Java Certification Exam Guide/McGrawwHill-JavaCertificationExamGuide.iso/pc/Web Links and Code/code/chap27/DB.java

Java Source | 1997-04-20 | 4.9 KB | 194 lines

import java.io.*;

/**
 * Record format is:
 * <num records>
 * <name> <seat #>
 * repeat as necessary
 */
public class DB {
   public static final String FILE_NAME = "Reservations.db";
   public static final int MAX_SEATS = 5;

   private static final int ACTIVE_REC = 0;
   private static final int DELETED_REC = 1;

   private boolean open;
   private RandomAccessFile file;

   public DB() throws IOException {
      try {
         File test = new File(FILE_NAME);
         boolean alreadyExisted = test.exists();
         file = new RandomAccessFile(FILE_NAME, "rw");
         open = true;

         if (!alreadyExisted) {
            FileHeader header = new FileHeader();
            writeHeader(header);
         }

      } catch (IOException x) {
         close();
         throw x;
      }
   }

   public synchronized String[] getPassengerList() throws IOException {
      String[] reservations = null;

      // PLACE YOUR CODE HERE.

      return reservations;
   }

   public synchronized int getSeat(String passenger)
      throws IOException, SeatNotFoundException
   {
      int seat = 0;

      // PLACE YOUR CODE HERE.

      return seat;
   }

   public synchronized void reservePassenger(String passenger, int seat)
      throws IOException, SeatTakenException
   {

      // PLACE YOUR CODE HERE.

   }

   public synchronized int[] getOpenSeats() throws IOException {
      PassengerRecord record;

      FileHeader header = readHeader();
      // Do the case where there are no seats:
      if (header.numPassengers == MAX_SEATS)
         return new int[0];

      // Do the case where all seats are still available.
      if (header.numPassengers == 0) {
         int[] open = {1, 2, 3, 4, 5};
         return open;
      }

      // Determine the open seats from the reserved seats.
      boolean[] reserved = new boolean[MAX_SEATS];
      String passenger;
      for (int i = 0; i < header.numPassengers; i++) {
         record = readNextRecord();
         reserved[record.getSeat() - 1] = true;
      }

      // Make the int array only as large as it needs to be.
      int[] open = new int[MAX_SEATS - header.numPassengers];
      int next = 0;
      for (int i = 0; i < MAX_SEATS; i++) {
         if (reserved[i] == false)
            open[next++] = i + 1;
      }

      return open;
   }

   public synchronized void deletePassenger(String name)
      throws IOException
   {
      FileHeader header = readHeader();

      boolean found = false;
      PassengerRecord record = null; // make the compiler happy
      for (int i = 0; i < header.numPassengers && !found; i++) {
         record = readNextRecord();
         if (name.equals(record.getName()))
            found = true;
      }

      // Delete at the position before we read the record to be deleted.
      if (found) {
         file.seek(record.getPtr());
         file.write(DELETED_REC);

         header.numPassengers--;
         writeHeader(header);
      }
   }

   public synchronized void close() {
      if (open) {
         try {
            file.close();
         } catch (IOException x) {
            System.out.println(x.getMessage());
         } finally {
            open = false;
         }
      }
   }

   public void finalize() throws Throwable {
      close();
      super.finalize();
   }

   private synchronized PassengerRecord readNextRecord()
      throws IOException
   {
      boolean looking = true;
      PassengerRecord record = new PassengerRecord();

      try {
         while (looking) {
            record.setPtr(file.getFilePointer());
            int type = file.readInt();
            record.setName(file.readUTF());
            record.setSeat(file.readInt());
            if (type == ACTIVE_REC)
               looking = false;
         }
      } catch (EOFException x) {
         System.out.println("read past the end of the file");
      }

      return record;
   }

   private synchronized void writeRecord(PassengerRecord record)
      throws IOException
   {
      FileHeader header = readHeader();
      header.totalRecords++;
      header.numPassengers++;
      writeHeader(header);

      file.seek(file.length());
      file.writeInt(ACTIVE_REC);
      file.writeUTF(record.getName());
      file.writeInt(record.getSeat());
   }

   private synchronized FileHeader readHeader() throws IOException {
      FileHeader header = new FileHeader();
      file.seek(0);
      header.totalRecords = file.readInt();
      header.numPassengers = file.readInt();
      return header;
   }

   private synchronized void writeHeader(FileHeader header)
      throws IOException
   {
      file.seek(0);
      file.writeInt(header.totalRecords);
      file.writeInt(header.numPassengers);
   }

}

class FileHeader {
   int totalRecords;
   int numPassengers;
}