java实现在原有日期时间上加几个月或几天

SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");//设置日期格式
Date date = new Date();
System.out.println(df.format(date));   // 当前系统时间        
Date newDate = stepMonth(date, 1);
System.out.println("当前时间前1个月的日期：" + df.format(newDate));
 
 
public static Date stepMonth(Date sourceDate, int month) {
    Calendar c = Calendar.getInstance();
    c.setTime(sourceDate);
    c.add(Calendar.MONTH, month);
 
    return c.getTime();
}

public static void main(String[] args) throws ParseException {
    String time = "2019-6-02 11:05:51";    //当前时间
    int num = 15;    //加的天数
    SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    Date  currdate = format.parse(time);
    System.out.println("初始的时间是：" + time);
    Calendar ca = Calendar.getInstance();
        ca.setTime(currdate);
    ca.add(Calendar.DATE, num);
    currdate = ca.getTime();
    String enddate = format.format(currdate);
    System.out.println("增加天数以后的时间：" + enddate);
 
}

public static void main(String[] args) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        String nowDateString = "2019-05-06 13:34:25";
        System.out.println("当前时间为:" + nowDateString);
        try {
            Date sourceDate = sdf.parse(nowDateString);
            Calendar calendar = Calendar.getInstance();
            calendar.setTime(sourceDate);
            calendar.add(Calendar.DATE,-5);
            Date targetDate = calendar.getTime();
            System.out.println("减去5天后的日期:"+sdf.format(targetDate));

        } catch (ParseException e) {
            e.printStackTrace();
        }

    }

calendar.add(Calendar.DAY_OF_MONTH,1);  
calendar.add(Calendar.DAY_OF_YEAR,1);  
calendar.add(Calendar.DATE,1);