HashMap vs. LinkHashMap

Problem

在報表的開發上，剛好遇到一個問題，我同時要餵資料給兩張iReport template(i.e. P1, P2)，然後後端用for loop將裡頭的資料抓出來，原本的方式:

1:  Map dataMap = new HashMap ();  
2:  dataMap.put("NIG071P1", p1Data);  
3:  dataMap.put("NIG071P2", p2Data);  

但是此方式，並沒有如預期的依照順序產生P1, P2，反而是先印出P2、再印出P1，這是因為HashMap並不會根據你insert的順序來將資料抓出來

Solution

改用LinkedHashMap (Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). )。
這是一個會根據我們insert的順序來把資料抓出來的Map，改寫如下，即可解決上述的問題：

1:  //LinkedHashMap defines the iteration ordering,   
2:  //which is normally the order in which keys were   
3:  //inserted into the map (insertion-order)  
4:  Map dataMap = new LinkedHashMap ();  
5:  dataMap.put("NIG071P1", p1Data);  
6:  dataMap.put("NIG071P2", p2Data);  

Reference: http://docs.oracle.com/javase/6/docs/api/java/util/LinkedHashMap.html