比较 Date 对象

Created: November-22, 2018

日历，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 之前的日期比较

在 Java 8 之前，可以使用 java.util.Calendar 和 java.util.Date 类来比较日期。Date 类提供了 4 种比较日期的方法：

after，before，compareTo 和 equals 方法比较每个日期的 getTime() 方法返回的值。

compareTo 方法返回正整数。

值大于 0：当 Date 在 Date 参数之后
值大于 0：当 Date 在 Date 参数之前时
值等于 0：当 Date 等于 Date 参数时

如示例所示，equals 结果可能会令人惊讶，因为如果未明确给出，则值（如毫秒）不会使用相同的值进行初始化。

自 Java 8 起

使用 Java 8，可以使用 java.time.LocalDate 来使用与 Date 一起使用的新 Object。LocalDate 实现 ChronoLocalDate ，它是可以插入 Chronology 或日历系统的日期的抽象表示。

要具有日期时间精度，必须使用 Object java.time.LocalDateTime 。LocalDate 和 LocalDateTime 使用相同的方法名称进行比较。

使用 LocalDate 比较日期与使用 ChronoLocalDate 不同，因为时间顺序或日历系统不考虑第一个。

因为大多数应用程序应该使用 LocalDate，所以示例中不包含 ChronoLocalDate。进一步阅读这里。

大多数应用程序应将方法签名，字段和变量声明为 LocalDate，而不是此[ChronoLocalDate]接口。

LocalDate 有 5 种方法来比较日期：

在 LocalDate 参数的情况下，isAfter，isBefore，isEqual，equals 和 compareTo 现在使用这个方法：

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 方法检查参数引用是否等于日期，而 isEqual 直接调用 compareTo0。

如果是 ChronoLocalDate 的其他类实例，则使用 Epoch Day 比较日期。大纪元日计数是一个简单的递增计数天，其中第 0 天是 1970-01-01(ISO)。