Develop Reference

Is there a way to add badge on app icon without showing a notification?

I've read about the notification badge icon here, but it is bound to a notification:
var notification = NotificationCompat.Builder(this#MainActivity, CHANNEL_ID)
.setContentTitle("New Messages")
.setContentText("You've received 3 new messages.")
.setSmallIcon(R.drawable.ic_notify_status)
.setNumber(messageCount)
.build()
Is there a way to change the count of the app badge icon to a customized number without showing a notification?

Yes, according to that documentation you can customize the badge count on Adroid 8.0 and higher (API 26+) and you already have it in your code;
.setNumber(messageCount)
messageCount is your customized number. Is this not working?

Android display multiple messages in a single notification

Currently, I receive notifications from a server. What happens is, if I get a notification, I dont see it for long time. When a new notification comes, I lose the previous one. I want a single notification with multiple messages something like this -

It is InboxStyle notification
Notification notif = new Notification.Builder(mContext)
.setContentTitle("5 New mails from " + sender.toString())
.setContentText(subject)
.setSmallIcon(R.drawable.new_mail)
.setLargeIcon(aBitmap)
.setStyle(new Notification.InboxStyle()
.addLine(str1)
.addLine(str2)
.setContentTitle("")
.setSummaryText("+3 more"))
.build();
Here is official document
check this example also - Inbox Style Notification Like Whatsapp

How to show notification from Android wear watchface

I am trying to show a notification from my Watch face, as I want to show additional information when the user first opens the watchface.
This is similar to the GMT watch face that is installed with the LG G Watch R, that asks the user to swipe left to select the timezone.
This code does not seem to work for me:
Notification.Builder builder = new Notification.Builder(context)
.setContentTitle("Title")
.setContentText("Content");
((NotificationManager) getSystemService(NOTIFICATION_SERVICE))
.notify(1, builder.build());
This was copied from one of Android wear's examples (was not used in a watchface example, just a regular wear app)

You are most likely missing the icon, you need to specify the icon for Android to show your notification. Try specifying everything that the example from the docs mentions:
Notification noti = new Notification.Builder(mContext)
.setContentTitle("New mail from " + sender.toString())
.setContentText(subject)
.setSmallIcon(R.drawable.new_mail)
.setLargeIcon(aBitmap)
.build();

Notification text is too long and is not showing full text

I'm developing an application for android 2.3.3. I am showing a notification but the text of this notification is too long and its not showing the full text and its being cut so how can I show my full text in my notification?
Here's the code:
String message = "Tonights Taraweeh consists of Alif Lam Mim and the first quarter of Sayaqul. The Surahs covered are Al-Fatiha(The Opening),and the first two-thirds of Al-Baqara(The Cow).";
Notification notification = new Notification();
notification.setLatestEventInfo(context, title, message, contentIntent);

Expanded notifications are only available from Android 4.1, read here
Android 2.3.3 uses the old notifications without expansion. You must user a shorter text, cut your text (and show the full text when user click on it) or adapt the text if you are showing the notification in Android 4.1 or older.

I short you have to set the BigTextStyle
Notification notification = new NotificationCompat.Builder(context, CHANNEL_ID)
.setSmallIcon(R.drawable.new_mail)
.setContentTitle(emailObject.getSenderName())
.setContentText(emailObject.getSubject())
.setLargeIcon(emailObject.getSenderAvatar())
.setStyle(new NotificationCompat.BigTextStyle()
.bigText(__BigTextHere___))
.build();

Android Notification- Display full message

My Android Application has to be able to send short alerts out to a large group of people. The obvious place to do this is in the notification center. The full notification shows up in the ticker without a problem, but in the notification center a user can only see the first couple words and then an elipsis. The notifications are not long at all, just 10-15 words at the most. How can I make the text wrap down to a new line?
My code to build the notifications is here
NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(this)
.setSmallIcon(R.drawable.splash)
.setContentTitle("Student Engauge")
.setContentText(extras.getString("message"))
.setAutoCancel(true)
.setTicker(extras.getString("message"));
final int notificationId = 1;
NotificationManager nm = (NotificationManager) getApplicationContext()
.getSystemService(Context.NOTIFICATION_SERVICE);
nm.notify(notificationId, mBuilder.build());

To show large chunk of text, use the BigTextStyle. Here is a sample code as given in BigTextStyle. This notification will one line of text and will expand to more lines if needed.
Notification noti = new Notification.Builder()
.setContentTitle("New mail from " + sender.toString())
.setContentText(subject)
.setSmallIcon(R.drawable.new_mail)
.setLargeIcon(aBitmap)
.setStyle(new Notification.BigTextStyle()
.bigText(aVeryLongString))
.build();
For android support library
Notification noti = new Notification.Builder()
.setContentTitle("New mail from " + sender.toString())
.setContentText(subject)
.setSmallIcon(R.drawable.new_mail)
.setLargeIcon(aBitmap)
.setStyle(new NotificationCompat.BigTextStyle()
.bigText(aVeryLongString))
.build();

For Android 4.1 and later devices, big view is the most suitable solution to show large amount of text. For pre 4.1 devices, you can use custom notification layout to show more data like mentioned here. But you should keep in mind two things:
From the official documentation
Caution: When you use a custom notification layout, take special care to ensure that your custom layout works with different device orientations and resolutions. While this advice applies to all View layouts, it's especially important for notifications because the space in the notification drawer is very restricted. Don't make your custom layout too complex, and be sure to test it in various configurations.
Custom notification layouts have some limitations. Too long texts are not shown fully, but 10-15 words probably fit to the custom layout. This answer has more info about the limitation of custom notification layouts