CS457 Syllabus & Progress

Reading the meta data information

Reading the Meta Data

As mentioned before:

To achieve "physical data independence" , the program must:

First read meta data to find out the format/structure of the actual data
Then access the actual data accordingly

You are warned that the procedure is quit tedious (a lot of "if"-statements)

Format of the meta data record is:

Name of a field 24 chars DataType of the field 4 chars Length of the field int

Sample meta data:

24 chars 4 chars int <-----------------> <-----> <----> SSN C 9 Name C 30 Department C 10 Salary I 4

We will use the following class to store one entry of meta data:

class DataDescription // Format of the Meta Data (description of the data) { String fieldName; String fieldType; int fieldSize; }

Next, we define an array of this class to store multiple entries of meta data:

/* ------------------------------------------------------------ (1) Variables used to store the DESCRIPTION of the data ------------------------------------------------------------ */ static DataDescription[] dataDes = new DataDescription[MAXNFIELDS]; static int n_fields; // Actual number of fields // This is a CONSTRUCTOR method for static variables static { // Need to create objects for the dataDes[] for ( int i = 0; i < MAXNFIELDS; i++ ) { dataDes[i] = new DataDescription(); // This is how you make // an array of objects in Java } }

Reading in the meta data is done as follows:

(The name of the meta data file is db-description )


   How to read in the Meta Data (stored in the file "db-description")

   public static void main(String[] args) throws IOException
   {
      DataFile descrFile = new DataFile("db-description");
                         // 1. Open the data description file

      /* -------------------------------------------------------
         Read in the data description and store them in the
         DataDes[] array (define in (1))
         ------------------------------------------------------- */

      n_fields = 0;                 // Count the actual number of fields in data     

      while ( true )
      {
         try
         {
            /* ======================================
	       Read the next field description entry
	       ====================================== */
            dataDes[n_fields].fieldName = descrFile.ReadString(24);
            dataDes[n_fields].fieldType = descrFile.ReadString(4);
            dataDes[n_fields].fieldSize = descrFile.ReadInt();

            System.out.println("Field: " + dataDes[n_fields].fieldName
                + ", type: " + dataDes[n_fields].fieldType
                + ", size: " + dataDes[n_fields].fieldSize);

            n_fields++;     // Count # fields in the format
         }
         catch ( IOException e )
         {
            /* ----------------------------------------------------------------
	       When you reach the end of the file, it will raise this exception
	       ---------------------------------------------------------------- */
            System.out.println("\nFinish reading data description file....\n");
            break;   // Exit loop: no more data !!!
         }
      }

      .....
  }

What did we achieve:

After reading the meta data from the meta data file "db-description" into the array variable dataDes[10], the program now know the format of the data file.
We are now ready to access the user data stored in the data file.

Example Program: (Demo above code)
- Prog file: click here
How to run the program: