How to schedule notifications using WorkManager?

You can show notification with the help of workmanager. Workmanger can be used for scheduling single occurrence task or periodic occurrence task as well as we can schedule task at a particular time.

OneTimeWorkRequest request= new OneTimeWorkRequest.Builder(CustomWorkerClass.class)
.setInitialDelay(delayedTime, TimeUnit.MILLISECONDS)
.addTag("TAG")
.build();

'delayedTime' -> calculate time when to trigger notification

for more implementation details click here. To read more regarding workmanager click here

According to the Android official documentation about worker manager: "These APIs let you create a task and hand it off to WorkManager to run immediately or at an appropriate time". In my app, I set a work to be activated 20 seconds after:

 // Schedule the time
 val request= OneTimeWorkRequestBuilder<YourWork>().setInitialDelay(20, TimeUnit.SECONDS).build()
    WorkManager.getInstance().enqueue(request)

As ianhanniballake stated

WorkManager isn't appropriate for work that needs to happen at a particular time.

But this can be set in near future of the exact time, depending on the state of the battery optimization.

An workaround will be like:

// Time to show notification at
LocalDateTime timeAt = LocalDate.now().atTime(20, 0);
LocalDateTime timeNow = LocalDateTime.now();

OneTimeWorkRequest.Builder workBuilder = new OneTimeWorkRequest.Builder(NotificationWorker.class);
// I just need to set an delay here
workBuilder.setInitialDelay(Duration.between(timeNow, timeAt));

// This is just to complete the example
WorkManager.getInstance().enqueueUniqueWork(UNIQUE_WORK_SHOW_NOTIFICATION,
                                    ExistingWorkPolicy.REPLACE,
                                    workBuilder.build());

WorkManager isn't appropriate for work that needs to happen at a particular time.

You should use AlarmManager, and specifically AlarmManagerCompat.setExactAndAllowWhileIdle(), to get a callback at a specific time.