JxBase

• JxBase - Copyright (c) 2019 Julien Férard (https://github.com/jferard)
• JDBF - Copyright (c) 2012-2018 Ivan Ryndin (https://github.com/iryndin)

JxBase is a fork from jdbf, a Java utility to read/write DBF files

User Guide

Read a DBF file

TODO

See the integration test example

Read a DBF file with MEMO fields

TODO

See the integration test example

Write a DBF file

TODO

See the integration test example

Write a DBF file with MEMO fields

TODO

See the integration test example

Design of an xBase database

The database is a simple set of files.

References

See:

• https://www.clicketyclick.dk/databases/xbase/format/
• http://www.dbase.com/KnowledgeBase/int/db7_file_fmt.htm
• http://devzone.advantagedatabase.com/dz/webhelp/Advantage12/index.html?master_advantage_isam_file_types.htm
• https://dbfread.readthedocs.io/en/latest/field_types.html

The DBF file

The main file. Contains the header and the records.

+------- Header ---------+
|       Metadata         |   <- XBaseMetaDataReader/Writer    
| Field Descriptor Array |   <- XBaseFieldDescriptorArrayReader/Writer
|        Optional        |   <- XBaseOptionalReader/Writer
+------------------------+
|        Records         |   <- XBaseRecordReader/Writer
+------------------------+

DBT file (memo: optional)

This file contains block of data.

+------------------------+  \
|         Header         |   |
+------------------------+   } <- XBaseMemoReader/Writer
|        Records         |   |
+------------------------+  /

NDX file (index: optional)

TODO

Design of JxBase

JxBase aims to build DBF format reader and writer by layers. Each dialect derives an existing format and adds fields, formats, files.

The covariance parameter problem

Whereas a GenericRecordReader has a GenericDialect parameter, a FoxProRecordReader needs a FoxProDialect parameter. A covariant parameter (the parameters type varies with the class type) is a real problem because it violates the Liskov substitution principle: the subclass FoxProRecordReader won't work with a GenericDialect parameter (see https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#Covariant_method_parameter_type). Usually, one solves this problem with generics:

    class GenericRecordReader<D extends GenericDialect> {
        ...
    }
    
    class FoxProRecordReader extends GenericRecordReader<FoxProDialect> {
        ...
    }

Currently, JxBase doesn't use generics for covariance because this would lead to a really complex situation. The fallaback used to achieve the covariance is a simple cast: we accept a GenericDialect and try to cast the instance into a FoxProDialect