1.判断当前日期是星期几：
Date date = new Date();
Calendar cal = Calendar.getInstance();
cal.setTime(date);
int week = cal.get(Calendar.DAY_OF_WEEK) - 1;//0是星期天，6是星期六
2.格式化日期格式：
Date date = new Date();
SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
String today = format.format(date);
3.跳转到指定日期：
Date date = new Date();
Calendar cal = Calendar.getInstance();
cal.setTime(date);
cal.add(Calendar.DAY_OF_YEAR, -1);
date = cal.getTime();
yesterday = format.format(date);
