Java 二维ArrayList不断添加新值

我在Java中发现了二维ArrayList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]

应该输出如下内容：

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

---

您两次添加相同的

childList

实例。所有操作都在该对象上完成。由于您添加了两次，因此一切都会给人一种发生两次的印象。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));

ArrayList父列表=新建ArrayList（）；
ArrayList childList=新建ArrayList（）；
//将日期添加到子列表
添加（日期1）；
//将“行”添加到父列表
parentList.add（新数组列表（子列表））；//复制
//将另一个日期添加到childList
添加（日期2）；
//将另一行添加到父列表
parentList.add（新数组列表（子列表））；//复制
System.out.println（parentList.get（0））；
System.out.println（parentList.get（1））；

@RohitJain:不，因为列表必须是累积的。所以我也需要第一个元素。

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

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));