Java根据日期计算周

//起始周为 周日

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Calendar cal = Calendar.getInstance();
Date time = sdf.parse("2017-1-20");
cal.setTime(time);
System.out.println("要计算日期为:" + sdf.format(cal.getTime()));
int dayWeek = cal.get(Calendar.DAY_OF_WEEK);// 从星期天开始为1~周六7
System.out.println(dayWeek);

if(dayWeek == 1) {
cal.add(Calendar.DATE, -1);
for(int i =1;i<=7;i++) {
cal.add(Calendar.DATE, 1);
System.out.println("--->"+sdf.format(cal.getTime()));
}
} else {
cal.add(Calendar.DATE, -(6-(7-dayWeek)));
cal.add(Calendar.DATE, -1);
for(int i =1;i<=7;i++) {
cal.add(Calendar.DATE, 1);
System.out.println("--->"+sdf.format(cal.getTime()));
}
}