Preferences

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Preferences
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Persistent storage

• Information is persistent if it is kept between
  one invocation of a program and the next
• Many programs keep user preferences in a
  persistent fashion
• Sometimes the user sets these explicitly (in,
  say, a dialog box)
• Sometimes these are implicit settings, such as
  window size and position
• To keep persistent information,
• Write it to a file whenever it is changed (or
  when the program quits), and
• Read it in again whenever the program starts up
• Java makes this very easy to do

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Types of preferences

• Java distinguishes user preferences, for a
  particular user, and system preferences, for
  everybody
• Each program may have its own preferences file
• Java saves and restores preferences for a
  particular class
• Preferences are kept in a hierarchical tree
  structure
• We will consider only the simplest version, in
  which preference settings act like a hash table

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Constructing a Preferences object

• import java.util.prefs.public class MyProgram
  ...
• private Preferences userPrefsprivate
  Preferences systemPrefs
• userPrefs Preferences.userNodeForPackage(MyProgr
  am.class)
• systemPrefs Preferences.systemNodeForPackage(MyP
  rogram.class)
• Note that MyProgram.class returns the Class of
  MyProgram

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Saving preferences

• The following are methods of your Preferences
  object
• void put(String key, String value)
• void putBoolean(String key, boolean value)
• void putByteArray(String key, byte value)
• void putDouble(String key, double value)
• void putFloat(String key, float value)
• void putInt(String key, int value)
• void putLong(String key, long value)
• void remove(String key) // remove this
  preference
• void clear() // remove all
  preferences
• void sync() // update
  preferences file now

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Getting saved preferences

• All getter methods must supply a default value,
  in case the preferences files does not exist or
  cannot be read
• The following are methods of your Preferences
  object
• String get(String key, String default)
• boolean getBoolean(String key, boolean default)
• byte getByteArray(String key, byte default)
• double getDouble(String key, double default)
• float getFloat(String key, float default)
• int getInt(String key, int default)
• long getLong(String key, long default)
• String keys()

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How does this work?

• Changes to the Preferences object (via put or
  putXXX) happen automatically--all file I/O is
  done for you
• You only need sync() if you want to force the
  file update to happen immediately rather than
  eventually
• We have treated the preferences file as if it
  were a flatfile however, its actually a tree
  structure
• If you want to know more, go to the API
• The preferences file is in XML format
• Java puts the preferences file in a
  system-dependent location on your computer
• You can export to/import from a particular file
• You can view/edit this XML directly if you like

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Final comments

• As described in these slides, a Preferences
  object is simply a hash table that is magically
  kept between invocations of your program
• As such, preferences are really easy to use
• Preference files are really intended to be used
  for small amounts of persistent data (for
  example, preferences!)
• For large amounts of data, you really should use
  a database instead

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The End