sample blackberry persistent store usage

package mypackage;

import java.util.Hashtable;

import net.rim.device.api.system.PersistentObject;
import net.rim.device.api.system.PersistentStore;
import net.rim.device.api.util.Persistable;

/**
 * Use this class like so:
 * <code>
 *    PersistentStoreHelper store = PersistentStoreHelper.getInstance();
 *    Vector msgs = (Vector)store.get("chatMsgs");
 *    if (msgs == null) {
 *        msgs = new Vector();
 *        store.put("chatMsgs", msgs);
 *    }
 *    msgs.addElement("this is a new chat message");
 *    store.put("someOtherKey", someOtherValue);
 *    // commit will save changes both to the msgs list, and to `someOtherValue` added
 *    store.commit();
 * </code>
 */
public class PersistentStoreHelper implements Persistable {  // only Persistable so inner class can be!

   private static final long KEY = 0x9df9f961bc6d6daL;
   /**
    * persistentHashtable is now an instance of a class that
    * only exists in your app
    */
   private MyAppsHashtable persistentHashtable;
   private PersistentObject persistentObject;

   /** PersistentStoreHelper is a singleton */
   private static PersistentStoreHelper instance = null;

   /** call this to get access to the methods of this class */
   public static PersistentStoreHelper getInstance() {
      if (instance == null) {
         instance = new PersistentStoreHelper();
      }
      return instance;
   }

   /** constructor private to force users to call getInstance() */
   private PersistentStoreHelper() {
      persistentObject = PersistentStore.getPersistentObject(KEY);
      if (persistentObject.getContents() == null) {
         persistentHashtable = new MyAppsHashtable();
         persistentObject.setContents(persistentHashtable);
      } else {
         persistentHashtable =
               (MyAppsHashtable)persistentObject.getContents();
      }
   }

   /** use this method to retrieve a saved value from the store, by its key */
   public Object get(String key) {
      return persistentHashtable.get(key);
   }

   /** use this method to add a value to be saved in the store */
   public void put(String key, Object value) {
      persistentHashtable.put(key, value);
   }

   /** call this when you want to make sure all recent changes are saved to device storage */
   public void commit() {
      persistentObject.setContents(persistentHashtable);
      persistentObject.commit();
   }

   private class MyAppsHashtable extends Hashtable implements Persistable {
      // don't need anything else ... the declaration does it all!
   }
}