- Next, we study a program that
access the
user data:
- we want to lookup records using the value of the first field.
- You will see again
that any manipulation on the
user data
in a
format independent manner,
the program must:
- Read the
meta data defining
the structure of the data
that is being accessed
- Then access the
user data by:
- accessing each field in the way according to the type of that data field
- Read the
meta data defining
the structure of the data
that is being accessed
- Again, for simplicity.
we are handling only two types of data:
- string and
- integer
- Here is the LookupData program:
class DataDescription { String fieldName; String fieldType; int fieldSize; } public class LookupData { static int MAXNFIELDS = 10; // Max. # fields in 1 record static int n_fields; // Actual number of fields /* ------------------------------------------------------------ (1) Variables used to store the DESCRIPTION of the data ------------------------------------------------------------ */ static DataDescription[] dataDes = new DataDescription[MAXNFIELDS]; // 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(); } } /* ------------------------------------------------------- (2) Variables used to store the ACTUAL data ------------------------------------------------------- */ static String[] c_buf=new String[MAXNFIELDS]; // Used to store String fields static int[] i_buf=new int[MAXNFIELDS]; // Used to store int fields public static void main(String[] args) throws IOException { Scanner in = new Scanner(System.in); /* =========================================================== We must first find out the STRUCTURE of the data file This information is stored in the data DESCRIPTION file =========================================================== */ /* ------------------------------------------------------- Read in the Meta Data and store them in the DataDes[] array (define in (1)) ------------------------------------------------------- */ DataFile descrFile = new DataFile("db-description"); // 1. Open the data description file n_fields = 0; // Count the actual number of fields in data while ( true ) { try { 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++; } catch ( IOException e ) { System.out.println("\nFinish reading data description file....\n"); break; // Read error: no more data !!! } } /* ============================================================ Here we read the data and print them out... ============================================================ */ DataFile dataFile = new DataFile("db-data"); // First open data file System.out.println( "The data file contains these records:\n"); while ( true ) { /* ============================================================ Read in all the (n_fields) fields ============================================================ */ try { for ( int i = 0; i < n_fields; i++ ) { /* ------------------------------------------------------ Read in the data depending on the TYPE of this field ------------------------------------------------------ */ if ( dataDes[i].fieldType.equals("I") ) { /* -------------------------------------------------------- Field i is an integer, use i_buf[i] to store the value -------------------------------------------------------- */ i_buf[i] = dataFile.ReadInt(); } else { /* -------------------------------------------------------- Field i is an String, use c_buf[i] to store the value -------------------------------------------------------- */ c_buf[i] = dataFile.ReadString( dataDes[i].fieldSize ); // We know the filed size !!! } } PrintRecord(); // Print the data read - see below for method } catch ( IOException e ) { System.out.print("\nDone.\n"); break; } }
/* ************************************************************ Here we do a look up operation on the first data field ************************************************************ */ int lookup_i = 0; // Stores an integer look up value String lookup_s = null; // Stores a String look up value System.out.println( "\nLookup data:\n"); /* ==================================================== Keep asking user to enter for new value to look up ==================================================== */ while ( true ) { System.out.print("Please enter " + dataDes[0].fieldName + " to lookup:"); /* =========================================================== Read in the value entered by the user It must have the same data type as the first data field !! =========================================================== */ if ( dataDes[0].fieldType.equals( "I" ) ) { lookup_i = in.nextInt(); // Read an int in.nextLine(); // Read the end of line char } else { lookup_s = in.nextLine(); // Read a String } /* ============================================================ Find the data record that contains the search value ============================================================ */ dataFile.rewind(); // Rewind data file while ( true ) { try { /* ===================================================== Read the next record (= read ALL fields !) ===================================================== */ for ( int i = 0; i < n_fields; i++ ) { if ( dataDes[i].fieldType.equals("I") ) { /* -------------------------------------------------------- Field i is an integer, use i_buf[i] to store the value -------------------------------------------------------- */ i_buf[i] = dataFile.ReadInt(); } else { /* -------------------------------------------------------- Field i is an String, use c_buf[i] to store the value -------------------------------------------------------- */ c_buf[i] = dataFile.ReadString( dataDes[i].fieldSize ); // We know the filed size !!! } } /* ===================================================== Check if the first field contains the search value ===================================================== */ if ( dataDes[0].fieldType.equals( "I" ) ) { if ( i_buf[0] == lookup_i ) { /* =============================================== Found !! Print and ask user for another value =============================================== */ System.out.print("Found: "); PrintRecord( ); break; } } else { if ( c_buf[0].equals( lookup_s ) ) { /* =============================================== Found !! Print and ask user for another value =============================================== */ System.out.print("Found: "); PrintRecord( ); break; } } } catch ( IOException e ) { System.out.print("Not found....\n"); break; } } } } }
- Example Program:
(Demo above code)
- Prog file: click here
- You will need the DataFile class to compile: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: javac LookupData.java
- To run: java LookupData
Demo:
cd /home/cs377001/demo/Phys-Data-Dependance/DB java LookupData
I have 2 sets of data: To use data set 1: cp db-description.1 db-description cp student-file.1 db-data record format: +-------------+--------------------------------+-------+ | ID | Name | level | +-------------+--------------------------------+-------+ 10 chars 30 chars 4 chars To use data set 2: cp db-description.2 db-description cp student-file.2 db-data record format: +-------------+----------------+-------------+-------+ | ID | Name | Address | level | +-------------+----------------+-------------+-------+ 6 chars 20 chars 20 chars 4 chars