package com;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.GregorianCalendar;

public class DateUtil {
	
	/**
	 * 獲取本月第一天
	 * @return 當前月第一天的日期
	 */
	public static String getMonthFirstDay() {
		Calendar cal = Calendar.getInstance();
		Calendar f = (Calendar) cal.clone();
		f.clear();
		f.set(Calendar.YEAR, cal.get(Calendar.YEAR));
		f.set(Calendar.MONTH, cal.get(Calendar.MONTH));
		String firstday = new SimpleDateFormat("yyyy-MM-dd").format(f.getTime());
		return firstday;

	}
	
	/**
	 * 獲取本月最后一天
	 * @return 當前月最后一天的日期
	 */
	public static String getMonthLastDay() {
		Calendar cal = Calendar.getInstance();
		Calendar l = (Calendar) cal.clone();
		l.clear();
		l.set(Calendar.YEAR, cal.get(Calendar.YEAR));
		l.set(Calendar.MONTH, cal.get(Calendar.MONTH) + 1);
		l.set(Calendar.MILLISECOND, -1);
		String lastday = new SimpleDateFormat("yyyy-MM-dd").format(l.getTime());
		return lastday;

	}
	
	/**
	 * 根據日期計算某月有多少天
	 * @param date 需要計算有多少天的日期
	 * @return 返回當前日期的天數
	 */
	public static int getDays(String date){
		SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");
		Calendar objCalendar = new GregorianCalendar();
		try {
			objCalendar.setTime(formatter.parse(date));
			int days = objCalendar.getActualMaximum(Calendar.DAY_OF_MONTH);
			return days;
		} catch (Exception e1) {
			e1.printStackTrace();
		}
		return 0;
	}
	

}


  

?

測試：

    package com.test;

import org.junit.Test;

import com.DateUtil;

public class DateTest {

	@Test
	public void test(){
		System.out.println("本月第一天是:"+DateUtil.getMonthFirstDay());	
	}
	@Test
	public void test2(){
		System.out.println("本月最后一天是:"+DateUtil.getMonthLastDay());
	}
	@Test
	public void test3(){
		String str = "2012-08-15";
		int days = DateUtil.getDays(str);
		System.out.println(str+"這個月共有"+days);
	}

}

  

?運行效果：

?

獲取當前月第一天、最后一天、計算某個月共有多少天