Why does this work? beginning question

From: ComieCreep (Mike97739_at_msn.com)
Date: 12/30/04 

Date: Thu, 30 Dec 2004 14:04:19 -0800

Hi,
     The following code works. The question I have concerns closing a file.
The line:
 if (dis !=null)
seems to say if dis is not empty. So the file should shut down after the
first line is read. I know I am misunderstanding this.
here's the code. Check out the last 5 lines or so

import java.io.*;

  class FileReadTest {

     public static void main (String[] args)
     {
 FileReadTest f = new FileReadTest();
        f.readMyFile();

     }

     void readMyFile() {

        DataInputStream dis = null;
        String record = null;
        int recCount = 0;

        try {

           File f = new File("D:/testfile2");
           FileInputStream fis = new FileInputStream(f);
           BufferedInputStream bis = new BufferedInputStream(fis);
           dis = new DataInputStream(bis);

           while ( (record=dis.readLine()) != null ) {
              recCount++;
              System.out.println(recCount + ": " + record);
           }

        } catch (IOException e) {
           // catch io errors from FileInputStream or readLine()
           System.out.println("Uh oh, got an IOException error!" +
e.getMessage());

        } finally {
           // if the file opened okay, make sure we close it
           if (dis != null) {
       try {
                 dis.close();
       } catch (IOException ioe) {
       }
           }
        }
     }
}
Thanks,
mike