Java: Preferences

The java.util.pref.Preferences class, which was added in Java 1.4 to provide improved functionality over the java.util.Properties class, is used to store and get persistent (remains on disk between program executions) hierarchical name/values pairs. These preferences are stored in an operating system dependent manner, eg in the Windows registry, a Mac preferences file, or in a Unix do file. Both system and user preferences can be obtained.

The method of using Preferences described below gets and maniupates a Preference object associated with the class of the current application, which gives each program a unique set of preferences.

Description

The following is only a summary of a few methods that use String values and are all at the level of the current program. Preferences can be hierarchical and support primitive values in addition to String.

java.util.pref.Preferences - Used to maintain persistent key/value pairs.
prefs = Preferences.userNodeForPackage(this.getClass());
Gets or create a new Preferences object associated with this program.
prefs.clear(); Clears all preferences for this Preference object.
prefs.flush(); Writes preferences to persistent storage (eg, disk). Not needed if program terminates normally. Can throw java.util.prefs.BackingStoreException exception.
prefs.set(key, s); Associates string value s with string key key
sval = prefs.get(key, defVal); Returns value associated with String key or the default value, defVal, if there is no value for that key.
keyArray = prefs.keys(); Returns String array of all keys in the current Preferences object.
prefs.remove(key); Returns value associated with string key.

Example

The following program illustrates the getting and saving of user name/value pairs. 
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/**
  * PrefTest.java - A simple program to top-level user preferences.
  * @version 2004-04-18 Rodenbach
  * @author Fred Swartz
*/

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

import java.util.prefs.Preferences;
import java.util.prefs.BackingStoreException;

/////////////////////////////////////////////////////////////////// PrefTest
class PrefTest extends JFrame {
    private JTextField _nameField = new JTextField(8);
    private JTextField _valueField= new JTextField(8);
    private JList      _prefList  = new JList(new String[] {});
    private Preferences _prefs;
    
    //========================================================== constructor
    PrefTest() {
        _prefs = Preferences.userNodeForPackage(this.getClass());
        setListFromPrefs();
        
        //-- Create and set attributes of widgets.
        JScrollPane scrollingList = new JScrollPane(_prefList);
        JButton newKeyButton = new JButton("Put Key/Value");
        JButton clearButton = new JButton("Clear All");
        JButton delSelectedButton = new JButton("Remove Selected");
        
        //-- Set action listeners.            
        newKeyButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                if (_nameField.getText().length() > 0) {
                    _prefs.put(_nameField.getText(), _valueField.getText()); 
                    _nameField.setText("");   // Clear fields after saving.
                    _valueField.setText("");
                    setListFromPrefs();  // Update display
                } else {
                    _nameField.setText("Key?");
                }
            }});
            
        clearButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                try {
                    _prefs.clear();
                    setListFromPrefs();  // Update display
                } catch (BackingStoreException ex) {
                    System.out.println(ex);
                }
            }});
            
        delSelectedButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                String selection = (String)_prefList.getSelectedValue();
                if (selection != null) {
                    String key = selection.substring(0, selection.indexOf("="));
                    _prefs.remove(key);  // Remove the selected key.
                    setListFromPrefs();  // Update display
                }
            }});
        //-- Layout widgets.
        JPanel buttonPanel = new JPanel(new FlowLayout());
        buttonPanel.add(newKeyButton);
        buttonPanel.add(new JLabel("Key"));
        buttonPanel.add(_nameField);
        buttonPanel.add(new JLabel("Value"));
        buttonPanel.add(_valueField);
        buttonPanel.add(delSelectedButton);
        buttonPanel.add(clearButton);
        
        JPanel content = new JPanel(new BorderLayout());
        content.add(buttonPanel  , BorderLayout.NORTH);
        content.add(scrollingList, BorderLayout.CENTER);
        
        this.setContentPane(content);
        this.pack();
    }//end constructor
    
    
    //===================================================== setListFromPrefs
    private void setListFromPrefs() {
        try {
            String[] keys = _prefs.keys();
            for (int i=0; i<keys.length; i++) {
                keys[i] += "=" + _prefs.get(keys[i], "ERROR");
            }
            _prefList.setListData(keys);
        } catch (BackingStoreException ex) {
            System.out.println(ex);
        }
    }
    
    //================================================================= main
    public static void main(String[] args) {
        JFrame window = new PrefTest();
        window.setTitle("PrefTest");
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.show();
    }//end main

}//end class PrefTest
Copyleft 2003 Fred Swartz MIT License, Last update: 2002-05-01