Blackberry Application design question - Singletons

I'm refactoring a Blackberry application and I have a scenario where I think I'm currently using a global variable, but I'm not sure if that's the right thing to do. Briefly, my scenario is the following -

My app first requires the user to login. The (uid, pass) are sent to a web service which determines if the login is valid and returns some additional data. I开发者_如何转开发 have a model object on my application that looks something like this - (After a succesfully calling login)

class UserDataModel
{
    private String username;
    private String password;
    private String fullName;
    private String age;
    ... 
    /* Getters and Setters */


}

I also have a UserPreferencesModel which contains all the preferences that the user has saved. (I need to back them up to our database / restore them across devices etc.)

Additionally, in what context are Globals generally used in the context of mobile development?

Thanks,

Teja.

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Well, I made a simple example how you can to use the RuntimeStore, I hope that this be of helpful

public class myData
{
long ID = 0xf46f5a7867d69ff0L;
String d1;
RuntimeStore runTS = RuntimeStore.getRuntimeStore();

public void setData(String _d1)
{
    try
    {
        syncronized (runTS)
        {
            runTS.put(ID, _d1);
        }
    }catch(Exception ex){}

}

public String getData()
{
    String s;
    try
    {
        s = (String)(RuntimeStore.getRuntimeStore().get(ID));   
    }catch(Exception ex){}
    return s;
}
}

---

There is nothing particularly special about BlackBerry in regards to using singletons. Of course, true constants should be just statics. And all of them should be final but Strings: there is a memory usage penalty if a static final String is reused often in your code.

What singleton gives you is the ability to replace or remove complex models with relatively long lifetime via a single point of control. In your example, DataModel is a good candidate. BlackBerry is a personal device, so there is a big chance this DataModel with user profile and, probably, additional data, will survive for the lifetime of the active application. So,

class UserDataModel
{
    private static UserDataModel singleton;

    public static void login() { 
      //get credentials
      //authenticate
      singleton = new UserDataModel(... user profile data...);
    }

    public static UserDataModel getInstance() { return singleton; }

    private String username;
    private String password;
    private String fullName;
    private String age;
    ... 
    /* Getters and Setters */


} 

This way of doing it is a valid, a little simplified, example. If something changes (say, server host), all you need to do is to replace singleton. Also, it opens up a possibility to use polymorphism, if UserDataModel implementation is different for different servers, etc. There are many benefits to it at the cost of one extra variable in a chain of accessors. Again, there is nothing special about BlackBerry here, this reasoning is valid in any Java application.

Why the example is simplified is because you need to think about threads. If there is even a remote chance that something somewhere will access getInstance() on a different thread than login(), you have to properly synchronize them (even though I was never able to break a simple object reference by accessing/updating it from different threads on BlackBerry).

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their are some scenarios when having static variable is good idea. like for Constant String fields.

here is the link to blackberry official Best practice document for writing efficient code for blackberry platform.

Black Berry: Best Practices: writing efficient code