How to automatic enable/disable push notification for certain time

I'm new at using an alarm service for Android. I want to create a medication reminder which is the medication information and number of repeated notification per day as the input data. However, I only want to enable the notification between 7 AM and 7 PM. So, the apps will notify within that time only and also the apps will start to notify at 7 AM.

Example: If the user wants to repeat the notification from the apps 3 times per day, then every 4 hours the apps will notify them to take the medicine (12 hours / 3 times = 4 hours).

I have tried using the while loop, however it somehow didn't worked as I expected where if the condition is true, then it will keep on notify even though it's not the time yet (this happened to my code).

So, here what I tried to do:

Calendar calendar = Calendar.getInstance();
calendar.setTimeInMillis(System.currentTimeMillis());
calendar.set(Calendar.HOUR_OF_DAY, 7);
while(calendar.get(Calendar.HOUR_OF_DAY) >= 0) {
    while (calendar.get(Calendar.HOUR_OF_DAY) >= 7 && calendar.get(Calendar.HOUR_OF_DAY) <= 19) {
           // the process here
    }
    calendar.add(Calendar.HOUR_OF_DAY, 1);
}

I'm sorry if my code is lacking in many ways. But, I really hope that you guys can help me or share with me any other possible solution. Thank you.

You are setting the calendar hour incorrectly.

Replace

Calendar calendar = Calendar.getInstance();
calendar.setTimeInMillis(System.currentTimeMillis());
calendar.set(Calendar.HOUR_OF_DAY, 7);
while(calendar.get(Calendar.HOUR_OF_DAY) >= 0) {
    while (calendar.get(Calendar.HOUR_OF_DAY) >= 7 && calendar.get(Calendar.HOUR_OF_DAY) <= 19) {
           // the process here
    }
    calendar.add(Calendar.HOUR_OF_DAY, 1);
}

With

Calendar cc = Calendar.getInstance();
int mHour = cc.get(Calendar.HOUR_OF_DAY);

if (mHour  >= 7 && mHour  <= 19) {
               // the process here
        }