二維ArrayList中不斷添加新值

Question

我發現在Java中與二維的ArrayList以下行爲：

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] 

如此看來到，即便在childList中已添加到parentList，新增項目到的childList也立即被添加到parentList。

我想出了這個問題，以下解決方案：

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] 

但我發現這個解決方案有點多餘和醜陋。

所以我想知道：這個問題有沒有更優雅的解決方案？

編輯：請注意，我需要childList累計。所以它應該包含所有元素，但是每次再添加一個元素，然後將其存儲在parentList中。

例如：

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

應該輸出類似：

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

Answer 1

您兩次添加相同childList實例。所有操作都在該對象上完成。既然你添加了兩次，所有事情都會給人兩次發生的印象。要解決該問題，請添加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));