# Java Date operation
## 前情提要
Java Date 物件底層都是透過TimeZone 來去做時區判斷，但是這會依賴系統時間，即使透過TimeZone setDefaultTimeZone 來去做完時區轉換的方式，但是這無法一勞永逸且不具有彈性的設計，所以才會有這篇文章的誕生。\
我這邊也不賣關子，我自己測試下來的結論，Date物件不好操作，日後有關時區問題，我是會棄用他的。
### 人狠，話不多，上扣！！
/**
* convert date String with time zone to LocalDateTime Object
*
* @param dateStr
* @param formatterPattern
* @param dateStrTimeZone
* @param toTimeZon
* @return
* @throws ParseException
*/
private static LocalDateTime convertDateStrWithTimeZone(String dateStr, String formatterPattern, TimeZone dateStrTimeZone, ZoneId toTimeZon) throws ParseException {
/**
* 這邊會發生兩件事情
* 1.SimpleDateFormat 認定字串 為 GMT+0
* 2.Date Object 將轉換的GMT+0 加上系統預設時區
* Ex. "2023-03-23 12:00:00" -> 是GMT+0 時區的時間
* 當下系統 default 時區是 GMT+8
* 轉換結果就會變成 2023-03-23 20:00:00[GMT+8]
*/
// Create a SimpleDateFormat object for parsing the date string
SimpleDateFormat sdf = new SimpleDateFormat(formatterPattern);
// Set the time zone of the SimpleDateFormat object to GMT+0
sdf.setTimeZone(dateStrTimeZone);
// Parse the date string into an Instant object and transfer to LocalDataTime
return sdf.parse(dateStr).toInstant().atZone(toTimeZon).toLocalDateTime();
}
/**
* @param dateTime
* @param fromZone
* @param toZone
* @return
*/
private static LocalDateTime convertTimeZoneUseZoneId(LocalDateTime dateTime, ZoneId fromZone, ZoneId toZone) {
ZonedDateTime originZoneDateTime = ZonedDateTime.of(dateTime, fromZone);
ZonedDateTime transZoned = originZoneDateTime.withZoneSameInstant(toZone);
return transZoned.toLocalDateTime();
}
/**
* @param dateTime
* @param fromOffset
* @param toOffset
* @return
*/
public static LocalDateTime convertTimeZoneUseZoneOffset(LocalDateTime dateTime, ZoneOffset fromOffset, ZoneOffset toOffset) {
long timeDifference = toOffset.getTotalSeconds() - fromOffset.getTotalSeconds();
return dateTime.plusSeconds(timeDifference);
}
其實 `convertTimeZoneUseZoneId`跟 `convertTimeZoneUseZoneOffset` 背後是繼承關係，所以基本上是一樣的東西，有可能差別在於效能上，但這部分我就沒有花太多時間著墨了。
### 測試！
```java
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.TimeZone;
public class DateTimeConvertWithTimeZoneUtil {
final static DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SSS");
static SimpleDateFormat sf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
final static String GMT0_STR = "GMT+0";
final static String GMT8_STR = "GMT+8";
final static ZoneId gmt8Zone = ZoneId.of(GMT8_STR);
final static ZoneId gmt0Zone = ZoneId.of(GMT0_STR);
final static TimeZone timeZoneGMT0 = TimeZone.getTimeZone(GMT0_STR);
final static TimeZone timeZoneGMT8 = TimeZone.getTimeZone(GMT8_STR);
final static TimeZone timeZoneGMT_M8 = TimeZone.getTimeZone("GMT-8");
public static void main(String[] args) throws ParseException {
// String dateStr = "1970-01-01 00:00:00.000";
String dateStr = "2023-01-05 8:00:00.000";
String dateStrPattern = "yyyy-MM-dd HH:mm:ss";
System.out.println("dateStr >> " + dateStr);
System.out.println("GMT+8 dateStr to GMT+0 LocalDateTime >>> " + df.format(convertDateStrWithTimeZone(dateStr, dateStrPattern, timeZoneGMT8
, gmt0Zone)));
System.out.println("GMT+8 dateStr to GMT+8 LocalDateTime >>> " + df.format(convertDateStrWithTimeZone(dateStr, dateStrPattern,
timeZoneGMT8, gmt8Zone)));
System.out.println("GMT+0 dateStr to GMT+8 LocalDateTime >>> " + df.format(convertDateStrWithTimeZone(dateStr, dateStrPattern,
timeZoneGMT0, gmt8Zone)));
System.out.println("GMT+0 dateStr to GMT+0 LocalDateTime >>> " + df.format(convertDateStrWithTimeZone(dateStr, dateStrPattern,
timeZoneGMT0, gmt0Zone)));
System.out.println("=================================================");
LocalDateTime GMT8LDT = convertDateStrWithTimeZone(dateStr, dateStrPattern, timeZoneGMT8, gmt8Zone);
LocalDateTime GMT0LDT = convertDateStrWithTimeZone(dateStr, dateStrPattern, timeZoneGMT0, gmt0Zone);
long start2 = System.currentTimeMillis();
for (int i = 18; i >= -18; i--) {
System.out.println("GMT+8 dateStr to GMT" + (i > 0 ? "+" + i : i) + " LocalDateTime >>> " +
df.format(convertTimeZoneUseZoneId(GMT8LDT, ZoneOffset.ofHours(8), ZoneOffset.ofHours(i))));
}
for (int i = 18; i >= -18; i--) {
System.out.println("GMT+0 dateStr to GMT" + (i > 0 ? "+" + i : i) + " LocalDateTime >>> " +
df.format(convertTimeZoneUseZoneId(GMT0LDT, ZoneOffset.ofHours(0), ZoneOffset.ofHours(i))));
}
System.out.println("two end: >>>" + (System.currentTimeMillis() - start2));
System.out.println("======== change ZoneId =======");
long start1 = System.currentTimeMillis();
for (int i = 18; i >= -18; i--) {
System.out.println("GMT+8 dateStr to GMT" + (i > 0 ? "+" + i : i) + " LocalDateTime >>> " + df.format(convertTimeZoneUseZoneOffset(GMT8LDT,
ZoneOffset.ofHours(8), ZoneOffset.ofHours(i))));
}
System.out.println(" reverse ~~ ");
for (int i = 18; i >= -18; i--) {
System.out.println("GMT+0 dateStr to GMT" + (i > 0 ? "+" + i : i) + " LocalDateTime >>> " + df.format(convertTimeZoneUseZoneOffset(GMT0LDT,
ZoneOffset.ofHours(0), ZoneOffset.ofHours(i))));
}
System.out.println("one end: >>>" + (System.currentTimeMillis() - start1));
}
}
```
## 結論
不斷的測試及驗證，最後發現Date 物件對於時區操作，並不是這麼靈活且適合，但我本身就不是那麼愛用Date，Java 8 過後新增的LocalDateTime 等等新增的時間類別，相較於Date 比較好用，但是呢～～～\
永遠都有但是工作上既有產品還是使用Date 所以你還是無法避免這狀況，只能將狀況變成最適合自己的操作方式，盡可能提高自己工作的效率。🔥
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