Java获取某年某周的第一天

1. 获取本周的第一天

public static String getFirstDay0fWeek() {
    Calendar calendar = Calendar.getInstance();
    //设置该周第一天为星期一
    calendar.set(Calendar.DAY_OF_WEEK,Calendar.MONDAY);

    return date2String(calendar.getTime(), null);
}

2. 获取本月的第一天

public static String getFirstDay0fMonth() {
    Calendar calendar = Calendar.getInstance();
    int dayOfMonth = calendar.get(Calendar.DAY_OF_MONTH);
    calendar.add(Calendar.DAY_OF_MONTH, -dayOfMonth + 1);

    return date2String(calendar.getTime(), null);
}

3. 获取今年的第一天

public static String getFirstDay0fYear() {
    Calendar calendar = Calendar.getInstance();
    int dayOfYear = Calendar.getInstance().get(Calendar.DAY_OF_YEAR);
    calendar.add(Calendar.DAY_OF_YEAR, -dayOfYear + 1);

    return date2String(calendar.getTime(), null);
}