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In android whenever we have to subclass an application object or extend it, in the following way:

public class AndroidApplication extends Application{
....
}

and then make the necessary changes in the manifest file. what is the need to write this line of code?

public class AndroidApplication extends Application{

private static AndroidApplication sInstance;
 public static AndroidApplication getInstance() {
  return sInstance;
}
....
}

Whenever you are going to access your Application object you are going to access it using the following code:

MyApplication myapp = (MyApplication)getApplicationContext();

The above is always going to return one static object of your Application class. What is the need to write the getinstance method? I have read it on several blogs and on android developer website that the above is the correct way of doing things buy i have never understood why?

The Application object is created in memory when the Application starts running. After this you cannot reinitialize it, meaning you cannot write this code:

MyApplication myapp = new MyApplication();

And even if you did you, the getInstance method would not help.

Thank you in advance.

user590849
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1 Answers1

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The single design pattern (also in a more general context, this design pattern is not really a good idea..) is well suited for the application object in android :

You are right, anyhow, this class will be instanciated once by the framework.

But a dev could still want to instanciate it, as you propose, using the new key word. And here, you seem to be missing something as the singleton design pattern doesn't only include a getInstance method, it also includes the definition of a private constructor, preventing the possibility of instanciating the class outside of the get instance method.

So, yes, the singleton pattern can be apllyed here and it's a good idea, just to enforce the fact that no developper should instanciate this class, the only instance will be created by the android framework.

Snicolas
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  • ok, that makes sense. But is there any other reason that i should not instantiate my class? – user590849 Feb 07 '12 at 14:06
  • The application lifecycle is given by the android framework. It would not make much sense to create a new object of this class as you could not bind its lifecycle to the lifecycle it has been designed to live in. – Snicolas Feb 08 '12 at 07:35