請問如何得到上一個月的最後一天呢？

public static  String ss(){
		Calendar   cal=Calendar.getInstance();//当前日期
		cal.set(Calendar.DATE,1);//设为当前月的1号
		cal.add(Calendar.DATE,-1);//减一天，变为上月最后一天
		SimpleDateFormat   simpleDateFormat   =   new   SimpleDateFormat( "yyyy-MM-dd ");
		return simpleDateFormat.format(cal.getTime());//输出2005-04-30
		
		
	}

package com.java.page;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Test {
	public static void main(String[] args) {

			Calendar   cal=Calendar.getInstance();//当前日期
		//	cal.set(Calendar.DATE,1);//设为当前月的1号
		//	cal.add(Calendar.DATE,-1);//减一天，变为上月最后一天
			System.out.println("现在的时间是："+Calendar.MONTH);
			int n=5;
			String[] str = {"1月31号","2月29号","3月31号","4月30号","5月31号","6月30号","7月31号","8月31号","9月30号","10月31号","11月30号","12月31号"};
			for(int i=0;i<12;i++){
				if((cal.get(Calendar.MONTH)+1)==i){
						System.out.println(str[i-1]);
				}
				
			}
			if(Calendar.MONTH==5){
				
				cal.set(Calendar.DAY_OF_MONTH, cal.getActualMaximum(Calendar.DAY_OF_MONTH));
				SimpleDateFormat   simpleDateFormat   =   new   SimpleDateFormat( "yyyy-MM-dd ");
				simpleDateFormat.format(cal.getTime());//输出2005-04-30
				
				System.out.println("现在的时间是："+simpleDateFormat.format(cal.getTime()));
			}
			
	}

}

package com.java.page;

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


public class Student {
	
	public  static void main(String args[])throws Exception{
	//	Calendar cal=Calendar.getInstance();
	//	cal.set(Calendar.MONTH, 6);
	//	cal.set(Calendar.DAY_OF_MONTH, cal.getActualMaximum(Calendar.DAY_OF_MONTH));
	//	System.out.println(cal.getTime());
		System.out.println(getLastDay());
		System.out.println(getLastDayOfLastMonth());
		System.out.println("---------"+ss());
		System.out.println("=========="+sd());
		
	}
	public static String getLastDay(){

		SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");
		  Calendar calendar = Calendar.getInstance();//此时打印它获取的是系统当前时间
		        calendar.add(Calendar.DATE, -1); //
		        Date theDate = calendar.getTime();
		        String s = df.format(theDate);
		        System.out.println(s);

		      return s;

		}
	public static Date getLastDayOfLastMonth() {
		Calendar calendar = new GregorianCalendar();
		calendar.set(Calendar.DATE, 1);
		calendar.set(Calendar.HOUR, 0);
		calendar.set(Calendar.MINUTE, 0);
		calendar.set(Calendar.SECOND, 0);
		calendar.set(Calendar.MILLISECOND, 0);
		long mils = calendar.getTime().getTime();
		mils -= 24 * 60 * 60 * 1000;
		return new Date(mils);
		}
	public static  String ss(){
		Calendar   cal=Calendar.getInstance();//当前日期
		cal.set(Calendar.DATE,1);//设为当前月的1号
		cal.add(Calendar.DATE,-1);//减一天，变为上月最后一天
		SimpleDateFormat   simpleDateFormat   =   new   SimpleDateFormat( "yyyy-MM-dd ");
		return simpleDateFormat.format(cal.getTime());//输出2005-04-30
		
		
	}
	public static String sd(){
		//Calendar   cal=Calendar.getInstance();
		Calendar   cal=new   GregorianCalendar(2000,2,1);
		cal.add(Calendar.MONTH,-1);
		return "2000年3月1号的上个月份最后一天是 "+cal.getActualMaximum(Calendar.DAY_OF_MONTH);
		
	}


}

package com.java.page;

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

public class Hello {
	public static void main(String args[]){
		Calendar cal=Calendar.getInstance();
		System.out.println(cal.get(Calendar.YEAR)+"年");
		System.out.println(cal.get(Calendar.MONTH)+1+"月");
		System.out.println(cal.get(Calendar.DATE)+"日");
		System.out.println("<br/>");
		java.util.Date date=new java.util.Date();
		SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM");
		String nowm=sdf.format(date);
		System.out.println("现在的时间是："+nowm);
	}

}