比較 Date 物件

Created: November-22, 2018

日曆，日期和 LocalDate

Version < Java SE 8

before，after，compareTo 和 equals 方法

//Use of Calendar and Date objects    
final Date today = new Date();
final Calendar calendar = Calendar.getInstance();
calendar.set(1990, Calendar.NOVEMBER, 1, 0, 0, 0);
Date birthdate = calendar.getTime();

final Calendar calendar2 = Calendar.getInstance();
calendar2.set(1990, Calendar.NOVEMBER, 1, 0, 0, 0);
Date samebirthdate = calendar2.getTime();

//Before example
System.out.printf("Is %1$tF before %2$tF? %3$b%n", today, birthdate, Boolean.valueOf(today.before(birthdate)));
System.out.printf("Is %1$tF before %1$tF? %3$b%n", today, today, Boolean.valueOf(today.before(today)));
System.out.printf("Is %2$tF before %1$tF? %3$b%n", today, birthdate, Boolean.valueOf(birthdate.before(today)));

//After example
System.out.printf("Is %1$tF after %2$tF? %3$b%n", today, birthdate, Boolean.valueOf(today.after(birthdate)));
System.out.printf("Is %1$tF after %1$tF? %3$b%n", today, birthdate, Boolean.valueOf(today.after(today)));
System.out.printf("Is %2$tF after %1$tF? %3$b%n", today, birthdate, Boolean.valueOf(birthdate.after(today)));

//Compare example
System.out.printf("Compare %1$tF to %2$tF: %3$d%n", today, birthdate, Integer.valueOf(today.compareTo(birthdate)));
System.out.printf("Compare %1$tF to %1$tF: %3$d%n", today, birthdate, Integer.valueOf(today.compareTo(today)));
System.out.printf("Compare %2$tF to %1$tF: %3$d%n", today, birthdate, Integer.valueOf(birthdate.compareTo(today)));

//Equal example
System.out.printf("Is %1$tF equal to %2$tF? %3$b%n", today, birthdate, Boolean.valueOf(today.equals(birthdate)));
System.out.printf("Is %1$tF equal to %2$tF? %3$b%n", birthdate, samebirthdate,
            Boolean.valueOf(birthdate.equals(samebirthdate)));
System.out.printf(
            "Because birthdate.getTime() -> %1$d is different from samebirthdate.getTime() -> %2$d, there are millisecondes!%n",
            Long.valueOf(birthdate.getTime()), Long.valueOf(samebirthdate.getTime()));

//Clear ms from calendars
calendar.clear(Calendar.MILLISECOND);
calendar2.clear(Calendar.MILLISECOND);
birthdate = calendar.getTime();
samebirthdate = calendar2.getTime();

System.out.printf("Is %1$tF equal to %2$tF after clearing ms? %3$b%n", birthdate, samebirthdate,
            Boolean.valueOf(birthdate.equals(samebirthdate)));

Version => Java SE 8

isBefore，isAfter，compareTo 和 equals 方法

//Use of LocalDate
final LocalDate now = LocalDate.now();
final LocalDate birthdate2 = LocalDate.of(2012, 6, 30);
final LocalDate birthdate3 = LocalDate.of(2012, 6, 30);

//Hours, minutes, second and nanoOfsecond can also be configured with an other class LocalDateTime
//LocalDateTime.of(year, month, dayOfMonth, hour, minute, second, nanoOfSecond);

//isBefore example
System.out.printf("Is %1$tF before %2$tF? %3$b%n", now, birthdate2, Boolean.valueOf(now.isBefore(birthdate2)));
System.out.printf("Is %1$tF before %1$tF? %3$b%n", now, birthdate2, Boolean.valueOf(now.isBefore(now)));
System.out.printf("Is %2$tF before %1$tF? %3$b%n", now, birthdate2, Boolean.valueOf(birthdate2.isBefore(now)));

//isAfter example
System.out.printf("Is %1$tF after %2$tF? %3$b%n", now, birthdate2, Boolean.valueOf(now.isAfter(birthdate2)));
System.out.printf("Is %1$tF after %1$tF? %3$b%n", now, birthdate2, Boolean.valueOf(now.isAfter(now)));
System.out.printf("Is %2$tF after %1$tF? %3$b%n", now, birthdate2, Boolean.valueOf(birthdate2.isAfter(now)));

//compareTo example
System.out.printf("Compare %1$tF to %2$tF %3$d%n", now, birthdate2, Integer.valueOf(now.compareTo(birthdate2)));
System.out.printf("Compare %1$tF to %1$tF %3$d%n", now, birthdate2, Integer.valueOf(now.compareTo(now)));
System.out.printf("Compare %2$tF to %1$tF %3$d%n", now, birthdate2, Integer.valueOf(birthdate2.compareTo(now)));

//equals example
System.out.printf("Is %1$tF equal to %2$tF? %3$b%n", now, birthdate2, Boolean.valueOf(now.equals(birthdate2)));
System.out.printf("Is %1$tF to %2$tF? %3$b%n", birthdate2, birthdate3, Boolean.valueOf(birthdate2.equals(birthdate3)));

//isEqual example
System.out.printf("Is %1$tF equal to %2$tF? %3$b%n", now, birthdate2, Boolean.valueOf(now.isEqual(birthdate2)));
System.out.printf("Is %1$tF to %2$tF? %3$b%n", birthdate2, birthdate3, Boolean.valueOf(birthdate2.isEqual(birthdate3)));

Java 8 之前的日期比較

在 Java 8 之前，可以使用 java.util.Calendar 和 java.util.Date 類來比較日期。Date 類提供了 4 種比較日期的方法：

after，before，compareTo 和 equals 方法比較每個日期的 getTime() 方法返回的值。

compareTo 方法返回正整數。

值大於 0：當 Date 在 Date 引數之後
值大於 0：當 Date 在 Date 引數之前時
值等於 0：當 Date 等於 Date 引數時

如示例所示，equals 結果可能會令人驚訝，因為如果未明確給出，則值（如毫秒）不會使用相同的值進行初始化。

自 Java 8 起

使用 Java 8，可以使用 java.time.LocalDate 來使用與 Date 一起使用的新 Object。LocalDate 實現 ChronoLocalDate ，它是可以插入 Chronology 或日曆系統的日期的抽象表示。

要具有日期時間精度，必須使用 Object java.time.LocalDateTime 。LocalDate 和 LocalDateTime 使用相同的方法名稱進行比較。

使用 LocalDate 比較日期與使用 ChronoLocalDate 不同，因為時間順序或日曆系統不考慮第一個。

因為大多數應用程式應該使用 LocalDate，所以示例中不包含 ChronoLocalDate。進一步閱讀這裡。

大多數應用程式應將方法簽名，欄位和變數宣告為 LocalDate，而不是此[ChronoLocalDate]介面。

LocalDate 有 5 種方法來比較日期：

在 LocalDate 引數的情況下，isAfter，isBefore，isEqual，equals 和 compareTo 現在使用這個方法：

int compareTo0(LocalDate otherDate) {
    int cmp = (year - otherDate.year);
    if (cmp == 0) {
        cmp = (month - otherDate.month);
        if (cmp == 0) {
            cmp = (day - otherDate.day);
        }
    }
    return cmp;
}

equals 方法檢查引數引用是否等於日期，而 isEqual 直接呼叫 compareTo0。

如果是 ChronoLocalDate 的其他類例項，則使用 Epoch Day 比較日期。大紀元日計數是一個簡單的遞增計數天，其中第 0 天是 1970-01-01(ISO)。