Question

I’m developing a j2me application based on codenameone. I implements some codes and Now I wanna add database to my application. After search a lot I found Storage class of codenameone that simplify database concept in mobile application.

In this appliaction I create a class for each entity (like person, city, …) and add “read” and “write” method to read and write data.

Some entity classes have 2 or more fields. So I must save and read them with Storage class.

How can I do this?

Here is my sample code:

package com.x.database;

import com.codename1.io.Storage;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Hashtable;
import java.util.Iterator;

public class Person {

    public Person(){
    }
    public Person(int pID, String pPersonNumber){
        ID = pID;
        PersonNumber = pPersonNumber;
    }

    public static String PERSON = "Person";

    private Storage store;
    private int ID;

    public int getID(){
        return ID;
    }

    public void setID(int pID){
        ID = pID;
    }

    private String PersonNumber;

    public String getPersonNumber(){
        return PersonNumber;
    }

    public void setPersonNumber(String pPersonNumber){
        PersonNumber = pPersonNumber;
    }

    public int getLastKeyNumber(){
        if(store == null) {
            store = Storage.getInstance();
        }

        Hashtable depHash = (Hashtable)store.readObject(PERSON);
        ArrayList<String> keyArray = (ArrayList<String>)depHash.keys();

        int i = 0;
        for (Iterator<String> it = keyArray.iterator(); it.hasNext();) {
            int tmp = Integer.parseInt(it.next());
            i = i < tmp ? tmp : i;
        }

        return i;
    }

    public void write(Person pPerson){
        if(store == null) {
            store = Storage.getInstance();
        }

        if(!store.exists(PERSON)) {
            Hashtable depHash = new Hashtable();
            try {
                depHash.put("0", pPerson);
            } catch (IOException ex) {
                System.out.println(ex.getMessage());
            }
            store.writeObject(PERSON, depHash);
        }
        else {
            Hashtable depHash = (Hashtable)store.readObject(PERSON);

            ArrayList<Person> depArray = (ArrayList<Person>)depHash.keys();

            for (Iterator<Person> it = depArray.iterator(); it.hasNext();) {
                Person tmp = it.next();

                if(!tmp.getPersonNumber().equals(pPerson)) {
                    depHash.put(String.valueOf(getLastKeyNumber()), pPerson.getPersonNumber());
                    store.writeObject(Person depHash);
                }
            }
        }
    }

    public ArrayList<Person> readAll(){
        Storage store = Storage.getInstance();

        if(!store.exists(PERSON)) {
            Hashtable depHash = (Hashtable)store.readObject(PERSON);

            return (ArrayList<Person>)depHash.elements();
        }
        return new ArrayList<Person>();
    }
}

In this code I have an error on write and read object on Storage.

How can I write one object in Storage and read it again?

Thanks in advance.

Answer 1

You can store a Vector or a Hashtable in storage and nest them as deeply as you like e.g.:

Vector v = new Vector();
Hashtable content = new Hashtable();
content.put("A", ...);
content.put("B", ...);
v.addElement(content);

Etc… You can add more hashes and they can contain Strings, numbers or byte arrays.

Alternatively you can implement the Codename One Externalizable interface which allows you to write an arbitrary class to storage. This poses one requirement though, you need to call Util.register in order to register the externalizable class name. There is a sample of this in the tipster demo.