How to run a method only once in Blackberry Java? - java

I am developing an application in Blackberry. I have to write a code or a method that runs only once. The method does an action on creating the database. Something similar to Oncreate method in Android. Help of any sort is appreciated.
A Y.

Create a singleton class and call the method from singleton class private constructor, or maintain a global flag and check that flag on method call.
There are many other ways but calling of a method is on you, and you can do any way for this.

If you want to run that code in single time of application life then,
use persistent store and add one flag, change the flag value after successfully running that code .
Both BlackBerry and Android provide the Persistent storage.

Related

Android - Static class method calls to server

Background -
I have an app that currently has a lot of methods in MainActivity to pull and upload data to firebase. It's quite a mess with the other app logic in there..
Problem -
I would like a separate class filled with server (firebase) access methods that can be called from any activity or fragment and run asynchronously. The class also needs access to context and be able to initialise and store variables.
PS. I'm using kotlin if that helps at all
Edit:
Have done more researching to find terms like "utility" and "static" classes which seems like an accurate way to go... If I create a static utility class with Async methods, will that achieve what I'm after? Also, can you initialise and hold variables in a static class?
Thanks :)
Solved
After more research and testing, I created an object that holds my methods and variables and just need to pass context into the relevant methods. Then just simply call objectname.methodname()

Is this a good reason to use a Singleton?

I'm making an Android app that will have the timetables of a local bus.
There are more than one timetable, the one that will be use depends on the day.
If it's a holiday I must use a special timetable, so I want to know when is a holiday and when is not.
The thing is that I'm creating a class that will handle this, it will try to retrieve information from memory or from a web api. Then some other classes will be able to communicate with this class, but it doesn't seem necessary to me to have more than one instance of this class, I could create just one instance and share it with the rest of the classes.
Could this class be a Singleton or it would be better if I create a normal class ?
In your case (retrieving info from memory), definitely avoid using a singleton class because it will highly likely be tied to your Activity context.
Your class will have a static reference to a class, therefore
it will be kept in memory when not needed.
singleton may be reinstantiated, or may use obsolete instance, with new instations of activities. You will lose control of the current variables.
diffent instances of the same activity class are highly likely to conflict with this class.
Examples of the same activity class several instantiation:
Change device orientation.
Running app from the webbrowser's, Google Play, file browser intent.
Besides, at some point, when you add functionality based on user reviews, your app will grow, you are likely want to refactor your class, break it into subclasses, put some of its methods into separate threads. It will no longer be easy to do.
It might seem fun while the app is small, and untested, but later, in Android specifically, you will run into a nightmite with unpredictable and hard to detect errors.
Because of Android's special way to recreate activity class, through onCreate, onResume etc. you will run into a nightmare, when the app will start living its own life.
You will no longer be able to rely on the assumption that the current singleton instantiation actually belongs to your current activity.
You may swap between orientations or run your app from different entry points (launcher, recent apps, google play), and it may reuse the variables actually prepared for a different activity instantiation.
If you need only one instance of the class, just create one instance of the class in the onCreate method - and that will make the app much more manageable.
One of the main advantages a Singleton class brings you is the fact that you are sure to have one and only one instance of an object doing some thing, and that it is instantiated only once (preferably at a specific point of your application, for instance at startup or only after certain other operations have been performed)
An example could be for instance a cache implementation: you want to make sure that all classes that need a certain cache read and write from the same object, that maybe is created and filled with information at startup time.
Your does not seem to be the case, unless you fetch the information you need when your application starts and then you keep them memorized for some reason: in this case you want to make sure your information is fetched one and only one time, to avoid wasting memory and elaboration time. Also, a Singleton is fine if you need to do some kind of operation when your class is instantiated, like opening a connection that then stays open.
On the other hand, if you just need a class with some method to call some external apis or database and you don't need to memorize any information in it, there is no reason to initialize a singleton.
If this is your case, why don't you try some static class/methods? They can be called like normal methods directly on the class with no need to instantiate objects or keeping a state, saving memory and avoiding side effects.

sending a stream of data from the main activity to a thread in a different class

Currently I have an app which has code in its main activity that reads data from an xbee. My problem is that I want to relay this data to 4 different threads in 4 different classes throughout my project. I looked into bundling it, but that seems like a one time data transfer, not a stream of it.
One idea I had was to write the data to a string called messages and then have a getMessages() function, but I ran into a problem calling a static method from a non-static class or vise versa.
Have you considered using SharedPreferences or extending Application? SharedPreferences will let you store basic types (String, int, boolean, etc) in persistent storage. Application can be extending to store variables / methods that can be accessed anywhere in your program. For example, MyApplication app = (MyApplication)getApplicationContext();
Are you looking to modify this stream of data in each of your threads, or simply read a value and modify data separately? You may want to setup some boolean flags to ensure you aren't accessing / modifying data that isn't safe to perform those operations on. Hope that helps! Let me know if I can provide a code example for clarity.
one way of doing it to simply make your main activity write into android.database.sqlite and others will simply read from it
since not sure how your code layout is I just refer you the documetation page :
http://developer.android.com/reference/android/database/sqlite/package-summary.html
You can create one handler for each of your threads and then post messages on all the handlers. Below is the link which creates an Handler and post messages from the handler reference:-
using a Looper in a Service is the same as using a separate thread?
You can implement observer pattern to solve this.Make the four classes as observers and the activity as subject.
Expose a api like onDataChanged(byte[] data) which will be called whenever your activity has some new data.
To learn more about observe pattern refer-https://en.wikipedia.org/wiki/Observer_pattern
You can also have a look at classic producer consumer problem if you want synchronization https://en.wikipedia.org/wiki/Producer%E2%80%93consumer_problem

Access a static variable from a service

My application has a service and my MainActivity has a static variable :
public static boolean appIsPlaying = false;
And my service code is something like this :
MainActivity.appIsPlaying = false;
This code works well but I'm not sure that it's a true way. So, would you help me if there is a problem with this solution?
Thanks
I don't really agree with the people who say let's just use getters and setters for everything, and that's the solution. Your solution is simple, but effective. There are no problems with this way of communication between a service and an activity per se. This isn't some public API, so using public fields isn't a definite no-no.
Problems can only arise if you have certain requirements which you have to fulfill. This from of interaction is the simplest, therefore it can't do much. For example, if you want to listen to the event of changing the value, then you will have a problem because you just set the value and don't notify the activity.
If that's the case, then you can bind to the service, and implement more complex interaction. Listening to the value changes could be done by storing listeners in the service and notifying them when changes happen.
There is not really the way to do it, there are more appropriate ones and less so. You have to choose one of them depending on your needs.
Put your static variable in a utility class and create static getters and setters with required validations. Making the class Single Instance is also preferred but what you are doing right is not without problems.

Can java access global events created using CreateEvent

I am trying to access a global event created by native code in my java client. I am using JNA for this purpose to call OpenEvent method of kernel32.dll. But the method always returns NULL and GetLastError returns 2, which is File not found.
So I was wondering if JVM can see these global events and if so is there any other approach I can use?
--
Vinzy
How do you call your openEvent?
I suppose it's something like this
int result = kernel32.OpenEvent( 10000, false, "Global\\nameOfEvent" ); //request for deletion
with the only difference you may be using objects as arguments, which, I suppose, is a matter of preference.
Maybe if you provide the code for the call we might be able to help you. Another thing to be asked is if you call CreateEvent in your native code somewhere. If you dig into the Windows API, you will notice that:
"The function succeeds only if some
process has already created the event
by using the CreateEvent function."
source : http://msdn.microsoft.com/en-us/library/ms684305(v=vs.85).aspx
Which in your situation mean you will be in a lot trouble if you were not the one creating the event. There is a way of obtaining a handle to an event you did not create but it's a bit more complicated and let's start by you providing a bit more information.
Cheers.
To sum it up:
If you don't call CreateEvent anywhere in your code you will have trouble when calling OpenEvent. To escape this problem you would basically have to find which process/thread holds the lock to the event and make it give it to your thread (the jvm's).
If you do call CreateEvent in your code then you should not have any problems obtaining a reference to your event and the culprit is somewhere else.
In any case, a bit more code would be nice.

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