Parse a JSON file in Java

Question

I am using Java to parse a JSON file into a hash map so that i can search by typing a date and then get the cost, tax and profit for that date.

So that if i write:

2001年09月18日 13:11:01

I get:

7.14, 1.81 and 31.10

Here is part of the JSON file:

{ 
 "stat":[ 
 { 
 "date":" 2001年09月18日 13:11:01 ",
 "cost":" 7.14 ",
 "tax":" 1.81 ",
 "profit":" 31.10 "
 },
 { 
 "date":" 2001年09月18日 14:15:02 ",
 "cost":" 7.80 ",
 "tax":" 0.99 ",
 "profit":" 30.20 "
 }
 ]
}

And my code using Jackson:

public static void main(String[] args) throws JsonGenerationException {
 try {
 ObjectMapper mapper = new ObjectMapper();
 JSONParser parser = new JSONParser();
 JSONObject jsonobj = (JSONObject) parser.parse(new FileReader("test.json")); 
 JSONArray jsons = (JSONArray) jsonobj.get("stat");
 Map<Integer, Map<String, String>> jsonmaps = new HashMap<Integer, Map<String, String>>();
 int i=0;
 for (Object j : jsons) {
 Map<String, String> map = new HashMap<String, String>();
 JSONObject o = (JSONObject) j;
 map = mapper.readValue(j.toString(), new TypeReference<Map<String, String>>() {
 });
 jsonmaps.put(i, map);
 i++;
 }
 System.out.println(jsonmaps.get(0).get("date"));
 } catch (IOException e) {
 e.printStackTrace();
 } catch (ParseException e) {
 // TODO Auto-generated catch block
 e.printStackTrace();
 }
 }

• Stephen Rauch: Welcome! Was something specific you were hoping to get from a review?

Answer 1

The initial value of a map variable is not used, you can avoid this assignment and inline the variable. o is not used too.

If indexes in a map is consequent then it is better to use a List instead of Map. So the part of the code can be rewritten following way:

List<Map<String, String>> jsonmaps = new ArrayList<>();
for (Object j : jsons) {
 jsonmaps.add(mapper.readValue(j.toString(),
 new TypeReference<Map<String, String>>() {}));
}

Answer 2

If you use a POJO, your code could be reduced to:

public class Stats {
 public List<Stat> stat;
}
public class Stat {
 public String date;
 public Double cost;
 public Double tax;
 public Double profit;
}
public static void main(String[] args)
 ObjectMapper mapper = new ObjectMapper();
 try {
 Stats stats = mapper.readValue(new File("test.json"), Stats.class);
 System.out.println(stats.stat.get(0).date);
 } catch (JsonParseException e) {
 e.printStackTrace();
 } catch (JsonMappingException e) {
 e.printStackTrace();
 } catch (IOException e) {
 e.printStackTrace();
 }
}

• Vogel612: storing a date in a String is generally a bad idea, especially since the stats are looked up by their date, which means your code can result in significant lookup time increases ... The POJO is a good idea, though. A Map<Date, Stat> is easily creatable from the list you have there
• gaborage: @Vogel612 I agree, state.date should be stored in LocalDateTime instead of String, thanks for the feedback.