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File Processing

File Processing. You've achieved success in your field when you don't know whether what you're doing is work or play. Warren Beatty (1937 - ). Classes. Classes entities within a system imported e.g. Turtle of same project write more than one class Instances objects created via new

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File Processing

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  1. File Processing You've achieved success in your field when you don't know whether what you're doing is work or play. Warren Beatty (1937 - )

  2. Classes • Classes • entities within a system • imported • e.g. Turtle • of same project • write more than one class • Instances • objects created via new • Instance variables • object’s memory • state • Methods • things object can do • behaviour • System execution via interacting objects

  3. Methods • Method calls • local • methodname ( paramlist ) • of another object • object . methodname ( paramlist ) • this . methodname ( paramlist ) • Behaviour depends on state • e.g. effect of forward depends on pen state and direction • methods refer to instance variables • can base actions on values of instance variables • loops, decisions • Role of constructor • well-defined initial state

  4. Case Study: Payroll System • Problem • Payroll report based on multiple binary data files • Analysis & design • entities • items (Employees) • details about item • Payroll report generator • process for generating report • Merge algorithm • Combine 2 binary data files into one.

  5. Employee Class • Class • not a main class • no method main • Execution • only when methods (constructor) called • Attributes/instance variables • Algorithms/methods • constructors • default • methods • accessor/updater • operations

  6. Persistent Objects • Lifetime extends beyond a single program execution • Binary I/O • Record (object) at-a-time • Serializable • serialVersionUID • BinaryOutputFile • writeObject() • subtype • upcasting to Object • always valid • BinaryDataFile • readObject() • must downcast to expected type • not a conversion • ClassCastException

  7. Creating a Binary File • MakeEmpFile class • only used once to bootstrap system • Employee records • data from a text file (ASCIIDataFile) • reads text record using Employee constructor • writes Employee object using writeObject

  8. Listing Contents of a Binary File • Working with internal representation • cannot view using text editor • must use same Employee class • or won’t be able to read • don’t make copy, use same file • programs in same package or import necessary classe(s) (e.g. Employee) • can be more than one main class in a package • Listing Employee File • program to read binary file and display as text • in same package (Payroll_System) • Creating the Employee File • Payroll3 assumes Employee objects are already in existence • must be a program to create Employee objects initially • in same package (Payroll_System)

  9. Payroll3 Class • Binary input • note: no use of Employee constructor – objects already exist • Processing algorithm • process to EOF • update record

  10. Class Design • Cohesion • Selective disclosure • only what client needs to know • hide representation and implementation • Accessor and updater methods • instance variables • private • if accessible, provide accessor method • Java convention: getxxx • if updateable, provide updater method • Java convention setxxx • pseudo attributes • Methods/Constructors • private vs public

  11. File Processing • large collections of data • sequential processing • batch • sequential devices • typically sorted by key • random processing • real-time/on-line • direct access device • access by key • key • primary key

  12. File Merge • combine two like files • maintain key order • basic algorithm • by hand • combine sorted piles • rationale • by computer • files & records • algorithm • improvements • sentinel • pattern • filter method

  13. E.g. Employee File Merge • HIGH_KEY • key for sentinel record • greater than all naturally occurring keys • termination condition • getRec • input filter • creates sentinel object at EOF • Equal keys? • unique records? • error situation vs duplicate keys vs combine records

  14. File Update • master file • transaction file • transactions • updating • random processing • immediate • sequential processing • batch transactions • batch processing • new master file

  15. Sequential File Update • sequential processing • batched transactions • backups • report generation • key comparison • master < trans • transaction applies to later master • master = trans • transaction applies to this master • master > trans • must apply to earlier master • no master for transaction • error • add new record • pattern

  16. The End

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