Jimmy's Blog » json

Java ME (J2ME) JSON Implementation For Array Object

jimmy — Tue, 13 Sep 2011 16:47:04 +0000

One of the comment in JSON ME tutorial (http://jimmod.com/blog/2010/03/java-me-j2me-json-implementation-tutorialsample/) asking about how to parse this kind of JSON :

{\”api_status\”: \”OK\”,\”threads\”: [{\"tid\": \"2\",\"title\": \"First Title\" }, { \"tid\": \"4\", \"title\": \"this is 2nd Title\"}]}
-246">

Latest JSON ME library : https://github.com/upictec/org.json.me/

From the JSON format we can easily see that the object have 2 property:

api_status : String
threads : Array of object

The Threads Object:

tid : Integer
title : String

My solution is not exactly the same because I have different approach on handling the arrays.

First I will make the threads class, I name it ThreadData. It should also implement the JSONable interface (see the 1st tutorial for the code).

class ThreadData implements JSONAble {
	private int tid;
	private String title;
	public int getTid() {
		return tid;
	}
	public void setTid(int tid) {
		this.tid = tid;
	}
	public String getTitle() {
		return title;
	}
	public void setTitle(String title) {
		this.title = title;
	}
	public void fromJSON(String jsonString) {
		try {
			JSONObject json = new JSONObject(jsonString);
			setTid(json.getInt("tid"));
			setTitle(json.getString("title"));
		} catch (JSONException ex) {
			ex.printStackTrace();
		}
	}
	public String toJSON() {
		JSONObject inner = new JSONObject();
		try {
			inner.put("tid", getTid());
			inner.put("title", getTitle());
		} catch (JSONException ex) {
			ex.printStackTrace();
		}
		return inner.toString();
	}
 
	public String toString() {
		return getTid() + "-" + getTitle(); 
	}
}

Above implementation is just like the 1st tutorial, the main class JSONable implementation is different. Please focus on the JSONArray usage.

class Response implements JSONAble {
 
	private String api_status;
	private ThreadData[] threads;
 
	public String getApi_status() {
		return api_status;
	}
	public void setApi_status(String apiStatus) {
		api_status = apiStatus;
	}
	public ThreadData[] getThreads() {
		return threads;
	}
	public void setThreads(ThreadData[] threads) {
		this.threads = threads;
	} 
 
	public void fromJSON(String jsonString) {
		try {
			JSONObject json = new JSONObject(jsonString);
			setApi_status(json.getString("api_status"));
			JSONArray jsonArray = json.getJSONArray("threads");
			int total = jsonArray.length();
			ThreadData[] threads = new ThreadData[total];
			for (int i=0;i<total;i++) {
				String threadsJSON = jsonArray.getString(i);
				threads[i] = new ThreadData();
				threads[i].fromJSON(threadsJSON);
			}
			setThreads(threads);
		} catch (JSONException ex) {
			ex.printStackTrace();
		}
	}
	public String toJSON() {
		JSONObject inner = new JSONObject();
		try {
			inner.put("api_status", getApi_status());
			JSONArray jsonArray = new JSONArray();
			ThreadData[] threads = getThreads();
			for (int i=0;i<threads.length;i++) {
				jsonArray.put(threads[i].toJSON());
			}
			inner.put("threads", jsonArray);
		} catch (JSONException ex) {
			ex.printStackTrace();
		}
		return inner.toString();
	}
 
	public String toString() {
		String response = getApi_status() + "-";
		ThreadData[] threads = getThreads();
		for (int i=0;i<threads.length;i++) {
			response += "[" + threads[i].toString() + "]";
		}
		return response; 
	}
}

So when you setup the startapp function:

	protected void startApp() throws MIDletStateChangeException {
		//Converting Object to JSON
		Response response = new Response();
		ThreadData[] threads = new ThreadData[2];
		threads[0] = new ThreadData();
		threads[0].setTid(2);
		threads[0].setTitle("First Title");
		threads[1] = new ThreadData();
		threads[1].setTid(4);
		threads[1].setTitle("this is 2nd Title");
		response.setApi_status("OK");
		response.setThreads(threads);
		System.out.println("Convert to JSON : "+response.toJSON());
 
 
		//Converting JSON to Object
		String sample = "{\"api_status\":\"SAMPLE\",\"threads\":[\"{\\\"tid\\\":10,\\\"title\\\":\\\"3rdTitle\\\"}\",\"{\\\"tid\\\":15,\\\"title\\\":\\\"this is 4th Title\\\"}\"]}";
		Response reponse = new Response();
		response.fromJSON(sample);
		System.out.println("Convert from JSON : "+ response);
	}

It will have output:

Convert to JSON : {"api_status":"OK","threads":["{\"tid\":2,\"title\":\"First Title\"}","{\"tid\":4,\"title\":\"this is 2nd Title\"}"]}
Convert from JSON : SAMPLE-[10-3rdTitle][15-this is 4th Title]

It demonstrate the sample for creating JSON string and parsing JSON string which have array/object inside the JSON.

Java ME (J2ME) JSON Implementation Tutorial/Sample

jimmy — Wed, 03 Mar 2010 05:48:25 +0000

JSON is one of the most popular format used for communicating between (Java ME) J2ME client and App Server. Strangely it’s hard to find JSON implementation sample in J2ME. So I decide to write a sample.

– Next Tutorial : Java ME (J2ME) JSON Implementation For Array Object –

The JSON library I used is from json.org : http://www.json.org/java/org.json.me.zip
(link reported not valid anymore, try the one I’ve uploaded : http://www.megaupload.com/?d=QDW9N225)

Extract it to your source code folder.

For example we have this User Class:

public class User{
   private int id;
   private String name;
   private String description;
}

To implement JSON to this class I usually create an Interface JSONAble:

public interface JSONAble {
String toJSON();
void fromJSON(String jsonString);
}

To make the User class is JSON compatible let’s add the JSONAble interface & implement it:

public class User implements JSONAble{
 
	private int id;
	private String name;
	private String description;
 
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getDescription() {
		return description;
	}
	public void setDescription(String description) {
		this.description = description;
	}
	public String toString() {
		return getId()+"-"+getName()+"-"+getDescription();
	}
 
	public void fromJSON(String jsonString) {
		try {
			JSONObject json = new JSONObject(jsonString);
			setId(json.getInt("id"));
			setName(json.getString("name"));
			setDescription(json.getString("description"));
		} catch (JSONException ex) {
			ex.printStackTrace();
		}
	}
	public String toJSON() {
		JSONObject inner = new JSONObject();
		try {
			inner.put("id", getId());
			inner.put("name", getName());
			inner.put("description", getDescription());
		} catch (JSONException ex) {
			ex.printStackTrace();
		}
		return inner.toString();
	}
}

To test it let’s create a MIDlet that have these startApp() implementation:

	protected void startApp() throws MIDletStateChangeException {
		//Converting Object to JSON
		User user = new User();
		user.setId(10);
		user.setName("jimmy");
		user.setDescription("testing json in j2me");
		System.out.println("Convert to JSON : "+user.toJSON());
 
		//Converting JSON to Object
		String sample = "{\"id\":99,\"name\":\"tester\",\"description\":\"This is JSON Data\"}";
		User user2 = new User();
		user2.fromJSON(sample);
		System.out.println("Convert from JSON : "+user2);
	}

As you can see the output results:
Convert to JSON : {"name":"jimmy","description":"testing json in j2me","id":10} Convert from JSON : 99-tester-This is JSON Data

The toJSON() function is the implementation how the object converted to JSON format. In J2ME we still implement it manually since we don’t have Java reflection which can enable automatically scan the variables.
What the JSON library help is building the JSON data. We just need to put the variables.

On the fromJSON() function what it does is getting out the data from JSON format and set it to class variables.

Next I will cover about converting more complex class structure.
Or create a fromJSON () & toJSON() generator (:?). Let see..

Thanks for reading this, although this is the basic hope it’ll help

– Next Tutorial : Java ME (J2ME) JSON Implementation For Array Object –