Calendar.getInstance().getActualMaximum(Calendar.DAY_OF_MONTH);

The above returns actual maximum for current month. For example it is February of leap year now, so it returns 29.

And to get last day as Date object:

Calendar cal = Calendar.getInstance();
cal.set(Calendar.DATE, cal.getActualMaximum(Calendar.DATE));

Date lastDayOfMonth = cal.getTime();