AndroidManifest.xml file

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="computervision.grewe.serviceoverlaywithcallbacktoactivity">

    <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />
    <uses-permission android:name="android.permission.INTERNET" />

    <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=".MainProcessingActivity" >
            <category android:name="android.intent.category.DEFAULT" />
        </activity>

        <service
            android:name=".HUD"
            android:enabled="true"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.BOOT_COMPLETED" />
            </intent-filter>
        </service>
    </application>

</manifest>

<<declaration of permissions

<<Main activity

<<Journaling Activity interface

<<Service

MainActvity.java

package computervision.grewe.serviceoverlaywithcallbacktoactivity;


        import android.annotation.TargetApi;
        import android.app.Activity;
        import android.content.Context;
        import android.content.Intent;
        import android.net.Uri;
        import android.os.Build;
        import android.provider.Settings;
        import android.support.v7.app.AppCompatActivity;
        import android.os.Bundle;
        import android.webkit.WebView;
        import android.webkit.WebViewClient;

//Dummy activity that has a TextView and WebView as interface
//important part is that this Android has a Service class called HUD that creates
// an overlay window that appears on top of any actively running application window
//not just this activity --it runs in the background.   You can use it to
//return to this app or do whatever
public class MainActivity extends AppCompatActivity {


    private int request_Code = 1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        //YOU MAY want to get rid of these --it opens the apps settings so user can turn on permission for overlay
        //The only way the user can return to the app is to hit the "back" button on the device
        //Purpose = In newer version of Android any app that creates an Overlay requires the user
        // to specifically set this permission on the app's settings on the device.  It is not enough
        // to request the permission in the AndroidManifest file

        requestSystemAlertPermission(MainActivity.this,1);
        isSystemAlertPermissionGranted(MainActivity.this);

        //start the service
        startService(new Intent(getApplicationContext(), HUD.class));


    }

    @Override
    protected void onStart() {
        super.onStart();
        setupView();

    }

    /**
     * method to setup the View with content in Activity
     */
    private void setupView(){

    }



   //brings up interface to settings for user to allow overlay
    public static void requestSystemAlertPermission(Activity context, int requestCode) {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.M)
            return;
        final String packageName = context == null ? context.getPackageName() : context.getPackageName();
        final Intent intent = new Intent(Settings.ACTION_MANAGE_OVERLAY_PERMISSION, Uri.parse("package:" + packageName));
        if (context != null)
            context.startActivityForResult(intent, requestCode);
        else
            context.startActivityForResult(intent, requestCode);
    }
    @TargetApi(23)
    public static boolean isSystemAlertPermissionGranted(Context context) {
        final boolean result = Build.VERSION.SDK_INT < Build.VERSION_CODES.M || Settings.canDrawOverlays(context);
        return result;
    }

    //callback function to get results from ACTION_MANAGE_OVERLAY_PERMISSION "activity" when it is done
    public void onActivityResult(int requestCode, int resultCode, Intent data)
    {
        if (requestCode == request_Code) {
            if (resultCode == RESULT_OK) {
                this.setupView();
            }
        }
    }
}

	        

HUD.java - the service class

package computervision.grewe.serviceoverlaywithcallbacktoactivity;



        import android.app.Service;
        import android.content.Intent;
        import android.os.IBinder;


        import android.app.Service;
        import android.content.Intent;
        import android.graphics.PixelFormat;
        import android.net.Uri;
        import android.os.IBinder;
        import android.view.Gravity;
        import android.view.LayoutInflater;
        import android.view.View;
        import android.view.WindowManager;
        import android.widget.FrameLayout;
        import android.widget.ImageButton;
        import android.widget.ImageView;
        import android.widget.LinearLayout;
        import android.widget.RelativeLayout;

/**
 * Class that is a background Service that creates an Overlay Window that can reside on top
 * of any running activity.
 * WHEN the interface is clicked on close it will launch the
 */
public class HUD extends Service {

    private WindowManager windowManager;
    private ImageView close;
    private LinearLayout overlayLayout;
    private ImageView overlayButton;


    /*
     @Override
     public IBinder onBind(Intent intent) {
         // Not used
         return null;
     }
     */
    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        throw new UnsupportedOperationException("Not yet implemented");
    }

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

        //using the WindowManager launch the Service  with interface overlay_layout
        windowManager = (WindowManager) getSystemService(WINDOW_SERVICE);
        LayoutInflater inflater = (LayoutInflater)getSystemService(LAYOUT_INFLATER_SERVICE);



        WindowManager.LayoutParams params = new WindowManager.LayoutParams(
                WindowManager.LayoutParams.WRAP_CONTENT,
                WindowManager.LayoutParams.WRAP_CONTENT,
                WindowManager.LayoutParams.TYPE_PHONE,
                WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE,
                PixelFormat.TRANSLUCENT);

        //will put the  "floating window = overlay" to top and left
        params.gravity = Gravity.TOP | Gravity.LEFT;
        params.x = 0;
        params.y = 100;

        //Inflate the ouverlay_layout.xml file that contains and Image "J" logo for "journaling"
        // and an "x" image representing a close of a window
        overlayLayout = (LinearLayout) inflater.inflate(R.layout.overlay_layout, null);
        close=(ImageView)overlayLayout.findViewById(R.id.close);
        overlayButton=(ImageView) overlayLayout.findViewById(R.id.overlayButton);

        //register listener for "J" logo ImageView widget that is part of this Service's "floating window
        //when clicked on it will lauch the Activity associated with this application as an Intent and then
        //stop THIS service from running
        overlayButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                //remove the view of the Overlay
                windowManager.removeView(overlayLayout);





                //restart the Activity MainActivity associated with this application
                Intent intent_start_mainProcessing_activity = new Intent(getApplicationContext(), MainProcessingActivity.class);
                intent_start_mainProcessing_activity.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

                startActivity(intent_start_mainProcessing_activity);


                //stop this background service
                stopService(new Intent(getApplicationContext(), HUD.class));
            }
        });

        //when the x image = "close"  it simply stops the service
        close.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                windowManager.removeView(overlayLayout);

                //simply stopping Service
                stopService(new Intent(getApplicationContext(), HUD.class));
                // startService(new Intent(getApplicationContext(), HUD.class)); //code to start service in case wanted to see it
            }
        });

        windowManager.addView(overlayLayout, params);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
       /* if (overlayLayout != null) windowManager.removeView(overlayLayout);*/
    }
}



/*

@Override
public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        throw new UnsupportedOperationException("Not yet implemented");
        }
*/

overlaylayout.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id="@+id/overlayButton"
        android:layout_width="54dp"
        android:layout_height="49dp"
        android:src="@drawable/overlaylogo" />

    <ImageView
        android:id="@+id/close"
        android:layout_width="45dp"
        android:layout_height="45dp"
        android:layout_gravity="right|top"
        android:src="@drawable/remove" />
</LinearLayout>

	        

MainProcessingActivity - the Journal Interface and processing it

package computervision.grewe.serviceoverlaywithcallbacktoactivity;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.webkit.WebView;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainProcessingActivity extends AppCompatActivity {


    Button button_enterJournal;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_processing);
        button_enterJournal = (Button) findViewById(R.id.button_SubmitJournal);
        button_enterJournal.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                //processing the journal entry
                String s =  getResources().getString(R.string.saving_journal_entry) + ": " + ((EditText) findViewById(R.id.journal_EditText)).getText();
                Toast.makeText(getApplicationContext(), s, Toast.LENGTH_LONG).show();

                //call method to restart the background Overlay Service
                //start the service
                startService(new Intent(getApplicationContext(), HUD.class));
            }

        });

    }



}

activity_main_processing.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="computervision.grewe.serviceoverlayexample.MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Journal"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintVertical_bias="0.0"
        android:id="@+id/textView"
        android:layout_marginLeft="8dp" />

    <Button
        android:id="@+id/button_SubmitJournal"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/enter"
        android:layout_marginTop="8dp"
        app:layout_constraintTop_toBottomOf="@+id/journal_EditText"
        android:layout_marginLeft="0dp"
        app:layout_constraintLeft_toRightOf="@+id/imageView_LoadedJournalImage"
        android:layout_marginRight="8dp"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintHorizontal_bias="0.81" />

    <EditText
        android:id="@+id/journal_EditText"
        android:layout_width="fill_parent"
        android:layout_height="121dp"
        android:layout_marginLeft="0dp"
        android:layout_marginTop="12dp"
        android:ems="10"
        android:inputType="textMultiLine"
        android:minLines="6"
        android:singleLine="false"
        android:text=""
        android:lines="20"
        android:scrollHorizontally="false"
        android:scrollbars="vertical"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView" />

    <ImageView
        android:id="@+id/imageView_LoadedJournalImage"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:srcCompat="@mipmap/ic_launcher"
        android:layout_marginTop="8dp"
        app:layout_constraintTop_toBottomOf="@+id/journal_EditText"
        android:layout_marginLeft="8dp"
        app:layout_constraintLeft_toLeftOf="parent" />


</android.support.constraint.ConstraintLayout>