java获取这周的开始时间

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public static void main(String[] args) {           Calendar c=Calendar.getInstance();         SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");         //今天         String today=df.format("今天时间："+c.getTime());         System.out.println(today);         //今天是周几         int dayOfWeek = c.get(Calendar.DAY_OF_WEEK);                //         dayOfWeek-=2;         //今天-1天（昨天）         c.add(Calendar.DAY_OF_MONTH, -dayOfWeek);           String yesterday=df.format(c.getTime());           System.out.println("周一时间   "+yesterday);           System.out.println("今天周几   "+dayOfWeek);     }