Persistent Store - Save, Read and Overwrite data

Question

I am trying to save 10 String values in PersistentStore in my BlackBerry App. The idea is to save the newest 10 items (String values) at any given time. When the first 10 values are saved and the 11th value is typed, it should:

Delete the first entry
Move the remaining 9 entries above the order
Save the 11th entry as the 10th value

This is the logic I want to follow. As the entries keep increasing, I will store maximum 10 entries which would be the latest 10 values. I tried saving the String values through the saveChatMsg() method:

 public void saveChatMsg()
 {
     if(xx<10)
     {
         PersistentStoreHelper.persistentHashtable.put("chatMsg"+xx,chatToSave);
         xx=xx+1;
         if(xx==10)
         {
             PersistentStoreHelper.persistentHashtable.put("xxValue",Integer.toString(0));
         }
         else
         {
             PersistentStoreHelper.persistentHashtable.put("xxValue",Integer.toString(xx));
         }

     }
 }

where xx is an int that goes through 0 upto 9. However, while this is saving the message, when I retrieve the message, it is not displayed in a chronological order. This method is called at 4 different places and so the 10 messages saved are not in the right order; newest message might appear as the 6th value instead of 10 etc. Kindly comment and advice how to implement.

score 1 · Accepted Answer · edited May 23 '17 at 12:21

1

If you want a list of 10 messages, I would use a Vector for that. Vectors have an order to them, and they are Persistable (*). You can remove the first (oldest) element from the Vector, and add a new one to the end.

It looks like your persistent store keeps one main Hashtable (which is good). Change your persistent model to be like this:

- Hashtable
    - Vector (key = "chatMsgs")
        - String
        - String
        - String
        - String
        - String
        - String
        - String
        - String
        - String
        - String

So, maybe something like this:

public void saveChatMsg(String newMsg) {
    Vector msgs = PersistentStoreHelper.persistentHashtable.get("chatMsgs");
    // add the new msg (to the end of the vector)
    msgs.addElement(newMsg);
    // delete old messages, if the vector is full
    while (msgs.size() > 10) {
        msgs.removeElementAt(0);
    }
    // store the modified vector back to the persistent store
    PersistentStoreHelper.persistentHashtable.put("chatMsgs", msgs);
    // I'm assuming your PersistentStoreHelper calls commit() somewhere in here
}

/** @param index - 0 is the oldest, 9 is the newest */
public String getChatMsg(int index) {
    Vector msgs = PersistentStoreHelper.persistentHashtable.get("chatMsgs");
    return (String)msgs.elementAt(index);
}

Edit:

(*) the BlackBerry API doc I linked to, and the BlackBerry Java Storage APIs documentation, both list java.util.Vector as a Persistable class. So does this answer/comment. However, the actual API javadoc for Vector does not say that it implements Persistable. I'm not in a position to run the code right now, but if a Vector of String objects doesn't work for you, you could always use a subclass of Vector, like ContentProtectedVector, that the API docs explicitly list as Persistable. Post a comment if you wind up needing to do that, for others' benefit.

edited May 23 '17 at 12:21

Community

1
1

answered Mar 31 '13 at 21:37

Nate

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thanks. This is helpful. However, I am encountering an issue with persistent. The string values that are saved as persistent, do not get removed when the App is deleted/uninstalled and then re-installed. Is it ok to ask this here or should I address this issue as a separate question? I ask here because of the String values I am saving to persistent. – Sarah Apr 01 '13 at 04:49
what is "chatMsgs" at this line of code: Vector msgs = PersistentStoreHelper.persistentHashtable.get("chatMsgs"). Do I need to create a vector in my PersistentStore class? – Sarah Apr 01 '13 at 05:34
@Sarah, yes, as you see in my little *picture* above, you should put a Hashtable into the PersistentStore. Inside the Hashtable should be a Vector, and inside the Vector should be 10 Strings. So, wherever you initialized your PersistentStore, you would insert a Vector for the key `"chatMsgs"`, instead of storing 10 String objects, all with their own key (`"chatMsg00"`, `"chatMsg01"`, etc.) – Nate Apr 01 '13 at 06:34
@Sarah, regarding your first question, [see the answer here](http://stackoverflow.com/a/9034232/119114), or [on blackberry.com here](http://supportforums.blackberry.com/t5/Testing-and-Deployment/Handle-stored-data-when-removing-an-application/ta-p/444832). If you want it deleted, make your Hashtable a subclass of your own (e.g. `public class SarahsHashtable extends Hashtable implements Persistable {`). – Nate Apr 01 '13 at 06:39
I have never worked with vectors in persistent store. Is "chatMsgs" the long value of key? Does this make sense; 'public void saveChatMsg1(String newMsg) { final long KEY = 0x9df9f961bc6d6daL; Vector msgs = (Vector) PersistentStoreHelper.persistentHashtable.get(KEY); // add the new msg (to the end of the vector) if( msgs == null ) { msgs = new Vector(); PersistentStoreHelper.persistentObject.setContents(msgs); PersistentStoreHelper.persistentObject.commit(); } msgs.addElement(newMsg);' – Sarah Apr 01 '13 at 08:02
@Sarah, I can't really comment on that, because I have no idea what your `PersistentStoreHelper` class does. Normally, you must first use the `long KEY` to call `obj = PersistentStore.getPersistentObject(KEY)`. Then, once you have the `PersistentObject`, you call `obj.getContent()`. In your case, `getContent()` should return a `Hashtable`. That doesn't change from your original code. What changes is that you shouldn't store strings directly in the Hashtable, by name/key (like `"chatMsg00"`). You should store a **Vector** in the Hashtable, and then put Strings into the Vector. – Nate Apr 01 '13 at 08:15
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/27325/discussion-between-sarah-and-nate) – Sarah Apr 01 '13 at 12:04
thanks for the explaination, I am able to get through with this.Regarding the links you provided for deleting the values from persistent store when the App is deleted/uninstalled. Please note that I am working on similar lines where I have a class public class PersistentStoreHelper extends Hashtable implements Persistable but this has not helped and the values are not deleted. Would you be able to comment on this or should this be posted as a [new question](http://stackoverflow.com/questions/15743380/delete-persistent-store-data-when-app-is-uninstalled-deleted). – Sarah Apr 01 '13 at 12:37
yes, please post a new question and show us the PersistentStoreHelper class. thanks! – Nate Apr 01 '13 at 18:41

Persistent Store - Save, Read and Overwrite data

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