这个随机出生日期生成器有什么问题?

问题描述

我正在尝试为 25 到 50 年前的出生日期编写一个随机的出生日期生成器.我坚持使用 Java 1.4，并试图通过 java.util.Calendar 完成此操作(是的，是的，我知道我应该使用 Joda 库).这是我所在的位置:

I'm trying to write a random date of birth generator for a date of birth between 25 and 50 years ago. I'm stuck with Java 1.4 and am attempting to accomlish this via java.util.Calendar (yes, yes, I know I should be using the Joda libraries). Here's where I'm at:

private static long ONE_YEAR_AS_MILLISECONDS = 365*24*60*60*1000;
private static long TWENTY_FIVE_YEARS_AS_MILLISECONDS = 25*ONE_YEAR_AS_MILLISECONDS;
private static long FIFTY_YEARS_AS_MILLISECONDS = 50*ONE_YEAR_AS_MILLISECONDS;

private static String generateDOB()
{
    //Equation for calculating a random number within a given range is as follows:  Min + (int)(Math.random() * ((Max - Min) + 1))
    long someTimeBetween25And50YearsInMilliSeconds = TWENTY_FIVE_YEARS_AS_MILLISECONDS +
                (long)(Math.random() * ((FIFTY_YEARS_AS_MILLISECONDS - TWENTY_FIVE_YEARS_AS_MILLISECONDS) + 1));
    Calendar dob = Calendar.getInstance();
    dob.setTimeInMillis(dob.getTimeInMillis() - someTimeBetween25And50YearsInMilliSeconds);
    StringBuffer sb = new StringBuffer();
    sb.append(dob.get(Calendar.YEAR)).append("/").append(dob.get(Calendar.MONTH)+1).append("/").append(dob.get(Calendar.DAY_OF_MONTH));
    return sb.toString();
}

它执行正常，但 100 个日期的输出使所有日期都在 2008 年或 2009 年:

It executes OK, but an output of 100 dates yeilds all of them in either 2008 or 2009:

2008/10/8
2008/9/22
2008/7/26
2008/8/20
2008/3/31
2008/10/1
2009/1/3
2009/2/2
2009/4/9
2008/9/20
...

但是，我需要 25-50 年前的日期.我错过了什么?

However, I need dates between 25-50 years ago. What am I missing?

推荐答案

这个问题很棘手，但是您的 ONE_YEAR_AS_MILLISECONDS 常量中出现数字溢出.

Quite a gotcha that problem, but you're getting a numeric overflow in your ONE_YEAR_AS_MILLISECONDS constant.

将整数文字更改为长整型:

Change the integer literals to longs:

private static long ONE_YEAR_AS_MILLISECONDS = 365L * 24L * 60L * 60L * 1000L;

现在对我来说很好用.

编辑:

因此，在我解决这个问题时，我没有使用 Calendar 类:

Consequently, while I was working this out, I didn't use the Calendar class:

Date d = new Date(System.currentTimeMillis() - someTimeBetween25And50YearsInMilliSeconds);

SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
return sdf.format(d);

这对我来说看起来更容易阅读.

That looks a bit easier to read to me.

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