Here you can find the source of nextDay(Date then)
public static Date nextDay(Date then)
//package com.java2s; //License from project: Apache License import java.util.Calendar; import java.util.Date; public class Main { private static final int[] incrementSequence = new int[] { Calendar.SECOND, Calendar.MINUTE, Calendar.HOUR_OF_DAY, Calendar.DAY_OF_MONTH, Calendar.MONTH, Calendar.YEAR, Calendar.ERA }; public static Date nextDay(Date then) { return next(then, Calendar.DAY_OF_MONTH, 1, 1); }//from w w w . ja va2 s. c o m public static Date nextDay() { return nextDay(new Date()); } public static Date next(Date then, int calendarField, int multiple, int minimalDelta) { int calendarFieldIncrementIndex = getIncrementSequenceIndex(calendarField); int nextSmallestCalendarField = calendarFieldIncrementIndex > 0 ? incrementSequence[calendarFieldIncrementIndex - 1] : Calendar.MILLISECOND; Calendar calThen = Calendar.getInstance(); calThen.setTime(then); calThen.add(nextSmallestCalendarField, minimalDelta); // always at least a second late if its a minute, a minute of its an hour, etc ... Calendar calResult = (Calendar) calThen.clone(); calResult.add(calendarField, multiple); for (int index = calendarFieldIncrementIndex - 1; index >= 0; --index) calResult.set(incrementSequence[index], incrementSequence[index] == Calendar.DAY_OF_MONTH ? 1 : 0); calResult.set(Calendar.MILLISECOND, 0); // we don't deal with sub second resolution if (calResult.before(calThen)) calResult.add(calendarField, 1); return calResult.getTime(); } public static Date next(Date then, int calendarField, int minimalDelta) { return next(then, calendarField, 1, minimalDelta); } private static int getIncrementSequenceIndex(int calendarFieldNumber) { for (int index = 0; index < incrementSequence.length; ++index) if (incrementSequence[index] == calendarFieldNumber) return index; return 0; } }