Hand-Up notification comes up with API level 21.
Hands-up notification is open small floating window on top of the screen only when device is active.

Hand-up notifications are High-priority notifications.Hands-up notification required to set a vibrate or ring-tone to make Heads-up work.

Hands-up notification are uses like alarm,incoming call,messages,battery low. etc.

Lets start with code:-

1. Create xml file of your activity like below.

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.android.basicnotifications.HandsUpActivity">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Notification demo for Hands Up Notification"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginTop="131dp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Show Notification"
        android:onClick="showNotification"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true" />

</RelativeLayout>

2. Open Java file of Activity.

3. Create showNotification method of button view.

public void showNotification(View view){}

4. Create Notification Builder Instance.

 NotificationCompat.Builder builder = new NotificationCompat.Builder(this);

5. Set Content text and small icon.

 builder.setContentText("Alarm");

        builder.setSmallIcon(R.drawable.ic_launcher);

6. Create Dismiss Intent and pending intent.

 Intent dismissIntent = new Intent(this, MainActivity.class);

        PendingIntent piDismiss = PendingIntent.getActivity(this, 0, dismissIntent, 0);

7. Set Snooze Intent and pending intent.

 Intent snoozeIntent = new Intent(this, MainActivity.class);
        PendingIntent piSnooze = PendingIntent.getActivity(this, 0, snoozeIntent, 0);

8. Set Defaults,priority and action.

 builder.setDefaults(Notification.DEFAULT_ALL) // must requires VIBRATE permission
                .setPriority(NotificationCompat.PRIORITY_HIGH) //must have priority to High, Max
                .addAction(R.drawable.ic_launcher,
                        getString(R.string.dismiss), piDismiss)
                .addAction(R.drawable.ic_launcher,
                        getString(R.string.snooze), piSnooze);

9. Create notificationManager instance.

 NotificationManager notificationManager = (NotificationManager) getSystemService(
                NOTIFICATION_SERVICE);
       

10. Notify Notification.

 notificationManager.notify(11, builder.build());

Happy Coding :D