Associates the specified value with the specified key in this map(optional operation). If the map previously contained . a mapping for this key, the old value is replaced by the specified value. (A map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true .))
public JSONObject accumulate (String key, Object value) 累積value到這個key下。這個方法同element()方法類似，特殊的是，如果當前已經存在一個value在這個key下那么一個JSONArray將會存儲在這個key下來保存所有累積的value。如果已經存在一個JSONArray，那么當前的value就會添加到這個JSONArray中
。相比之下replace方法會替代先前的value
Accumulate values under a key. It is similar to the element method except that if there is already an object stored
under the key then a JSONArray is stored under the key to hold all of the accumulated values. If there is already a
JSONArray, then the new value is appended to it. In contrast, the replace method replaces the previous value.
public JSONObject element (String key, Object value) 將鍵/值對放到這個JSONObject對象里面。如果當前value為空(null)，那么如果這個key存在的話，這個key就會移除掉。如果這
個key之前有value值，那么此方法會調用accumulate()方法。
Put a key/value pair in the JSONObject. If the value is null, then the key will be removed from the JSONObject if it is
present. If there is a previous value assigned to the key, it will call accumulate.
轉載：http://ljhzzyx.blog.163.com/blog/static/3838031220126810430157/