如何从URL获取json数据而不丢失Java中的空格？

Question

I'm learning Java, so I decided to toy around with the GW2 with the hope of coding something useful: API: http://wiki.guildwars2.com/wiki/API:Main

I'm trying to get the following data: https://api.guildwars2.com/v1/world_names.json into a Java String, this is my code:

    public static void main(String[] args) {

    URL url = null;
    String jsonData = "";
    try {
        url = new URL("https://api.guildwars2.com/v1/world_names.json");
        InputStream is = url.openStream();
        Scanner scan = new Scanner(is);


        while(scan.hasNext()) {
            jsonData += scan.next();
        }

        scan.close();
    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }



    System.out.println(jsonData);

}

The problem is I'm losing the format of the Strings (I'm missing the blank characters that separate words). This is what you see if you access the URL:

[{"id":"1009","name":"Fort Aspenwood"}, {"id":"1005","name":"Maguuma"}, {"id":"1008","name":"Jade Quarry"},

This is what I get in my String:

{"id":"1009","name":"FortAspenwood"}, {"id":"1005","name":"Maguuma"}, {"id":"1008","name":"JadeQuarry"}

How can I fix that? am I doing something wrong?

My final goal is to convert this data to an object, and then be able to ask for an specific ID or NAME to the api and get more data, such as maps or events, but first things first, since I'm learning and I can't get the String right..

Thank you for reading,

Answer 1

Right from the documentation:

A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace.

You don't want any sort of tokenization, so you can borrow a one-liner from Stupid Scanner tricks :

final String jsonData = new Scanner(is).useDelimiter("\\A").next();

which consumes the entire input stream in one line.

---

NB if you do stick with using a loop, you should use a StringBuilder instead of concatenation ( jsonData += scan.next(); ) because that operation has quadratic asymptotic runtime.

However, I strongly recommend that you use Jackson for all your real-world JSON processing uses.

Answer 2

As I can see you are doing:

while(scan.hasNext()) {
   jsonData += scan.next();
}

next() returns the next token which is separated by spaces, new lines etc. Now since you have to get the whole line from your JSON data, you can do:

while(scan.hasNextLine()) {
   jsonData += scan.nextLine();
}

Answer 3

One way to approach this problem is to use a library, like Jackson, that knows how to parse json input. Here is an example program that goes out to the url you provided and creates a List of DataTransferObject.

This code depends on http://mvnrepository.com/artifact/org.codehaus.jackson/jackson-mapper-asl/1.9.13

This is not strictly speaking the answer to your question but is another approach.

package stackoverflow;

import org.codehaus.jackson.JsonNode;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.type.TypeReference;

import java.io.IOException;
import java.net.URL;
import java.util.List;

public class JsonParser {

    private static final String urlString = "https://api.guildwars2.com/v1/world_names.json";
    public static void main(String[] args) throws IOException {
        JsonParser parser = new JsonParser();

        URL url = new URL(urlString);

        ObjectMapper mapper = new ObjectMapper();

        JsonNode root = mapper.readTree(url.openStream());
        List<DataTransferObject> dtoList = mapper.readValue(root, new TypeReference<List<DataTransferObject>>(){});

        for(DataTransferObject dto : dtoList) {
            System.out.println("DTO: " + dto);
        }
    }

    public static class DataTransferObject {
        public int id;
        public String name;

        @Override
        public String toString() {
            return "ID: " + id + " NAME: " + name;
        }
    }

}