如何使用Gson API为复杂嵌套的json结构提取特定的键值对

Question

My final aim is to have a map with the a authored name and the extracted value from the Json below (english-value for param1 and param2). eg: {(SW: 5000 w),(RW: 5800 W)}

string =  

[
    {
        "title":"Specifications",
        "sort":1,
        "specValues":[
            {
            "name":"xxx",
            "type":"yyy",
            "englishValue":"xx.0 lb",
            "metricValue":"yy.0 kg",
            "sort":4
            },
            {
            "name":"Param1",
            "type":"measurement",
            "englishValue":"5000.0 W",
            "metricValue":"5000.0 W",
            "sort":99
            },
            {
            "name":"Param2",
            "type":"measurement",
            "englishValue":"5800.0 W",
            "metricValue":"5800.0 W",
            "sort":99
            },
            {
            "name":"Frequency",
            "type":"measurement",
            "englishValue":"60.0 Hz",
            "metricValue":"60.0 Hz",
            "sort":99
            }
        ]
    }
]

Here is my code to extract it and the errors below it:

public Map<String, String> parseGenGson(String jsonLine) {
     Map <String,String> generatorMap = new LinkedHashMap <String,String>();
     try {
        JsonElement jelement = new JsonParser().parse(jsonLine);

        JsonArray  jarray1 = jelement.getAsJsonArray();
        JsonArray jarray = (JsonArray) jarray1.get(2);
        JsonObject jobject1 = jarray.getAsJsonObject();
        JsonArray jarray2 = jobject1.getAsJsonArray("specValue");
        JsonObject jobject = jarray2.get(1).getAsJsonObject();
        String result1 = jobject.get("englishValue").toString();
        generatorMap.put("Running Watts",result1);
        jobject = jarray.get(2).getAsJsonObject();
        String result2 = jobject.get("englishValue").toString();
        generatorMap.put("Starting Watts",result2);         
        log.info("Map of generators" + generatorMap);
        return generatorMap;
     } catch (Exception e){
         log.error(e.getMessage());
         return generatorMap;
     }
    }

Error:

Index: 2, Size: 1

Something is off with the structure I am using, I used GSon API. Any ideas?

Answer 1

The first line is the one that is throwing the error:

JsonArray jarray = (JsonArray) jarray1.get(2);

jarray1 contains just one element (your root object, the one with properties title , sort , specValues ) so the error you get makes sense.

Try something like this:

Map<String, String> generatorMap = new LinkedHashMap<String, String>();
try {
    JsonElement jelement = new JsonParser().parse(jsonLine);

    JsonArray jarray1 = jelement.getAsJsonArray();
    JsonObject rootObject = (JsonObject) jarray1.get(0);
    JsonArray specValues = rootObject.getAsJsonArray("specValues");

    JsonObject first = (JsonObject) specValues.get(0);
    JsonObject second = (JsonObject) specValues.get(1);

    String result1 = first.get("englishValue").toString();
    generatorMap.put("Running Watts", result1);
    String result2 = second.get("englishValue").toString();
    generatorMap.put("Starting Watts", result2);
    return generatorMap;
} catch (Exception e) {
    log.error(e.getMessage());
    return generatorMap;
}

Answer 2

    JsonArray  jarray1 = jelement.getAsJsonArray();
    JsonArray jarray = (JsonArray) jarray1.get(0);
    JsonObject jobject1 = jarray.getAsJsonObject();
    JsonArray jarray2 = jobject1.getAsJsonArray("specValue");
    JsonObject jobject = jarray2.get(0).getAsJsonObject();
    String result1 = jobject.get("englishValue").toString();
    generatorMap.put("Running Watts",result1);
    jobject = jarray.get(1).getAsJsonObject();

Reading the docs JsonArray get method is inherited from java.util.List so the first element is the index 0. Try this and let me know!