org.gjt.qcis.persist
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Sub-framework for managing persistent objects - those that can be stored within a file system or transmitted over a network. Internally, a persistent object is essentially just a changeable one for which readers and writers have been registered. After explaining the basic typing concepts for persistent objects, and some interface considerations, we explain the approach this package takes to supporting persistence. To understand this package, you have to comprehend three notions of 'type' for a persistent object. - The implementation type - synonymous with the Java class used to represent the persistent object during execution.
- The external type -- the format in which the persistent object was (or will be) represented in the file system. This type is represented by a Java class defining a reader/writer pair, which implicitly define a file format.
- The file type, a somewhat confusing term synomymous in this case with file extension -- the list of characters after some dot in the file name. For simplicity, this version of the package assumes the last dot, but if we were ever to handle UNIX .tar.gz files, that might require some prior dot instead.
- The file filter type -- the list of file types expected by a specific file filter in the user interface. There are two special cases:
- A list of varying lengths of file extension, meant to represent the same external type but allowing for operating system limitations, such as ".tif, .tiff" or ".htm, .html".
- A list of arbitrary file extensions grouped for some purpose related to user interface design, e.g. "Image files: .jpg, .gif"
In an ideal world there would be a one-to-one correspondence between these things. Unfortunately, this turns out not to be the case. - The same implementation type and file type pair may correspond to several external types. Thus version 6 of a word processor may require several writers to emit earlier versions of the format.
- The same file type might correspond to different implementation type and external type pairs. Thus a file with extension .DOC might be an MS Word file, or a text file emitted by a UNIX document formatter.
- A user might insist on controlling the complete name of a file, and thus have an arbitrary apparent file type.
Swing provides a JFileChooser class, and an ExampleFileFilter class, that make it easy to associate a set of file filters with a file choice dialog, where each filter can recognize an arbitrary list of extensions, and the user can choose which specific filter to apply.
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