firebaseCM/fcm/app/src/main/java/com/example/fcm/MainActivity.java

package com.example.fcm;

import androidx.appcompat.app.AppCompatActivity;

import android.annotation.SuppressLint;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.os.Build;
import android.os.Bundle;
import android.util.Log;
import android.widget.Button;
import android.widget.Toast;
import com.android.volley.toolbox.JsonObjectRequest;
import com.example.fcm.databinding.ActivityMainBinding;
import com.google.firebase.iid.FirebaseInstanceId;
import com.google.firebase.messaging.FirebaseMessaging;

import org.json.JSONException;
import org.json.JSONObject;

import java.util.HashMap;
import java.util.Map;
import java.util.Objects;


public class MainActivity extends AppCompatActivity {

    final private String FCM_API = "https://fcm.googleapis.com/fcm/send";
    final private String serverKey = "key=" + "AAAA446tEwQ:APA91bEAqpEFQ0ufbp9WSFIGV68P8kUgNlnKzuEHIcWLeS1YlMwJewRdnAGRae0VpDzriag7ZQbMUMPDiT-LD1kTXLQFM73C9DC13iN4nn9gEYCPVp4MN8JHFm20m1BxS8zWZcKDYFcz";
    final private String contentType = "application/json";
    private static final String TAG = "MainActivity";

    String NOTIFICATION_TITLE;
    String NOTIFICATION_MESSAGE;
    String TOPIC;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ActivityMainBinding binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());

        Button buttonSend = findViewById(R.id.sendButton);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            // Create channel to show notifications.
            String channelId  = getString(R.string.default_notification_channel_id);
            String channelName = getString(R.string.default_notification_channel_name);
            NotificationManager notificationManager =
                    getSystemService(NotificationManager.class);
            notificationManager.createNotificationChannel(new NotificationChannel(channelId,
                    channelName, NotificationManager.IMPORTANCE_LOW));
        }

        // If a notification message is tapped, any data accompanying the notification
        // message is available in the intent extras. In this sample the launcher
        // intent is fired when the notification is tapped, so any accompanying data would
        // be handled here. If you want a different intent fired, set the click_action
        // field of the notification message to the desired intent. The launcher intent
        // is used when no click_action is specified.
        //
        // Handle possible data accompanying notification message.
        // [START handle_data_extras]
        if (getIntent().getExtras() != null) {
            for (String key : getIntent().getExtras().keySet()) {
                Object value = getIntent().getExtras().get(key);
                Log.d(TAG, "Key: " + key + " Value: " + value);
            }
        }
        // [END handle_data_extras]

        binding.subscribeButton.setOnClickListener(v -> {
            Log.d(TAG, "Subscribing to topic");
            // [START subscribe_topics]
            FirebaseMessaging.getInstance().subscribeToTopic("testForMDT")
                    .addOnCompleteListener(task -> {
                        String msg = getString(R.string.msg_subscribed);
                        if (!task.isSuccessful()) {
                            msg = getString(R.string.msg_subscribe_failed);
                        }
                        Log.d(TAG, msg);
                        Toast.makeText(MainActivity.this, msg, Toast.LENGTH_SHORT).show();
                    });
            // [END subscribe_topics]
        });

        binding.logTokenButton.setOnClickListener(v -> {
            // Get token
            // [START retrieve_current_token]
            FirebaseInstanceId.getInstance().getInstanceId()
                    .addOnCompleteListener(task -> {
                        if (!task.isSuccessful()) {
                            Log.w(TAG, "getInstanceId failed", task.getException());
                            return;
                        }

                        // Get new Instance ID token
                        String token = Objects.requireNonNull(task.getResult()).getToken();

                        // Log and toast
                        @SuppressLint({"StringFormatInvalid", "LocalSuppress"}) String msg = getString(R.string.msg_token_fmt, token);
                        Log.d(TAG, msg);
                        Toast.makeText(MainActivity.this, msg, Toast.LENGTH_SHORT).show();
                    });
            // [END retrieve_current_token]
        });

        buttonSend.setOnClickListener(v -> {
        TOPIC = "/topics/testForMDT"; //topic must match with what the receiver subscribed to
        NOTIFICATION_TITLE = "Test";
        NOTIFICATION_MESSAGE = "Gregor";

        JSONObject notification = new JSONObject();
        JSONObject notifcationBody = new JSONObject();
        try {
            notifcationBody.put("title", NOTIFICATION_TITLE);
            notifcationBody.put("message", NOTIFICATION_MESSAGE);

            notification.put("to", TOPIC);
            notification.put("data", notifcationBody);
        } catch (JSONException e) {
            Log.e(TAG, "onCreate: " + e.getMessage() );
        }
        sendNotification(notification);
        });
    }

    private void sendNotification(JSONObject notification) {
        JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(FCM_API, notification,
                response -> {
                    Log.i(TAG, "onResponse: " + response.toString());
                    //edtTitle.setText("");
                    //edtMessage.setText("");
                },
                error -> {
                    Toast.makeText(MainActivity.this, "Request error", Toast.LENGTH_LONG).show();
                    Log.i(TAG, "onErrorResponse: Didn't work");
                }){
            @Override
            public Map<String, String> getHeaders() {
                Map<String, String> params = new HashMap<>();
                params.put("Authorization", serverKey);
                params.put("Content-Type", contentType);
                return params;
            }
        };
        MySingleton.getInstance(getApplicationContext()).addToRequestQueue(jsonObjectRequest);
    }
}