---

Problems with traditional file processing

---

• Problem with the traditional File Processing Approach

  • A severe short-coming:

    Structure of the data is encoded inside the program

    Explanation:

    Notice that in the above programs that the structure of the student record: class StudentRecord // Format of a student record { String ID; // 10 chars String name; // 30 characters String level; // 4 characters } is embedded inside the code (statements) of the programs: myFile.WriteString( x.ID, 10 ); myFile.WriteString( x.name, 30 ); myFile.WriteString( x.level, 4 ); As a result, the programs can only be used to process data of one specific format !!! In other words: If you want to process data of a different format, you will need to write a different program

  ---

  ---

  ---

  ---

• Processing a data file with a different record format
  • Suppose the student (record) information in the data file is changed to:

    +-------------+----------------+-------------+-------+ | ID | Name | Address | level | +-------------+----------------+-------------+-------+ 6 chars 20 chars 20 chars 4 chars

    The new record structure is:

    class StudentRecord { String ID; // 6 chars String name; // 20 chars String address; // 20 chars String level; // 4 chars }

    ---

    ---

  • Important consequence:

    The program statements inside the computer program used to write the data file must now be: myFile.WriteString( x.ID, 6 ); myFile.WriteString( x.name, 20 ); myFile.WriteString( x.address, 20 ); myFile.WriteString( x.level, 4 ); The statements must access the data exactly as the structure of the data !!! Also, the program statements used inside the computer program used to read the data file must be as follows: x.ID = myFile.ReadString(6); x.name = myFile.ReadString(20); x.address = myFile.ReadString(20); x.level = myFile.ReadString(4);

    ---

  • Here are the new programs that:                                                
    • Prog file that write student records in the new format: click here
    • Prog file that lookup student records in the new format: click here
    • You 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 CreateStuFile2.java   or   javac LookupStuFile2.java To run:          java CreateStuFile2   or   java LookupStuFile2

    ---

    ---

  • Example: I insert the following 4 records in the new format

    Student file content: 123, John Smith, North Street 12, 4 234, James Bond, West Street 89, 3 333, George, East Ln 21, 2 444, Peter Pan, Never Land, 3

    This is how the records are stored:

    0000000 1 2 3 \0 \0 \0 J o h n S m i t h 0000020 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 N o r t h 0000040 S t r e e t 1 2 \0 \0 \0 \0 \0 4 \0 0000060 \0 \0 2 3 4 \0 \0 \0 J a m e s B o 0000100 n d \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 W e s t 0000120 S t r e e t 8 9 \0 \0 \0 \0 \0 \0 0000140 3 \0 \0 \0 3 3 3 \0 \0 \0 G e o r g e 0000160 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 E a 0000200 s t L n 2 1 \0 \0 \0 \0 \0 \0 \0 \0 0000220 \0 \0 2 \0 \0 \0 4 4 4 \0 \0 \0 P e t e 0000240 r P a n \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 0000260 N e v e r L a n d \0 \0 \0 \0 \0 \0 0000300 \0 \0 \0 \0 3 \0 \0 \0

    When you use the old program to do lookup on the student-file, you will get:

    cheung@aruba> java LookupStuFile // use the old program !!! Student file content: 123J, SmithNort, 2 !!!!! 4, James Bond, eet 89, 333Geo, st L n 21, 2444,

    The old program cannot access the fields correctly because their position have moved !!!

  ---

  ---

  ---

  ---

  ---

  ---

  ---

  ---

• Changes to programs when you add (or delete) data fields from the record structure
  • Fact:

    The information need of companies/organizations will change over time

    Conclusion:

    The information structure used by a company may change over time

    ---

  • What to do to adapt programs to use (access) the new record format:

    Modify (= edit !!) the existing programs to handle the new record format Compile all program files so it can process the new record format !!!

  ---

  ---

  ---

  ---

• Physical Data Dependence
  • Physical Data Dependence:

    Physical Data Dependence = the phenomenon where a computer program contains read/write statements that reflects the structure of the data record that the computer program is accessing

    Example:

    When the structure of the data that the program is accessing is: class DataFormat { int x; double y; String z; // 10 characters string } then the program will contain the follow statements to read the data: intVar = file.ReadInt(); doubleVar = file.ReadDouble(); stringVar = file.ReadString(10); And if the structure of the data that the program is accessing is: class DataFormat { double y; // Swapped ! int x; String z; // 10 characters string } then the program must contain the follow statements to read the new data format: doubleVar = file.ReadDouble(); // Must swap statements !! intVar = file.ReadInt(); stringVar = file.ReadString(10);

    ---

    ---

  • Fact:

    A program that exhibit physical data dependence can only be use to access that (one) specific data file format !!!!

  • It also means that when we change the structure of the data , the existing programs cannot access the new data file

    When we change the data format, we will need to change all programs that access the data and re-compile these programs.

    Needless to say that this is a system maintainance nightmare

  ---

  ---

  ---

  ---

• Enter Databases.....
  • Databases were primarily designed to tackle the above short-comings in file processing

  • You will learn in this course how to use (relational) databases to avoid duplication of information

  • You will also learn (next) how databases achieve "Physical Data Independence"

  • And much more....