Complete Program

The class File could be used to check if a file already exists (and then not used as the destination). The program could be improved by using that class. But for now, here is the complete program:

import java.io.*;
class CopyBytes
{
  public static void main ( String[] args ) 
  {
    DataInputStream  instr;
    DataOutputStream outstr;
    if ( args.length != 3 || !args[1].toUpperCase().equals("TO") )
    {
      System.out.println("java CopyBytes source to destination");
      return;
    }

    try
    {
      instr = 
        new DataInputStream(
          new BufferedInputStream(
            new FileInputStream( args[0] )));
      outstr = 
        new DataOutputStream(
          new BufferedOutputStream(
            new FileOutputStream( args[2] )));

      try
      {
        int data;
        while ( true )
        {
          data = instr.readUnsignedByte() ;
          outstr.writeByte( data ) ;
        }
      }
      catch ( EOFException  eof )
      {
        outstr.close();
        instr.close();
        return;
      }
    }

    catch ( FileNotFoundException nfx )
    {
      System.out.println("Problem opening files" );
    }
    catch ( IOException iox )
    {
      System.out.println("IO Problems" );
    }

  }
}

A computer's operating system comes with a file copy command, so there is no practical use for this program. But it is a foundation for many programs that are of practical use. Various data transformations can be done by slipping in some statements between the reading and the writing of the byte.