Head-UP Notification wird nicht angezeigt

[kukuk]

Hallo alle zusammen, ich bin jetzt alle Tutorials durchgegangen und ich bekomme es nicht hin, das auch nur eine Head-UP Notification angezeigt wird. kann mir jemand helfen. Ich verwende folgenden Code:

Intent intent = new Intent(MainActivity.this, MainActivity.class);
PendingIntent pi = PendingIntent.getActivity(MainActivity.this,0,intent,0);
Notification.Builder builder = new Notification.Builder(this);
builder.setContentTitle("Title")
.setContentText("Test")
        .setSmallIcon(R.mipmap.ic_launcher)
        .setLargeIcon(BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher))
        .setContentIntent(pi)
        .setPriority(Notification.PRIORITY_HIGH);
NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
notificationManager.notify(0,builder.build());

Mein 2. Versuch sieht so aus: Auch das funktioniert nicht.

button.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        mNotificationManager = (NotificationManager) getSystemService(MainActivity.this.NOTIFICATION_SERVICE);





        mNotificationManager.notify(NOTIFICATION_ID, createNotification(
                true));


        createNotification(true);

    }
});
}



private Notification createNotification(boolean makeHeadsUpNotification) {
    Notification.Builder notificationBuilder = new Notification.Builder(this)
            .setSmallIcon(R.drawable.ic_launcher_background)
            .setPriority(Notification.PRIORITY_DEFAULT)
            .setCategory(Notification.CATEGORY_MESSAGE)
            .setContentTitle("Sample Notification")
            .setContentText("This is a normal notification.");
    if (makeHeadsUpNotification) {
        Intent push = new Intent();
        push.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        push.setClass(this, MainActivity.class);

        PendingIntent fullScreenPendingIntent = PendingIntent.getActivity(this, 0,
                push, PendingIntent.FLAG_CANCEL_CURRENT);
        notificationBuilder
                .setContentText("Heads-Up Notification on Android L or above.")
                .setFullScreenIntent(fullScreenPendingIntent, true);
    }
    return notificationBuilder.build();
}