JSON Array iteration in Android/Java

Question

I am building an android app that needs to download and synchronise with an online database, I am sending my query from the app to a php page which returns the relevant rows from a database in JSON format.

can someone please tell me the best way to iterate through a JSON array?

I receive an array of objects:

[{json object},{json object},{json object}]

What is the simplest piece of code I could use to access the JSONObjects in the array?

EDIT: now that I think of it the method I used to iterate the loop was:

for (String row: json){
     id = row.getInt("id");
     name = row.getString("name");
     password = row.getString("password");
}

So I guess I had was somehow able to turn the returned Json into and iterable array. Any Ideas how I could achieve this?

I apologise for my vaguness but I had this working from an example I found on the web and have since been unable to find it.

Answer 1

I think this code is short and clear:

int id;
String name;
JSONArray array = new JSONArray(string_of_json_array);
for (int i = 0; i < array.length(); i++) {
    JSONObject row = array.getJSONObject(i);
    id = row.getInt("id");
    name = row.getString("name");
}

Is that what you were looking for?

Answer 2

I have done it two different ways,

1.) make a Map

        HashMap<String, String> applicationSettings = new HashMap<String,String>();
        for(int i=0; i<settings.length(); i++){
            String value = settings.getJSONObject(i).getString("value");
            String name = settings.getJSONObject(i).getString("name");
            applicationSettings.put(name, value);
        }

2.) make a JSONArray of names

    JSONArray names = json.names();
    JSONArray values = json.toJSONArray(names);
    for(int i=0; i<values.length(); i++){
        if (names.getString(i).equals("description")){
            setDescription(values.getString(i));
        }
        else if (names.getString(i).equals("expiryDate")){
            String dateString = values.getString(i);
            setExpiryDate(stringToDateHelper(dateString)); 
        }
        else if (names.getString(i).equals("id")){
            setId(values.getLong(i));
        }
        else if (names.getString(i).equals("offerCode")){
            setOfferCode(values.getString(i));
        }
        else if (names.getString(i).equals("startDate")){
            String dateString = values.getString(i);
            setStartDate(stringToDateHelper(dateString));
        }
        else if (names.getString(i).equals("title")){
            setTitle(values.getString(i));
        }
    }

Answer 3

Unfortunately , JSONArray doesn't support foreach statements, like:

for(JSONObject someObj : someJsonArray) {
    // do something about someObj
    ....
    ....
}

Answer 4

When I tried @vipw's suggestion, I was faced with this exception: The method getJSONObject(int) is undefined for the type JSONArray

This worked for me instead:

int myJsonArraySize = myJsonArray.size();

for (int i = 0; i < myJsonArraySize; i++) {
    JSONObject myJsonObject = (JSONObject) myJsonArray.get(i);

    // Do whatever you have to do to myJsonObject...
}

Answer 5

If you're using the JSON.org Java implementation, which is open source, you can just make JSONArray implement the Iterable interface and add the following method to the class:

@Override
public Iterator iterator() {
    return this.myArrayList.iterator();
}

This will make all instances of JSONArray iterable, meaning that the for (Object foo : bar) syntax will now work with it (note that foo has to be an Object, because JSONArrays do not have a declared type). All this works because the JSONArray class is backed by a simple ArrayList, which is already iterable. I imagine that other open source implementations would be just as easy to change.

Answer 6

On Arrays, look for:

JSONArray menuitemArray = popupObject.getJSONArray("menuitem"); 

Answer 7

You are using the same Cast object for every entry. On each iteration you just changed the same object instead creating a new one.

This code should fix it:

JSONArray jCastArr = jObj.getJSONArray("abridged_cast");
ArrayList<Cast> castList= new ArrayList<Cast>();

for (int i=0; i < jCastArr.length(); i++) {
    Cast person = new Cast();  // create a new object here
    JSONObject jpersonObj = jCastArr.getJSONObject(i);

    person.castId = (String) jpersonObj.getString("id");
    person.castFullName = (String) jpersonObj.getString("name");

    castList.add(person);
}
details.castList = castList;

Answer 8

While iterating over a JSON array (org.json.JSONArray, built into Android), watch out for null objects; for example, you may get "null" instead of a null string.

A check may look like:

s[i] = array.isNull(i) ? null : array.getString(i);

Answer 9

Alternatively, especially on Android where there is no iterator(), you could define your own iterator :

import java.util.Iterator;
import org.json.JSONArray;
import org.json.JSONException;

public final
class JSONArrayIterator<E> implements Iterator<E> {
    private JSONArray jsonArray;
    private int l;
    private int i;

    JSONArrayIterator(JSONArray jsonArray) {
        this.jsonArray = jsonArray;
        this.l = jsonArray.length();
        this.i = 0;
    }
    @SuppressWarnings("unchecked")
    public E next() {
        try {
            Object o = this.jsonArray.get(this.i++);
            return (E)o;
        } catch (JSONException ex) {
        }
        return null;
    }

}

It works when you assume the element types of your array are all the same. This might not be considered a good practice in many cases, so you might want to add more type-checking.

But then you can then use :

for (JSONObject row: new JSONArrayIterator<JSONObject>(jsonArray)){
     id = row.getInt("id");
     name = row.getString("name");
     password = row.getString("password");
}