在JSON对象上使用分组操作类型

Question

I have JSON data in the following format.

 [{
    "id": 16966,
    "post": "This is about road!",
    "category": "road",
},
.
.
.]

I want to group JSON entries according to their categories. So, I will get all road related entries in one datastructure, (say list). I know that I can put the data into Mongo DB or even a relational database and do querying. Without doing that, is there some easy method to do this?

Answer 1

If you gonna read the entire JSON file, an easy way is to first read all the entries into a List<Data> and then group them in a Map<String, List<Data>> .

class Data {
    private int id;
    private String post;
    private String category;

    //getter, equals, hashcode, toString, etc.
}

and then:

public class Test {    
    public static void main(String args[] ) throws Exception {
        Gson gson = new Gson();
        List<Data> list = gson.fromJson(new BufferedReader(new FileReader("myJson.json")), new TypeToken<List<Data>>(){}.getType());
        Map<String, List<Data>> groupedMap = list.stream().collect(Collectors.groupingBy(Data::getCategory));

        groupedMap.forEach((k, v) -> System.out.println(k + " => " + v));
    }    
}

which outputs:

road => [Data [id=16966, post=This is about road!, category=road], Data [id=16965, post=This is about road!, category=road]]
land => [Data [id=16961, post=This is about land!, category=land]]

I added some entries to the file. I guess you could write your own deserializer too to get rid of the step when you have a temporary list and store directly in the map, but you asked for an easy way :-). Also note that I'm using java-8 and Gson, but you can also achieve this without it (but you'll write more code).

Answer 2

Take a look on JXPath. This library allows running XPath queries on collections of java objects. So, you can map JSON to your java model using one of popular JSON parser (eg Jackson or Gson) and then JXPath to run XPath queries on your collection.

For more information refer here: http://commons.apache.org/proper/commons-jxpath/