可以使用java.util.Calendar来取得相关日期信息

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

public class DateDemo {

    static SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");

    public static void main(String[] args) throws ParseException {
        String date = "2011-04-25 22:28:30";
        System.out.println(date);
        System.out.println("年份：" + getYear(date));
        System.out.println("月份：" + getMonth(date));
        System.out.println("日期：" + getDay(date));
        System.out.println("月初日期是: " + getMinMonthDate(date));
        System.out.println("月末日期是: " + getMaxMonthDate(date));
    }

    /**
     * 获取日期年份
     * @param date
     * @return
     * @throws ParseException
     */
    public static int getYear(String date) throws ParseException{
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(dateFormat.parse(date));
        return calendar.get(Calendar.YEAR);
    }

    /**
     * 获取日期月份
     * @param date
     * @return
     * @throws ParseException
     */
    public static int getMonth(String date) throws ParseException{
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(dateFormat.parse(date));
        return (calendar.get(Calendar.MONTH) + 1);
    }

    /**
     * 获取日期号
     * @param date
     * @return
     * @throws ParseException
     */
    public static int getDay(String date) throws ParseException{
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(dateFormat.parse(date));
        return calendar.get(Calendar.DAY_OF_MONTH);
    }
    /**
     * 获取月份起始日期
     * @param date
     * @return
     * @throws ParseException
     */
    public static String getMinMonthDate(String date) throws ParseException{
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(dateFormat.parse(date));
        calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMinimum(Calendar.DAY_OF_MONTH));
        return dateFormat.format(calendar.getTime());
    }

    /**
     * 获取月份最后日期
     * @param date
     * @return
     * @throws ParseException
     */
    public static String getMaxMonthDate(String date) throws ParseException{
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(dateFormat.parse(date));
        calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
        return dateFormat.format(calendar.getTime());
    }
}

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

public class DateDemo {

	static SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
	
	public static void main(String[] args) throws ParseException {
		String date = "2011-04-25 22:28:30";
		System.out.println(date);
		System.out.println("年份：" + getYear(date));
		System.out.println("月份：" + getMonth(date));
		System.out.println("日期：" + getDay(date));
		System.out.println("月初日期是: " + getMinMonthDate(date));
		System.out.println("月末日期是: " + getMaxMonthDate(date));
	}

	/**
	 * 获取日期年份
	 * @param date
	 * @return
	 * @throws ParseException
	 */
	public static int getYear(String date) throws ParseException{
		Calendar calendar = Calendar.getInstance();
		calendar.setTime(dateFormat.parse(date));
		return calendar.get(Calendar.YEAR);
	}
	
	/**
	 * 获取日期月份
	 * @param date
	 * @return
	 * @throws ParseException
	 */
	public static int getMonth(String date) throws ParseException{
		Calendar calendar = Calendar.getInstance();
		calendar.setTime(dateFormat.parse(date));
		return (calendar.get(Calendar.MONTH) + 1);
	}
	
	/**
	 * 获取日期号
	 * @param date
	 * @return
	 * @throws ParseException
	 */
	public static int getDay(String date) throws ParseException{
		Calendar calendar = Calendar.getInstance();
		calendar.setTime(dateFormat.parse(date));
		return calendar.get(Calendar.DAY_OF_MONTH);
	}
	/**
	 * 获取月份起始日期
	 * @param date
	 * @return
	 * @throws ParseException
	 */
	public static String getMinMonthDate(String date) throws ParseException{
		Calendar calendar = Calendar.getInstance();
		calendar.setTime(dateFormat.parse(date));
		calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMinimum(Calendar.DAY_OF_MONTH));
		return dateFormat.format(calendar.getTime());
	}
	
	/**
	 * 获取月份最后日期
	 * @param date
	 * @return
	 * @throws ParseException
	 */
	public static String getMaxMonthDate(String date) throws ParseException{
		Calendar calendar = Calendar.getInstance();
		calendar.setTime(dateFormat.parse(date));
		calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
		return dateFormat.format(calendar.getTime());
	}
}

lxbccsu (中级程序员) 2011-03-25

不过对日期时间的操作处理，joda-time提供更好的支持
joda-time官网：
http://joda-time.sourceforge.net/

joda-time简介：
http://www.ibm.com/developerworks/cn/java/j-jodatime.html