Question

我在Java中发现了使用二维ArrayLists的以下行为：

ArrayList<ArrayList<Date>> parentList = new ArrayList<ArrayList<Date>>();
ArrayList<Date> childList = new ArrayList<Date>();

//Adding a date to childList
childList.add(date1);

//Adding a 'row' to parentList
parentList.add(childList);

//Adding another date to childList
childList.add(date2);

//Adding another row to parentList
parentList.add(childList);

System.out.println(parentList.get(0));
System.out.println(parentList.get(1));

//Expected output:   
//   [date1]
//   [date1, date2]

//Real output:   
//   [date1, date2]
//   [date1, date2]

所以看来，尽管将childList添加到parentList，但是新添加到childList的项目也立即被添加到parentList中。

我想出了以下针对此问题的解决方案：

ArrayList<ArrayList<Date>> parentList = new ArrayList<ArrayList<Date>>();
ArrayList<Date> childList = new ArrayList<Date>();
ArrayList<Date> cacheList = new ArrayList<Date>();

//Adding a date to childList
childList.add(date1);

//Adding a 'row' to parentList
parentList.add(childList);

//Saving all current dates in cacheList
cacheList = childList;
childList = new ArrayList<Date>();

for (int i = 0; i < cacheList.size(); i++)
{
    childList.add(cacheList.get(i));
}

cacheList = new ArrayList<Date>();

//Adding another date to childList
childList.add(date2);

//Adding another row to parentList
parentList.add(childList);

System.out.println(parentList.get(0));
System.out.println(parentList.get(1));

//Expected output:   
//   [date1]
//   [date1, date2]

//Real output:   
//   [date1]
//   [date1, date2]

但我发现这个解决方案有点多余和丑陋。

所以我想知道：这个问题有更优雅的解决方案吗？

编辑：请注意，我需要childList累积。所以它应该包含所有元素，但每次添加一个元素，然后它就会存储在parentList中。

例如：

for (int i = 0; i < parentList.size(); i++)
{
     System.out.println(parentList.get(i));
}

应输出如下内容：

[date1]
[date1, date2]
[date1, date2, date3]
[date1, date2, date3, date4]
etc.

Answer 1

您要添加两次相同的childList实例。所有操作都在该对象上完成。由于您添加了两次，所以一切都给人以两次发生的印象。要解决此问题，请添加childList的副本，如下所示：

ArrayList<ArrayList<Date>> parentList = new ArrayList<ArrayList<Date>>();
ArrayList<Date> childList = new ArrayList<Date>();

//Adding a date to childList
childList.add(date1);

//Adding a 'row' to parentList
parentList.add(new ArrayList<Date>(childList)); // COPY!

//Adding another date to childList
childList.add(date2);

//Adding another row to parentList
parentList.add(new ArrayList<Date>(childList)); // COPY!

System.out.println(parentList.get(0));
System.out.println(parentList.get(1));

二维ArrayList不断添加新值

1 个答案: