JSON API Android解析教程

Question

So I've been following [this tutorial][1] and watched through it and written all the code (except for a few changes to suit my needs) and whenever I launch the app using an emulator it will display "Progress start" with the loading animation underneath it (like it's supposed to) then it crashes. I messed around with it for a couple of minutes but couldn't figure out how to fix it since I'm still pretty new to Android development.

Error log:

    08-03 17:41:39.293  10235-10255/? E/JSON Parser﹕ Error parsing data org.json.JSONException: Value {"address":"mc.hypixel.net","port":25565,"online":true,"players":{"max":35053,"online":22162,"sample":[]},"motd":"§aHypixel Network\n§c§l2 NEW GAMES! §bPixel Painters §e+ §bMega Skywars","version":{"name":"Hypixel BungeeCord","protocol":4},"ping":203} of type org.json.JSONObject cannot be converted to JSONArray
08-03 17:41:39.295  10235-10255/? E/AndroidRuntime﹕ FATAL EXCEPTION: AsyncTask #1
    Process: in.untie.hypixelmobile, PID: 10235
    java.lang.RuntimeException: An error occured while executing doInBackground()
            at android.os.AsyncTask$3.done(AsyncTask.java:304)
            at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
            at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
            at java.util.concurrent.FutureTask.run(FutureTask.java:242)
            at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
            at java.lang.Thread.run(Thread.java:818)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int org.json.JSONArray.length()' on a null object reference
            at in.untie.hypixelmobile.MainActivity$ProgressTask.doInBackground(MainActivity.java:80)
            at in.untie.hypixelmobile.MainActivity$ProgressTask.doInBackground(MainActivity.java:40)
            at android.os.AsyncTask$2.call(AsyncTask.java:292)
            at java.util.concurrent.FutureTask.run(FutureTask.java:237)
            at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
            at java.lang.Thread.run(Thread.java:818)
08-03 17:41:41.007  10235-10235/? E/WindowManager﹕ android.view.WindowLeaked: Activity in.untie.hypixelmobile.MainActivity has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView{a3a3b8a V.E..... R......D 0,0-1026,348} that was originally added here
            at android.view.ViewRootImpl.<init>(ViewRootImpl.java:363)
            at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:271)
            at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:85)
            at android.app.Dialog.show(Dialog.java:298)
            at in.untie.hypixelmobile.MainActivity$ProgressTask.onPreExecute(MainActivity.java:55)
            at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:591)
            at android.os.AsyncTask.execute(AsyncTask.java:539)
            at in.untie.hypixelmobile.MainActivity.onCreate(MainActivity.java:37)
            at android.app.Activity.performCreate(Activity.java:5990)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

MainActivity.java

   package in.untie.hypixelmobile;

import android.app.ListActivity;
import android.app.ProgressDialog;
import android.content.Context;
import android.os.AsyncTask;
import android.os.Bundle;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.HashMap;

public class MainActivity extends ListActivity {

    private Context context;
    private static String url = "http://api.razex.de/server/status/mc.hypixel.net:25565";

    private static final String PMAX = "Max";
    private static final String PCURRENT = "Current";
    private static final String PING = "Ping";

    ArrayList<HashMap<String, String>> jsonlist = new ArrayList<HashMap<String, String>>();

    ListView lv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        new ProgressTask(MainActivity.this).execute();
    }

    private class ProgressTask extends AsyncTask<String, Void, Boolean> {

        private ProgressDialog dialog;
        private ListActivity activity;
        private Context context;

        public ProgressTask(ListActivity activity) {
            this.activity = activity;
            context = activity;
            dialog = new ProgressDialog(context);
        }

        @Override
        protected void onPreExecute() {
            this.dialog.setMessage("Progress start");
            this.dialog.show();
        }

        @Override
        protected void onPostExecute(Boolean result) {
            if (dialog.isShowing()) {
                dialog.dismiss();
            }

            ListAdapter adapter = new SimpleAdapter(context, jsonlist,
                    R.layout.list_item, new String[]{PMAX, PCURRENT, PING}
                        , new int[]{R.id.ping, R.id.max, R.id.current});

            setListAdapter(adapter);

            lv = getListView();

        }

        @Override
        protected Boolean doInBackground(String... params) {

            JSONParser jParser = new JSONParser();
            JSONArray json = jParser.getJSONFromURL(url);

            for(int i = 0; i < json.length(); i++) {

                try {

                    JSONObject c = json.getJSONObject(i);
                    String ping = c.getString("ping");
                    String pcurrent = c.getString("online");
                    String pmax = c.getString("max");

                    HashMap<String, String> map = new HashMap<String, String>();

                    map.put(PING, ping);
                    map.put(PCURRENT, pcurrent);
                    map.put(PMAX, pmax);
                    jsonlist.add(map);

                } catch(JSONException e) {

                    e.printStackTrace();

                }
            }
            return false;
        }
    }
}

JSONParser.java

package in.untie.hypixelmobile;

import android.util.Log;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.StatusLine;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

public class JSONParser {

    static InputStream iStream = null;
    static JSONObject jarray = null;
    static String json = "";

    public JSONParser() {

    }

    public JSONObject getJSONFromURL(String url) {

        StringBuilder builder = new StringBuilder();
        HttpClient client = new DefaultHttpClient();
        HttpGet httpGet = new HttpGet(url);

        try {

            HttpResponse response = client.execute(httpGet);
            StatusLine statusLine = response.getStatusLine();
            int statusCode = statusLine.getStatusCode();

            if(statusCode == 200) {

                HttpEntity entity = response.getEntity();
                InputStream content = entity.getContent();
                BufferedReader reader = new BufferedReader(new InputStreamReader(content));

                String line;

                while((line = reader.readLine()) != null) {
                    builder.append(line);
                }

            } else {
                Log.e("==>", "Failed to download file");
            }

        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        try {
            jarray = new JSONObject(builder.toString());
        } catch(JSONException e) {
            Log.e("JSON Parser", "Error parsing data " + e.toString());
        }

        return jarray;

    }

}

Answer 1

change

JSONArray json = jParser.getJSONFromURL(url);

to

JSONObject json = jParser.getJSONFromURL(url);

The

for(int i = 0; i < json.length(); i++) {

with

JSONObject c = json.getJSONObject(i);

loop is also irrelevant.

Reason : You are not getting JSON Array in response, you are getting just object. As a quick rule : between { is JSONObject, between [ is JSONArray (where you would use loop to cycle through). Your only array there is sample but there is nothing inside it.

Also note you have players object inside main object! You need to get object players before accessing the max

EDIT :

        JSONParser jParser = new JSONParser();
        JSONObject json = jParser.getJSONFromURL(url);

        //for loop useless

            try {

                //JSONObject c = json.getJSONObject(i); is useless. you are not getting i-th object of array because there is no loop and no array

                String ping = json.getString("ping");
                String pcurrent = json.getString("online"); //if returns true/false use getBoolean instead of getString?

                //max is inside of another object. get it first to get to it.
                JSONObject playersObject = json.getJSONObject("players");
                String pmax = playersObject.getString("max");

                HashMap<String, String> map = new HashMap<String, String>();

                map.put(PING, ping);
                map.put(PCURRENT, pcurrent);
                map.put(PMAX, pmax);
                jsonlist.add(map);

            } catch(JSONException e) {

                e.printStackTrace();

            }