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

public class Test {
     public static void main(String[] args) {
         SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
         Date currentdate=new Date();
         String systime= format.format(currentdate);
         System.out.println("当前系统时间：====="+systime);
         String curtime=getBeforeOneDay(currentdate);
         System.out.println("当前系统时间的前一天的时间：====="+curtime);
                 System.out.println("得到某个时间的前一天的时间：====="+getBeforeOneDayByStringDate("20210407"));
     }
     /**
     * 获取当前系统时间的前一天时间
     * @param dateStr
     * @return Date
     * @return
     */
    public static String getBeforeOneDay(Date currentdate) { 
            SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            Calendar calendar = Calendar.getInstance();  
            calendar.setTime(currentdate);  
            calendar.add(Calendar.DAY_OF_MONTH, -1);  
            currentdate = calendar.getTime();
            String dayTime=format.format(currentdate);
            return dayTime;  
    }

public static String getBeforeOneDayByStringDate(String dateString) { 
            SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            Date d;
            try {
                d = format .parse(dateString);
                Calendar calendar = Calendar.getInstance();
                calendar.setTime(d);
                calendar.add(Calendar.DAY_OF_MONTH, -1);  // 在当前日基础上-1
                System.out.println(format .format(calendar.getTime()));
                //获取String类型的时间
                
            } catch (ParseException e) {
                e.printStackTrace();
            }
            return format .format(calendar.getTime());  
    }


}