String time="2012-05-12，18:00";
DateFormat f= new SimpleDateFormat("yyyy-MM-dd HH:mm");
Date date = f.parse(time);
Date date2 = f.parse("2012-05-13 8:01");
int i = date.compareTo(date2);
/*the value 0 if the argument Date is equal to this Date;
a value less than 0 if this Date is before the Date argument; 
and a value greater than 0 if this Date is after the Date argument.*/

//date.before(date2);
//date.after(date2);

------解决方案--------------------

package leedp.login;

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

public class DateCompare {
	public static void main(String[] args) throws ParseException {
		/*
		 * the value 0 if the argument Date is equal to this Date; a value less
		 * than 0 if this Date is before the Date argument; and a value greater
		 * than 0 if this Date is after the Date argument.
		 */
		String time = "2012-05-12，18:00";
		time = time.replace("，", " ");
		DateFormat f = new SimpleDateFormat("yyyy-MM-dd HH:mm");
		Date date1 = f.parse(time);
		Date date2 = f.parse("2012-05-13 8:01");
		int i = date1.compareTo(date2);
		System.out.println(i);
		Calendar c = Calendar.getInstance();
		Date date = null;
		try {
			date = new SimpleDateFormat("yyyy-MM-dd HH:mm").parse("2012-05-12 18:00");
		} catch (ParseException e) {
			e.printStackTrace();
		}
		c.setTime(date);
		int day = c.get(Calendar.DATE);
		c.set(Calendar.DATE, day - 1);

		String dayBefore = new SimpleDateFormat("yyyy-MM-dd  HH:mm").format(c
				.getTime());
		System.out.println(dayBefore);

	}

}


------解决方案--------------------

探讨

那我想在请问一下，如果现在的时间是个变量，就比如是当天，那前面那个比较，就是当天的前一天，如何获取，当天的前一天了？比如，今天是2012-05-12，18:00昨天就是2012-05-11，18:00如何获取这个时间了？


------解决方案--------------------
具体代码你自己会写的，简单说就是将string转成date,在java里面date类型可以直接比较的。