ArrayList的引用传递的日期对象？

问题描述

我传递日期的的ArrayList 但是当我修改日期对象，所有的日期 ArrayList的内部> 将发生变化。这是一个例子：

I pass dates to an ArrayList but when I change the Date object, all the Dates inside the ArrayList will change. This is an example:

Date currentDate = new Date("6/10/2011");
ArrayList<Date> datesList = new ArrayList();

currentDate.setDate(currentDate.getDate() + 1);
datesList.add(currentDate);

currentDate.setDate(currentDate.getDate() + 1);
datesList.add(currentDate);

currentDate.setDate(currentDate.getDate() + 1);
datesList.add(currentDate);

System.out.println(datesList.toString());

这会打印：

[Mon Jun 13 00:00:00 EDT 2011, Mon Jun 13 00:00:00 EDT 2011, Mon Jun 13 00:00:00 EDT 2011]

为什么出现这种情况，如何我可以解决这个问题的任何想法？

Any idea of why this happening and how could I solve it?

推荐答案

这是因为变量的currentdate 引用日期的单个实例，您已添加到列表中多次。当你调用 currentDate.setDate（currentDate.getDate（）+ 1）你只是更新了同一个对象的状态，并且每次调用时间 datesList 。新增（的currentdate）相同的对象添加到的ArrayList 。

This is because the variable currentDate references a single instance of Date, which you have added to the list many times. When you call currentDate.setDate(currentDate.getDate() + 1) you're simply updating that same object's state, and every time you call datesList.add(currentDate) that same object is added to the ArrayList.

另外请注意，和的是德precated方法。你应该考虑使用日期操作：

Also note that setDate() and getDate() are deprecated methods. You should look into using a Calendar for date manipulation:

Calendar cal = Calendar.getInstance();
cal.set(2011, 5, 10);

ArrayList<Date> datesList = new ArrayList<Date>();

datesList.add(cal.getTime());

cal.add(Calendar.DATE, 1);
datesList.add(cal.getTime());

cal.add(Calendar.DATE, 1);
datesList.add(cal.getTime());

System.out.println( datesList.toString());

或更好，但。

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