bug fix: crash when string is null

Readme.md

@@ -20,7 +20,7 @@ Just add these lines in your `build.gradle`
 
 ```
 dependencies {
-    compile 'nctu.fintech:appmate:1.3.0'
+    compile 'nctu.fintech:appmate:1.3.1'
 }
 ```
 

lib/build.gradle

@@ -2,8 +2,8 @@ apply plugin: 'com.android.library'
 
 def groupId = 'nctu.fintech'
 def artifactId = 'appmate'
-def code = 8
-def version = '1.3.0'
+def code = 9
+def version = '1.3.1'
 
 android {
     compileSdkVersion 22

lib/src/main/java/nctu/fintech/appmate/Tuple.java

@@ -71,8 +71,8 @@ private Core getCore() {
         return mCore;
     }
 
-   private JsonPrimitive getElem(String key) {
-        return (JsonPrimitive) mData.get(key);
+   private JsonElement getElem(String key) {
+        return mData.get(key);
     }
 
     /**
@@ -258,7 +258,12 @@ public Set<Map.Entry<String, String>> entrySet() {
      * @throws ClassCastException 當該值無法被轉型為 {@link String} 型別
      */
     public String get(String key) {
-        JsonPrimitive primitive = getElem(key);
+        JsonElement element = mData.get(key);
+        if (element.isJsonNull()) {
+            return null;
+        }
+
+        JsonPrimitive primitive = element.getAsJsonPrimitive();
         if (primitive.isString()) {
             return primitive.getAsString();
         } else {