import java.io.*;
import java.util.*;

public class Doubler
{
    private ObjectInputStream inputStream = null;
    private ObjectOutputStream outputStream = null;

    /**
     Doubles the integers in one file and puts them in another file.
    */
    public static void main(String[] args)
    {
        Doubler twoTimer = new Doubler( );
        twoTimer.connectToInputFile( );
        twoTimer.connectToOutputFile( );
        twoTimer.timesTwo( );
        twoTimer.closeFiles( );
        System.out.println("Numbers from input file");
        System.out.println("doubled and copied to output file.");
    }

    public void connectToOutputFile( )
    {
        String outputFileName =
                      getFileName("Enter output file name:");
        try
        {
            outputStream = new ObjectOutputStream(
                        new FileOutputStream(outputFileName));
        }
        catch(IOException e)
        {
            System.out.println("Error opening output file "
                                             + outputFileName);
            System.out.println(e.getMessage( ));
            System.exit(0);
        }
    }

    public void connectToInputFile( )
    {
        String inputFileName =
                     getFileName("Enter input file name:");
        try
        {
            inputStream =
              new ObjectInputStream(
                             new FileInputStream(inputFileName));
        }
        catch(FileNotFoundException e)
        {
            System.out.println("File " + inputFileName
                                       + " not found.");
            System.exit(0);
        }
        catch(IOException e)
        {
            System.out.println("Error opening input file "
                                             + inputFileName);
            System.exit(0);
        }
    }

    private String getFileName(String prompt)
    {
        String fileName = null;
        System.out.println(prompt);
        Scanner keyboard = new Scanner(System.in);
        fileName = keyboard.next( );
        return fileName;
    }

    public void timesTwo( )
    {
        int next;
        try
        {
            while (true)
            {
                next = inputStream.readInt( );
                outputStream.writeInt(2*next);
            }
        }
        catch(EOFException e)
        {
            //Do nothing. This just ends the loop.
        }
        catch(IOException e)
        {
            System.out.println(
                   "Error: reading or writing files.");
            System.out.println(e.getMessage( ));
            System.exit(0);
        }
    }

    public void closeFiles( )
    {
        try
        {
            inputStream.close( );
            outputStream.close( );
        }
        catch(IOException e)
        {
            System.out.println("Error closing files "
                                        + e.getMessage( ));
            System.exit(0);
        }
    }
}