RanFile.java

//== Create a RandomAccessFile ==
// Creates a RandomAccessFile with names and salaries (money)
// The program allocates 50 bytes for each recored -
//    40 bytes for the name field
//    10 bytes for the salary field
// The commands .writeUTF and .readUTF use the following system:
//    First a two bytes tell the length of the following string,
//     and the following bytes contain the string (1 char per byte)
// The double value occupies 8 bytes.
// So there are 42 bytes for the name = 2 bytes for length, 40 bytes for data.
// If shorter strings are recorded, the extra bytes are empty (wasted).
//=============================================================================

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;

public class RanFile extends EasyApp     // EasyApp contains IBIO
                                          // + standard constructor stuff
{ public static void main(String[] args)
   { new RanFile(); }                    // Instantiate object = start program

   Button bWrite = addButton("Write Record",30,30,100,50,this);
   Button bRead   = addButton("Read Record",130,30,100,50,this);

   public void actionPerformed(ActionEvent evt)
   {
       Object source = evt.getSource();
       if (source == bWrite) { writeRecord();}
       if (source == bRead) { readRecord(); }
   }

   public void writeRecord()
   {
       try
       {
           RandomAccessFile file = new RandomAccessFile("salaries.dat","rw");
           long pos = inputLong("Type the record number for saving the data:");

           String name = input("Type the employees's name:");
           if (name.length() > 40) { name = name.substring(0,40);}

           double salary = inputDouble("Type the employee's salary:");

           file.seek(50*pos);
           file.writeUTF(name);
           file.seek(50*pos + 42);
           file.writeDouble(salary);
       }
       catch (IOException e) { output(e.toString());}
   }

   public void readRecord()
   {
       try
       {
           RandomAccessFile file = new RandomAccessFile("salaries.dat","r");
           long pos = inputLong("Type the record number for reading the data:");

           file.seek(50*pos);
           String name = file.readUTF();
           file.seek(50*pos + 42);
           double salary = file.readDouble();

           output("Record #" + pos + " = " + name + " : " + salary);
       }
       catch (IOException e) { output(e.toString());}
   }
}