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Cannot find getApplicationContext() in class extended from FirebaseMessagingService

I want to send and receive remote notification through Firebase but my custom class could not find getApplicationContext() method.
From my understanding, FirebaseMessagingService extends Service, which extendsContextWrapper` which extends Context.
So I don't know getApplicationContext() is not working. Any help will be appreciated!
import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;
public class FirebaseMessageService extends FirebaseMessagingService {
#Override
public void onMessageReceived(RemoteMessage remoteMessage) {
if(remoteMessage.getNotification() != null){
String title = remoteMessage.getNotification().getTitle();
String text = remoteMessage.getNotification().getBody();
NotificationHelper.displayNotification(getApplicationContext(), title, text);
}
super.onMessageReceived(remoteMessage);
}
}
Here's my AndroidManifest.xml file.
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.mealz">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme.NoActionBar">
<activity android:name=".Activities.UserActivity"></activity>
<activity android:name=".Activities.LoginActivity" />
<activity android:name=".Activities.SignupActivity" />
<activity android:name=".Activities.SearchRecipeActivity" />
<activity android:name=".Activities.RecipeDetailActivity" />
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<service android:name=".Notifications.FirebaseMessageService">
<intent-filter>
<action android:name="com.google.firebase.MESSAGING_EVENT" />
</intent-filter>
</service>
</application>
</manifest>

You can achieve this in many way one way is to keep a weak refrence of context make a app class extending android.app.application and on it's create method and make a variable of weak refrence of context and assign it to getApplicationContext() and you may use this context anywhere you want.
public class app extends android.app.Application{
private static WeakReference<Context> referenceCntx;
#Override
public void onCreate() {
super.onCreate();
referenceCntx = new WeakReference<>(getApplicationContext());
}
public static Context getApplicationCntx(){
return referenceCntx.get();
}
}
And don't forget to add this line in manifest
<application
android:name=".app"

Incorrect Activity when running App Android Studio

I have starting building an app in Android studio. I have established the MainPage as the launcher activity in the manifest.xml.
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/Theme.AppCompat.NoActionBar">
<activity android:name=".MainPage">
android:screenOrientation="portrait"
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".courseSelect" />
<activity android:name=".profile1" />
<activity android:name=".stats1" />
<activity android:name=".ReviewRounds" />
<activity android:name=".ReferFriends" />
<activity android:name=".RangeMode" />
</application>
I have double checked that the run configuration is set to 'Default' and yet the app is running a different activity, entitled courseSelect. It is also not running some code correctly on the NumberPicker. Even though I've set the picker to have a min, max, and default, the picker only shows 0 and will not scroll. The two issues seem to be related somehow, in terms of what activity is being run.
this is the courseSelectCode:
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.NumberPicker;
import co.ceryle.segmentedbutton.SegmentedButtonGroup;
public class courseSelect extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_course_select);
//Hole Picker
NumberPicker holePicker = (NumberPicker)findViewById(R.id.holePicker);
holePicker.setMaxValue(18);
holePicker.setMinValue(1);
holePicker.setWrapSelectorWheel(false);
holePicker.setValue(1);
SegmentedButtonGroup sbg = (SegmentedButtonGroup) findViewById(R.id.segmentedButtonGroup);
sbg.setOnClickedButtonPosition(new SegmentedButtonGroup.OnClickedButtonPosition() {
#Override
public void onClickedButtonPosition(int position) {
// if(position == 0)
}
});
}
}
I tried to set the run configuration specifically to the MainPage activity and it still opens in the courseSelect Page.
EDIT: on request, here is my MainPage.java code:
public class MainPage extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_course_select);
Window g = getWindow();
g.setFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION, WindowManager.LayoutParams.TYPE_STATUS_BAR);
setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
}
...
}

edit the Mainfest.xml, in order to enforce portrait layout there already:
<activity android:name=".MainPage"
android:screenOrientation="portrait"
android:configChanges="orientation|keyboardHidden">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
...which makes this code merely useless (styles.xml can also be used for window styles):
Window g = getWindow();
g.setFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION, WindowManager.LayoutParams.TYPE_STATUS_BAR);
setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
also update setContentView( R.layout.activity_course_select ); to the proper resource.
because it starts the MainPage Activity, but then inflates the wrong XML file.
one "suggested edit" before was to swap the order of setContentView() and the paragraph below ...which I've rejected, because setting it in Manifest.xml appeared more organized (less code).

First of all There is a mistake in your manifest file
you wrote screenOrientation attribute outside the opening tag
<activity android:name=".MainPage">
android:screenOrientation="portrait"
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
it should be
<activity
android:name=".MainPage"
android:screenOrientation="portrait">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
and you used wrong xml to setContenView

Try this and error was in 9th line because your code line is outside of the tag:
<activity android:name=".MainPage">
android:screenOrientation="portrait" // error
Do this :
<activity android:name=".MainPage"
android:screenOrientation="portrait"> // After doing this no error
and do also this thing :
public class MainPage extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_course_select); // error
do this :
public class MainPage extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.MAIN_PAGE_ACTIVITY_NAME); // no error

Your launcher activity is MainPage but you are calling the layout of courseselect activity inside MainPage activity's onCreate method on this line
setContentView(R.layout.activity_course_select);
Change it to your MainPage layout
setContentView(R.layout.yourMainPageLayout);

Android.app.Application cannot be cast to

Despite i followed some StackOverflow's tips, I can't get rid of this error:
Android.app.Application cannot be cast to com.dostalgia.airshootingrogatti.GlobalClass
I inserted GlobalClass in manifest file, as already said in S.O.
This is the Global Class Code:
public class GlobalClass extends AppCompatActivity {
private int StatoContatore1;
public int getStatoContatore1() {
return StatoContatore1;
}
public void setStatoContatore1(int statoContatore1) {
StatoContatore1 = statoContatore1;
}
}
And this is the activity (Not the main) that calls and generate error
package com.dostalgia.airshootingrogatti;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
public class Postazione1 extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_postazione1);
final GlobalClass globalVariable = (GlobalClass) getApplicationContext();
}
}
This is my Mainfest file
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.dostalgia.airshootingrogatti">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".Postazione1"></activity>
<activity android:name=".GlobalClass"></activity>
</application>
</manifest>
My goal is to use and manage global variables in different activities!
Thanks for your reply!

My goal is to use and manage global variables in different activities!
You are welcome to carefully use static fields. This would not require you to try creating a custom Application subclass, as you appear to be attempting to do here.
That being said, if you really want to do what I think you are trying to do:
Step #1: Have GlobalClass extend android.app.Application, not AppCompatActivity
Step #2: Remove the <activity> element in the manifest for GlobalClass
Step #3: Add an android:name attribute to the <application> element in the manifest, pointing to your GlobalClass class (e.g., android:name="com.dostalgia.airshootingrogatti.GlobalClass")

Error : MainActivity is not an enclosing class

I create a personnal component view and when we click on this, an other activity starts. There is my manifest.
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="fr.freshkamekentrainement.skrt">
<application
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:theme="#style/AppTheme">
<activity android:name=".Splash"
android:theme="#style/Splash"
android:screenOrientation="portrait"
android:configChanges="orientation">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".MainActivity"
android:screenOrientation="portrait"
android:configChanges="orientation"/>
<activity android:name=".niveaux.LilUziVert_GrowUp"
android:screenOrientation="portrait"
android:configChanges="orientation"/>
</application></manifest>
there is my intent
public class Niveauview extends RelativeLayout {
Intent intentNiveau;
//Code
#Override
public void onFinishInflate() {
super.onFinishInflate();
setOnClickListener(new OnClickListener() {
#Override
public void onClick(View view) {
intentNiveau = new Intent(MainActivity.this,LilUziVert_GrowUp.class);
startActivity(intentNiveau);
}
});
}}
I get error:
MainActity is not an enclosing class
Notice that NiveauView and MainActivity isn't in the same package (but they are public). Where does the problem come from? When i try new Intent(this,LilUziVert_GrowUp.class); i have an error too.

You can only access Foo.this if you're in a inner class of Foo. Otherwise you need to be passed in an instance of Foo instead. In the case of a View, every view has an instance of a Context you can get by calling getContext(). So you need to call intentNiveau = new Intent(getContext(),LilUziVert_GrowUp.class);

Illegal access exception while sharing Global data

i want to share the global data across activities and i have followed
this link
.but i am not getting how to declare it in my manifest. i am posting my manifest code, i have tried it in different ways, but still getting the error. please tell me how to resolve it.
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.helloandroid"
android:versionCode="1"
android:versionName="1.0">
<uses-sdk android:minSdkVersion="7" />
<application android:icon="#drawable/icon" android:label="#string/app_name"
android:name=".Myapp">
<activity android:name=".AndroidtestActivity"
android:label="#string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
This is my main activity
public class AndroidtestActivity extends Activity
{
/** Called when the activity is first created. */
public static final String PREFS_NAME = "MyPrefsFile";
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
EditText et = (EditText) findViewById(R.id.text1);
Myapp appState = ((Myapp)getApplicationContext());
String s= appState.getState();
et.setText(s);
}
}
and my Myapp class is
class Myapp extends Application {
private String myState;
public String getState(){
return myState;
}
public void setState(String s){
myState = "hello world";
}
}
i am getting error in the line
Myapp appState = ((Myapp)getApplicationContext());
illegal access exception ,please tell me how to resolve this problem
please help me with this.

Merge the application tags (i.e. put the android:name in the first one and delete the rest):
<application android:icon="#drawable/icon" android:label="#string/app_name"
android:name=".MyApp">
<activity android:name=".AndroidtestActivity"
android:label="#string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
Then use getApplication() instead of getApplicationContext().