Thread: Can't seem to understand whats wrong with my code! Help!!

I'm doing this assigment for my java class. Everything compiles without a problem but when i run the code it gives me an error. I will attach the code and error to see if you guys know whats the problem.

The code:

import java.io.DataInputStream;
import java.io.DataOutputStream;
import javax.swing.JFileChooser;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import javax.swing.JOptionPane;

public class BasicFile
{
File f;

public BasicFile()
{
JFileChooser choose = new JFileChooser(".");
choose.setFileSelectionMode(JFileChooser.FILES_AND _DIRECTORIES);

int status = choose.showOpenDialog(null);

try
{
if (status != JFileChooser.APPROVE_OPTION)
throw new IOException();
f = choose.getSelectedFile();

if (!f.exists())
throw new FileNotFoundException();
}
catch(FileNotFoundException e)
{
display(e.toString(), "File not found ....");
}
catch(IOException e)
{
display(e.toString(), "Approve option was not selected");
}
}

void getFile()
{
String s = "";

File f2 = new File(f.getPath(), f.getName());

File[] files = f.listFiles();

System.out.println("Absolute Path: " + f.getAbsolutePath());
System.out.println("Files: " + f.getName());
System.out.println("File Size: " + f.length() + "Bytes");
}

void backUp()
{
JFileChooser jfc = new JFileChooser(".");
File f = jfc.getSelectedFile();
File out = jfc.getSelectedFile();

DataInputStream dis = null;
DataOutputStream dos = null;

try
{
dis = new DataInputStream(new FileInputStream(f));
dos = new DataOutputStream(new FileOutputStream(out));

while(true)
{
dos.writeByte(dis.readByte());
}
}
catch(IOException e)
{
display(e.toString(), "File couldn't be duplicated");
}
finally
{
try
{
dis.close();
dos.close();
}
catch(IOException e)
{
}
}
}

void display(String msg, String s)
{
JOptionPane.showMessageDialog(null, msg, s, JOptionPane.ERROR_MESSAGE);
}
}

The error:

Exception in thread "main" java.lang.NullPointerException
at BasicFile.backUp(BasicFile.java:81)
at TestFile.main(TestBasicFile.java:21)
Java Result: 1

I made line 81 bold in the code above. This code was given to me by my professor so i really do not understand why this is giving me an error when i run it. Any help will be much appreciated. Thanks in advance.

Thank you for the help i will try that.