performance - 为什么此Java代码具有此年龄验证日期比较？

Question

Here is a response到question有关使用Java计算年龄的信息。

/**
 * This Method is unit tested properly for very different cases , 
 * taking care of Leap Year days difference in a year, 
 * and date cases month and Year boundary cases (12/31/1980, 01/01/1980 etc)
**/

public static int getAge(Date dateOfBirth) {

    Calendar today = Calendar.getInstance();
    Calendar birthDate = Calendar.getInstance();

    int age = 0;

    birthDate.setTime(dateOfBirth);
    if (birthDate.after(today)) {
        throw new IllegalArgumentException("Can't be born in the future");
    }

    age = today.get(Calendar.YEAR) - birthDate.get(Calendar.YEAR);

    // If birth date is greater than todays date (after 2 days adjustment of leap year) then decrement age one year   
    if ( (birthDate.get(Calendar.DAY_OF_YEAR) - today.get(Calendar.DAY_OF_YEAR) > 3) ||
            (birthDate.get(Calendar.MONTH) > today.get(Calendar.MONTH ))){
        age--;

     // If birth date and todays date are of same month and birth day of month is greater than todays day of month then decrement age
    }else if ((birthDate.get(Calendar.MONTH) == today.get(Calendar.MONTH )) &&
              (birthDate.get(Calendar.DAY_OF_MONTH) > today.get(Calendar.DAY_OF_MONTH ))){
        age--;
    }

    return age;
}

这段代码可以正常工作，但是为什么要进行以下比较：
(birthDate.get(Calendar.DAY_OF_YEAR) - today.get(Calendar.DAY_OF_YEAR) > 3)

我到目前为止已经创建了一个巨大的电子表格，其中包含一年中的全天差异，以尝试查看它可能涵盖的情况，但是我看不到其他比较未涵盖的任何内容。谁能解释包括此比较的目的？以某种方式更有效吗？

Answer 1

以下来自ThreetenBP的代码示例（Java-8的反向端口）支持以下语句：不需要进行年度检查：

@Override 
public long until(Temporal endExclusive, TemporalUnit unit) { 
LocalDate end = LocalDate.from(endExclusive); 
    if (unit instanceof ChronoUnit) { 
         switch ((ChronoUnit) unit) { 
             case DAYS: return daysUntil(end); 
             case WEEKS: return daysUntil(end) / 7; 
             case MONTHS: return monthsUntil(end); 
             case YEARS: return monthsUntil(end) / 12; 
             case DECADES: return monthsUntil(end) / 120; 
             case CENTURIES: return monthsUntil(end) / 1200; 
             case MILLENNIA: return monthsUntil(end) / 12000; 
             case ERAS: return end.getLong(ERA) - getLong(ERA); 
         } 
         throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit); 
     } 
     return unit.between(this, end); 
} 

[...]     

private long monthsUntil(LocalDate end) { 
   long packed1 = getProlepticMonth() * 32L + getDayOfMonth();  // no overflow 
   long packed2 = end.getProlepticMonth() * 32L + end.getDayOfMonth();  // no overflow 
   return (packed2 - packed1) / 32; 
} 

case YEARS: return monthsUntil(end) / 12;行（表达式 birthday.until(today, YEARS)和 YEARS.between(birthday, today)是等效的-一个委派给另一个）使用与OP引用的以下简化代码相同的算法，并且不引用任何年度检查：

age = today.get(Calendar.YEAR) - birthDate.get(Calendar.YEAR);

if (birthDate.get(Calendar.MONTH) > today.get(Calendar.MONTH)) {
    age--;
}else if ((birthDate.get(Calendar.MONTH) == today.get(Calendar.MONTH )) &&
          (birthDate.get(Calendar.DAY_OF_MONTH) > today.get(Calendar.DAY_OF_MONTH ))){
    age--;
}

问题出现了：为什么要进行年度检查？

a）海报原本是认真对待年度想法的，后来又忘了清理

b）张贴者希望“改善”表演

以下Java-8代码演示了如果认真对待并作为完整版本，则说明基于天的算法的问题（库的选择在此无关紧要，仅算法很重要）：

LocalDate birthday = LocalDate.of(2001, 3, 6);
LocalDate today = LocalDate.of(2016, 3, 5); // leap year

int age = today.getYear() - birthday.getYear();
if (birthday.getDayOfYear() > today.getDayOfYear()) {
    age--;
}
System.out.println("age based on day-of-year: " + age); // 15 (wrong)
System.out.println("age based on month and day-of-month: " 
  + ChronoUnit.YEARS.between(birthday, today)); // 14 (correct)

结论：

您引用的拟议的年度条款仅是噪音，因为算法的其余部分与Java-8的功能相对应。年度检查可能源自某些较早的基于年度建议代码的版本，并且尚未被清除。

为了回答您的最后一个问题：像这样不必要的检查不是好办法。就性能而言是高效的（尽管我们在这里谈论微优化）。