创建Java对象一般问题

发布于 2024-10-10 19:09:32 字数 432 浏览 5 评论 0原文

哪个对于内存管理或任何其他原因更好，或者这两种情况是否相同：

Calendar currentDateTime = Calendar.getInstance();
int i= foo.getSomething(currentDateTime);
Bar bar= foo.getBar(currentDateTime);

另一个代码块：

int i= foo.getSomething(Calendar.getInstance());
Bar bar= foo.getBar(Calendar.getInstance());

一般问题是，是否更好地获取对象的实例，然后在需要时使用该实例，或者使每次需要时都会调用 getInstance() 。而且，如果不处理单例，而是制作一个普通的 POJO，答案会改变吗？

原文

Which is better for memory management, or any other reason, or are these two scenarios the same:

Calendar currentDateTime = Calendar.getInstance();
int i= foo.getSomething(currentDateTime);
Bar bar= foo.getBar(currentDateTime);

The other code block:

int i= foo.getSomething(Calendar.getInstance());
Bar bar= foo.getBar(Calendar.getInstance());

The general question is, is it better to get an instance of an object, then use that instance when needed, or make the getInstance() call each time when needed.
And, does the answer change if not dealing with a singleton, but making a plain POJO?

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岁月打碎记忆 2024-10-17 19:09:33

使用更少的内存和更少的 CPU
使用更多的内存和更多的 CPU

垃圾收集器可以在最后一次使用对 Calendar 实例的引用后立即收集它。

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花桑 2024-10-17 19:09:32

需要明确的是，Calendar 实际上并不是单例。每次调用 Calendar.getInstance() 时都会返回一个新对象。

这意味着您问题的答案取决于函数 getSomething() 和 getBar() 是否具有导致 foo 存储新 Calendar 实例的副作用。一般来说，良好的编程实践表明情况不会如此。

编辑：但是，每次调用 Calendar.getInstance() 时，您可能会得到不同的日期。这可能是一个重要的细节，具体取决于您的函数正在执行的操作。

编辑 2：它还取决于您执行上述过程的频率。正如另一个答案指出的那样，实例化 Calendar 对象可能会很密集。如果只做两次，那么缓存也没关系。如果您经常这样做，那么您可以考虑改变您的方法或进行一些缓存。

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别靠近我心 2024-10-17 19:09:32

日历是一个非常昂贵的对象（我所知道的所有库中最昂贵的日期对象之一）。调用 getInstance() 也非常昂贵。如果您必须使用日历，您可以考虑缓存它。这实际上取决于您为什么需要它。

获取和存储当前时间的最有效方法是使用 long 原语。

long currentDateTime = System.currentTimeMillis();

如果您在内部使用 GMT 时间，则可以使用 EDIT 存储当前日期

int currentDay = (int)(System.currentTimeMillis() / 86400000);

：值得在您的计算机上进行测试，而 getInstance() 相对昂贵，但仍然相当快。在我的旧盒子上大约需要 20 微秒。在快速机器上，currentTimeMillis() 可能需要 140 纳秒。

下面打印

Calendar.getInstance() took on average 20088 ns. java.util.GregorianCalendar[time=1294086899359 ... deleted ...]
System.currentTimeMillis() took on average 938 ns. 1294086899377

代码

int runs = 10000;
long start = System.nanoTime();
Calendar cal = null;
for(int i=0;i<runs;i++)
    cal = Calendar.getInstance();
long time = System.nanoTime() - start;
System.out.println("Calendar.getInstance() took on average "+time/runs+" ns. "+cal);

long start2 = System.nanoTime();
long now = 0;
for(int i=0;i<runs;i++)
    now = System.currentTimeMillis();
long time2 = System.nanoTime() - start2;
System.out.println("System.currentTimeMillis() took on average "+time2/runs+" ns. "+now);

Calendar is a very expensive object (one of the most expensive date objects in any library I know of). Calling getInstance() is very expensive too. If you have to use Calendar you could look at caching it. It really depends on why you need it.

The most efficient way to obtain and store the current time is using a long primitive.

long currentDateTime = System.currentTimeMillis();

If you use GMT time internally, you can store the current day with

int currentDay = (int)(System.currentTimeMillis() / 86400000);

EDIT: It is worth testing on your machine, while getInstance() is relatively expensive but it is still fairly fast. On my old box it takes ~20 micro-seconds. On a fast machine currentTimeMillis() can take 140 nano-seconds.

The following prints

Calendar.getInstance() took on average 20088 ns. java.util.GregorianCalendar[time=1294086899359 ... deleted ...]
System.currentTimeMillis() took on average 938 ns. 1294086899377

code

int runs = 10000;
long start = System.nanoTime();
Calendar cal = null;
for(int i=0;i<runs;i++)
    cal = Calendar.getInstance();
long time = System.nanoTime() - start;
System.out.println("Calendar.getInstance() took on average "+time/runs+" ns. "+cal);

long start2 = System.nanoTime();
long now = 0;
for(int i=0;i<runs;i++)
    now = System.currentTimeMillis();
long time2 = System.nanoTime() - start2;
System.out.println("System.currentTimeMillis() took on average "+time2/runs+" ns. "+now);

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