Where to put Firebase.setAndroidContext() function

As seen in the sample applications of Firebase you should place it inside your Application.

package com.firebase.androidchat;

import com.firebase.client.Firebase;

/**
 * @author Jenny Tong (mimming)
 * @since 12/5/14
 *
 * Initialize Firebase with the application context. This must happen before the client is used.
 */
public class ChatApplication extends android.app.Application {
    @Override
    public void onCreate() {
        super.onCreate();
        Firebase.setAndroidContext(this);
    }
}

Source

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Firebase.getAndroidContext()

After setting the Application context once you can use it where ever you need it. You can retrieve it as often as you like, everywhere. I would also recommend to use the Firebase.getAndroidContext() instead of storing it into variables to prevent MemoryLeaks

To quote (step 4 of) the Firebase quickstart documentation:

The Firebase library must be initialized once with an Android Context. This must happen before any Firebase reference is created or used.

Create MyApplication.java:

public class MyApplication extends android.app.Application {

    @Override
    public void onCreate() {
        super.onCreate();

        //Previous versions of Firebase
        Firebase.setAndroidContext(this);

        //Newer version of Firebase
        if(!FirebaseApp.getApps(this).isEmpty()) {
            FirebaseDatabase.getInstance().setPersistenceEnabled(true);
        }
    }
}

And update name parameter value in your AndroidManifest.xml:

<application 
        android:label="@string/app_name"
        android:name=".MyApplication">
...
</application>

I do not know FireBase but i know Android.. A Context is a global information about an application environment. Your Activity is a Context so i am pretty sure Firebase.getAndroidContext() retrieves your Application Context which is getApplicationContext(), Since that seems sensible.

Should I only call this function once in Application or once in each Activty?

call it whenever you need a Context with respects to FireBase codes- but i think will suit best if you call it in your Application class

If the answer for question 1 is only once, then where should I put it ?

what if its not once? where do you call it? i guess you will call it anywhere you need Context right? so do that irrespective of question 1's answer, but you can fall on Class.this , getBaseContext() or View.getContext() anytime