Question

I don’t really get the idea behind how this whole thing works really, so if I have some class A that need the context of a class B which extends Activity, how do i get that context?

I’m searching for a more efficient way than giving the context as a parameter to class A constructor. For example if class A is going to have millions of instances then we would end up having millions of redundant pointer to Context while we should be able somehow to have just one somewhere and a getter function…

Answer 1

You can use Application class(public class in android.application package),that is:

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.

To use this class do:

public class App extends Application {

    private static Context mContext;

    public static Context getContext() {
        return mContext;
    }

    public static void setContext(Context mContext) {
        this.mContext = mContext;
    }

    ...

}

In your manifest:

<application
        android:icon="..."
        android:label="..."
        android:name="com.example.yourmainpackagename.App" >
                       class that extends Application ^^^

In Activity B:

public class B extends Activity {

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.sampleactivitylayout);

        App.setContext(this);
                  ...
        }
...
}

In class A:

Context c = App.getContext();

Note:

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.