] temp = new byte[21];
        for (int i = 0; i < 20; i++)
        {
            temp[i]= is.readByte();
            if (temp[i] == '\0')
                temp[i] = ' ';
        }
        
        String convert = new String(temp);
        country = convert;
        
        for (int i = 0; i < 16; i++)
            popData[i] = is.readInt();
    }
    
    public String getName()
    {
        return country;
    }
    
    public int getPopulation(int y)
    {
        int dat = 0;
        if (y < 1986 || y > 2001)
            dat = 0;
        else
        {
            if (y == 1986)
                dat = 0;
            else if (y == 1987)
                dat = 1;
            else if (y == 1988)
                dat = 2;
            else if (y == 1989)
                dat = 3;
            else if (y == 1990)
                dat = 4;
            else if (y == 1991)
                dat = 5;
            else if (y == 1992)
                dat = 6;
            else if (y == 1993)
                dat = 7;
            else if (y == 1994)
                dat = 8;
            else if (y == 1995)
                dat = 9;
            else if (y == 1996)
                dat = 10;
            else if (y == 1997)
                dat = 11;
            else if (y == 1998)
                dat = 12;
            else if (y == 1999)
                dat = 13;
            else if (y == 2000)
                dat = 14;
            else if (y == 2001)
                dat = 15;
        }
        return popData[dat];
    }
}

public class Asst02
{
    public static void main(String[] args) throws IOException
    {
        long numRec;
        
        numRec = (new File("popdata.bin")).length() / 84;
        Record[] rec = new Record[(int)numRec];
        DataInputStream in = new DataInputStream(new FileInputStream("popdata.bin"));
        
        for (int i = 1; i < numRec; i++)
        {
            rec[i].read(in);
        }
        
        System.out.println(rec[1]);
    }
}

When I run the program it gives me a NullPointerException in main at the for loop. What am I doing wrong?

This is exactly the sort of thing that RandomAccessFile is meant for.

Um, no. I don't know if you see it but I am using DataInputStream to read the binary file sequentially and store all the data into an object array.

In any case, you're getting an NPE, because each element of the array is null until you actually define something there. I.E.

Object[] i = new Object[3];

is the same as doing

Object[] i = { null, null, null };

add a

rec[i] = new Record()

before the call to read.

RandomAccessFile is still the way to go here. Especially if you're using it as a "DB".

Edited 6 Years Ago by masijade: n/a

The entire else block code can be replaced with dat = y - 1986; .

BTW, how are you persisting the data? DataOutputStream or plain old FileOutputStream?

Edited 6 Years Ago by ~s.o.s~: n/a

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