Develop Reference

Android Application without Classes?

I have seen a JS talk about Redux recently. The speaker programmed a working application without any classes, just with const variables. He even excluded classes in his lint file (https://github.com/MartinSeeler/redux-presentation/tree/master/demo).
So I tried to do the same in my Android apps, which are written in Kotlin. Unfortunately, I get the following error when I try to make my MainActivity class an object:
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{blog.app.niklas.flyingbytes/blog.app.niklas.flyingbytes.MainActivity}: java.lang.IllegalAccessException: void blog.app.niklas.flyingbytes.MainActivity.<init>() is not accessible from java.lang.Class<android.app.Instrumentation>
Caused by: java.lang.IllegalAccessException: void blog.app.niklas.flyingbytes.MainActivity.<init>() is not accessible from java.lang.Class<android.app.Instrumentation>
Is there a way to work around this? I really like the idea of having as many singleton objects as possible in my app.
Thank you in advance,
Niklas

You can't replace Activity class by kotlin object on Android, for the same reason you don't instantiate Activities by calling it's constructor:
activities are managed by the platform.
When you ask (via an Intent) the platform for an Activity, the platform will try to instantiate it by calling it's constructor with zero parameter. Which doesn't exist for an object, giving the mentioned error.
That's said, some developer dodge around the frame given by Android, like mortar, by having only one activity and using custom logic for navigation between views.

As Geoffrey said, the Android platform manages your activities, but that also means that the platform is responsible for keeping track of your Activity's instances - which is the purpose of Singleton.
So, while you can't make your Activity an object, you can still make it a Singleton by using the android:launchMode attribute in your AndroidManifest:
<activity
android:name=".MainActivity"
android:label="#string/app_name"
android:launchMode="singleInstance">
For more information on the launchMode attribute (including other possible values), check out the Official Documentation

application's name attribute - What is it for?

Inside my android manifest file I have an application tag and inside is name property/label.For what is this used for ?

If you are referring to the android:name XML attribute, then, quoting the documentation, it is:
The fully qualified name of an Application subclass implemented for the application. When the application process is started, this class is instantiated before any of the application's components.
The subclass is optional; most applications won't need one. In the absence of a subclass, Android uses an instance of the base Application class.

Application class in the Android is used for global settings or running entrance.
In the Android reference it describes the Application class: "Base class for those who need to maintain global application state. You can provide your own implementation by specifying its name in your AndroidManifest.xml's tag, which will cause that class to be instantiated for you when the process for your application/package is created." more

Class extending Application in Android project library?

I have a project (in Eclipse) which I've turned into an Android Project Library so as to re-use some of the code in another similar project. I think I've shot myself in the foot however as I'm getting the error:
Unable to start activity ComponentInfo{com.test.scroller1/com.lib.scrolltest.ScrollTestActivity}: java.lang.ClassCastException: android.app.Application cannot be cast to com.lib.scrolltest.resAppVars
com.lib.scrolltest is my Project Library which instantiates a class extending Application (resAppVars). In the onCreate() method I call:
mRav = (resAppVars) getApplicationContext ();
This way, I can use methods in the mRav object which would otherwise be a lot of duplicated code in other classes (such as passing a query to a generic select statement which returns an ArrayList of results).
What's the problem here? It seems I've hit a limitation in the way I've implemented the Application class.

Calling getApplicationContext() returns the Application object for the current application (i.e. the application that owns the activity that onCreate() is running inside of).
Unless you're doing something strange, you don't get to pick which Application class is used. There's even a note in the documentation for Application saying not to do this:
There is normally no need to subclass Application. In most situation,
static singletons can provide the same functionality in a more modular
way. If your singleton needs a global context (for example to register
broadcast receivers), the function to retrieve it can be given a
Context which internally uses Context.getApplicationContext() when
first constructing the singleton.
You should just create a regular shared class inside of your library project. Or if you don't have a need for the special functionality library projects offer, you can also just use a regular .jar file.
And if you need shared state, just make it a singleton. ;)

Although this is a very old post. I encountered the same problem and solved it. So I thought I'll post the solution for everyone.
It turns out that I forgot to declare the subclassed application name in the manifest file.
The android:name should point to the extended app class, even if it is defined in the referenced library.
<application
android:allowBackup="true"
android:icon="#drawable/ic_launcher"
android:label="#string/app_name"
android:theme="#style/AppTheme"
android:name="com.example.lib.MyApp">
After I added that I could use the extended app with (<cast>) getApplication() anywhere in my project.

Android Application sub-class constructor not being called

I have a class in my Android application that sub-classes the AndroidApplication object. The documents say:
public void onCreate ()
Since: API Level 1 Called when the
application is starting, before any
other application objects have been
created. Implementations should be as
quick as possible (for example using
lazy initialization of state) since
the time spent in this function
directly impacts the performance of
starting the first activity, service,
or receiver in a process. If you
override this method, be sure to
call super.onCreate().
I placed a breakpoint on my sub-class's constructor and when I run my application, it is never reached. Naturally, when I call the sub-class's getInstance() method from other code it returns NULL since the instance variable is (supposed to be) initialized when the constructor is called.
Can anyone tell me what is wrong? I would assume from the docs that I don't have to create an instance of the AndroidApplication sub-class myself, or do I? Am I supposed to modify my manifest file somehow to add the AndroidApplication sub-class and if so, how?
-- roschler

I'm posting the answer here for others. Yes you need to add the name of your Application object sub-class's name to the Android manifest. For Eclipse users, the easiest way to do this is to open the AndroidManifest.xml file, select the Application tab in the manifest editor, and use the Browse button next to the Name field to find your Android Application object sub-class name and select it. The manifest file will be updated properly to register it. I just did that and it worked.

I had a problem of not having a . before my application class name.
Should be:
android:name=".MyApp"
since the MyApp class is in the package defined in the manifest.

Android: this.getApplication() returns NULL pointer

I'm running the following line in an Activity, which is within the same application, but in a different package:
AppObject appObj = (AppObject)this.getApplication();
// FYI: AppObject is my extension class of Application.
It returns only a null pointer, while when I move it to the "main" package and run it from there it returns the application reference as expected.
I've defined the activity in my AndroidManifest.xml with the full qualified class name, since it is in another package: <activity android:name="com.foo.bar.TestActivity"></activity>
Update: As suggested in a question below android:name="AppObject" was already in the <application> tag of the AndroidManifest.xml

It's important to call getApplication() in the activity's onCreate() method, not in the constructor.

You need the update application tag to AndroidManifest.xml with your class name, which is extended from Application, with proper package name.
<application android:name=".AppObject">
As per Application tag google docs, Base class for those who need to maintain global application state. You can provide your own implementation by specifying its name in your AndroidManifest.xml's tag, which will cause that class to be instantiated for you when the process for your application/package is created.
here

Just run into the same thing, having refactored all my code still got the same issue, noticed that I was setting the local mApplication variable in the constructor, it should go in the onCreate(), I think all the objects in the manifest may be constructed first before getApplication() is setup so you need to call getApplication() in or after onCreate() has been called. Haven't refactored all my code back again to see if this works for different packages (sigh).

I think, it isn't a null pointer, but your function which you want to use next in the class AppObject maybe wrong.