有時在解析json資料中的欄位key是動態可變的時候，由於Gson是使用靜態註解的方式來設定實體物件的，因此我們很難直接對返回的型別來判斷。但Gson在解析過程中如果不知道解析的欄位，就會將所有變數儲存在一個Map中，我們只要例項化這個map就能動態地取出key和value了。先給出一段jsondata，這是天氣預報的資料，其中day_20151002這種key是隨日期而變化的，在實體類中就不能當做靜態變數來處理，我們就通過map來取出其對映物件。{
“resultcode”: “200”,
“reason”: “successed!”,
“result”: {
“sk”: {
“temp”: “24”,
“wind_direction”: “東北風”,
“wind_strength”: “2級”,
“humidity”: “28%”,
“time”: “17:38”
},
“today”: {
“temperature”: “15℃~26℃”,
“weather”: “多雲轉晴”,
“wind”: “東北風微風”,
“week”: “星期日”,
“city”: “桂林”,
“date_y”: “2015年10月11日”,
“dressing_index”: “舒適”,
“dressing_advice”: “建議著長袖T恤、襯衫加單褲等服裝。年老體弱者宜著針織長袖襯衫、馬甲和長褲。”,
“uv_index”: “弱”,
“comfort_index”: “”,
“wash_index”: “較適宜”,
“travel_index”: “較適宜”,
“exercise_index”: “較適宜”,
“drying_index”: “”
},
“future”: {
“day_20151011”: {
“temperature”: “15℃~26℃”,
“weather”: “多雲轉晴”,
“wind”: “東北風微風”,
“week”: “星期日”,
“date”: “20151011”
},
“day_20151012”: {
“temperature”: “16℃~27℃”,
“weather”: “晴轉多雲”,
“wind”: “微風”,
“week”: “星期一”,
“date”: “20151012”
},
“day_20151013”: {
“temperature”: “16℃~26℃”,
“weather”: “多雲轉晴”,
,
“wind”: “微風”,
“week”: “星期二”,
“date”: “20151013”
},
“day_20151014”: {
“temperature”: “17℃~27℃”,
“weather”: “晴”,
“wind”: “北風微風”,
“week”: “星期三”,
“date”: “20151014”
},
“day_20151015”: {
“temperature”: “17℃~28℃”,
“weather”: “晴”,
“wind”: “北風微風”,
“week”: “星期四”,
“date”: “20151015”
},
“day_20151016”: {
“temperature”: “17℃~30℃”,
“weather”: “晴”,
“wind”: “北風微風”,
“week”: “星期五”,
“date”: “20151016”
},
“day_20151017”: {
“temperature”: “17℃~30℃”,
“weather”: “晴”,
“wind”: “北風微風”,
“week”: “星期六”,
“date”: “20151017”
}
}
},
“error_code”: 0
}12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667686970717273747576777879808182相關的實體類如下：public class FutureDay {
private String temperature;
private String weather;
private String wind;
private String week;
private String date;
}1234567public class Result {
private Sk sk;
private Today today;
private Map<String,FutureDay> future;
}12345public class Sk {
private String temp;
private String wind_direction;
private String wind_strength;
private String humidity;
private String time;
}1234567public class Today {
private String temperature;
private String weather;
private String week;
private String city;
private String date_y;
private String dressing_index;
private String dressing_advice;
private String uv_index;
private String comfort_index;
private String wash_index;
private String travel_index;
private String exercise_index;
private String drying_index;
}
12345678910111213141516public class Response {
private String resultcode;
private String reason;
private String error_code;
private Result result;
}123456具體解析過程如下程式碼所示：import java.io.BufferedReader;
import java.io.FileReader;
import java.util.Map;
import weather.*;
import com.google.gson.Gson;

public class GsonParseDynamicKey {
public static void main( String args []){
String jsondata = readJsonFile();//從檔案中讀取出json字串，並打印出來
Gson gson = new Gson();
System.out.println(“Start Gson parse jsondata”);
Response response = gson.fromJson(jsondata, Response.class);
System.out.println(response.toString());
System.out.println(response.getResult().getSk().toString());
System.out.println(response.getResult().getToday().toString());

    Map<String, FutureDay> future = response.getResult().getFuture(); //對動態的key，來建立map，間接從中取出實體類futrue。
    System.out.println("Keyset method");                     //這裡取出value的方法有兩種keySet() entrySet().都給出了遍歷的方法
    for (String key:future.keySet()){                        //遍歷取出key，再遍歷map取出value。
        System.out.println("key:"+key); 
        System.out.println(future.get(key).toString());
    }

    System.out.println("Entryset method");
    for (Map.Entry<String,FutureDay> pair:future.entrySet()){//遍歷取出鍵值對，呼叫getkey()，getvalue()取出key和value。
         System.out.println("key:"+pair.getKey());
         System.out.println(pair.getValue().toString());
   }    
 

}123456789101112131415161718192021222324252627282930這裡順便一提遍歷Map的兩種方法keySet()，entrySet()的差別。
keySet()方法返回的是key的集合set，entrySet()返回的是鍵值對的集合set。雖然兩者從set遍歷取出元素的方法是一樣的，但是根據這個元素取出value的效率有些不同。前者取出的元素是key，還要去原map中遍歷取出value。
後者取出的元素是鍵值對，直接呼叫getkey()，getvalue()方法就能快速取出key和value。顯然在map中存在大量鍵值對時，使用entrySet()來取出value的效率更高。
本文參考：http://blog.csdn.net/Chaosminds/article/details/49049455