Java 时间选择器问题

我在安卓系统中有一个计时器来设置一个小时的闹钟（AlarmManager）。问题是我发现了这个问题

 public void onTimeSet( TimePicker view, int hourOfDay, int minute )
 { // Some logical code here }

因此，在我的闹钟中，用户可以设置通知开始前的时间，5分钟、10分钟或30分钟。所以

我怎样才能得到小时和分钟，以缩短闹钟发出通知前的时间

例如：

User set alarm: 13:30 and 10 Minutes before, i need to get 13:20.

Another example: user sets 2:5AM and 30 minutes before, i need to get 1:35AM

首先，创建：

Calendar cal = Calendar.getInstance();
cal.set(year, month, day, hourOfDay, minute);
long exact_alarm_time = cal.getTimeInMillis();

// if user want to set before 10 minutes
long before = 10*60*1000;

// now subtract the before time from current time
long newTime = exact_alarm_time - before;

Calendar cNew = Calendar.getInstance();
cal.setTimeInMillis(newTime);
int year = cal.get(Calendar.YEAR);
int month = cal.get(Calendar.MONTH);
int date = cal.get(Calendar.DATE);
int min = cal.get(Calendar.MINUTE);
int hour = cal.get(Calendar.HOUR);

// now set alarm for the new time

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你有没有看到在没有领域，我要检查thx！只要读一遍就行了