Java數字、日期、字符串之間的轉換

數字格式化

private static void f() {
    //保留兩位小數：四舍五入
    DecimalFormat decimalFormat = new DecimalFormat(".##");
    String format = decimalFormat.format(12.123476);
    //12.12
    System.out.println(format);

    //保留兩位小數：百分比形式
    decimalFormat=new DecimalFormat(".##%");
    String format1 = decimalFormat.format(12.123546);
    //1212.35%
    System.out.println(format1);
    //每三位加入,分隔符 同時不保留位數（四舍五入）
    decimalFormat=new DecimalFormat(",###");
    String format2 = decimalFormat.format(1235467890.56);
    //1,235,467,891
    System.out.println(format2);
     //每三位加入,分隔符 同時保留3位小數（四舍五入）
    decimalFormat=new DecimalFormat(",###.###");
    String format3 = decimalFormat.format(123456.123456);
    //123,456.123
    System.out.println(format3);
}

日期格式化

LocalDateTime now = LocalDateTime.now();
DateTimeFormatter pattern = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
//LocalDateTime--->字符串
String format = pattern.format(now);
System.out.println("LocalDateTime轉成字符串:\t" + format);
LocalDateTime parse = LocalDateTime.parse(format, pattern);
System.out.println("字符串轉成LocalDateTime:\t" + parse);

/*Calendar和String的互相轉換*/
Calendar calendar = Calendar.getInstance();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
//Calendar--->Date--->String
Date date = calendar.getTime();
System.out.println(sdf.format(date));//2020-07-10 19:17:42
//String--->Date--->Calendar
String str="2020-07-10 19:17:42";
Date date1 = sdf.parse(str);
calendar.setTime(date1);
System.out.println(calendar);

日期格式之間的轉換

private static void f8() {
    //LocalDateTime類
    LocalDateTime now = LocalDateTime.now();
    System.out.println(now);//2020-07-10T16:49:57.962

    /*LocalDateTime可以直接轉換成LocalDate，LocalTime*/
    LocalDate localDate = now.toLocalDate();
    LocalTime localTime = now.toLocalTime();

    System.out.println(localDate);//2020-07-10
    System.out.println(localTime);//16:52:54.369

    //Calendar類
    Calendar calendar = Calendar.getInstance();
    calendar.set(2020,1,1,12,0,0);

    /*Calendar轉成Date*/
    Date date = calendar.getTime();
    System.out.println(date);//Sat Feb 01 12:00:00 CST 2020

    /*Date轉成Calendar*/
    Calendar calendar1 = Calendar.getInstance();
    calendar1.setTime(date);
    System.out.println(calendar1);
..
　　//Date--->Instant
　　Instant instant = Instant.ofEpochMilli(date.getTime());
　　System.out.println(instant);//2020-07-10T11:32:27.831Z
　　//String--->Instant
　　Instant parse1 = Instant.parse("2020-07-10T11:32:27.831Z");
　　System.out.println(parse1);//2020-07-10T11:32:27.831Z

個人覺得比較有用的：

• Calendar轉成Date

  • Date date = calendar.getTime();

• Date轉成Calendar

  • Calendar calendar1 = Calendar.getInstance();

  • calendar1.setTime(date);

•  終極大殺器：日期類先轉成String，再由String轉成其他日期類
  • 舉個例子：LocalDateTime不好直接轉成Calendar，可以通過String，Date進行中轉
    

    LocalDateTime localDateTime = LocalDateTime.of(2020, 7, 11, 12, 0);
    //LocalDateTime--->String
    String string = localDateTime.format(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"));
    //String--->Date
    Calendar calendar = Calendar.getInstance();
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    Date parse = sdf.parse(string);
    //Date--->Calendar
    //即：LocalDateTime--->String，String--->Date，Date--->Calendar
    calendar.setTime(parse);