Cannot cast to an Extended Class in Java

Question

I looked around for a similar issue but couldn't find anything that matched it.

I'm trying to extend the built-in JSONObject to add some functionality, like so:

public class MyJSONObject extends JSONObject {

    // Easily return an integer from a JSONObject, handling when the value is null.
    //
    public Integer getIntegerUnlessNull(String key) throws JSONException {
        String key_value = this.getString (key);

        if ( key_value.equals("null") ) {
            return null;

        } else {
            return Integer.parseInt( key_value );
        }
    }
}

However, when I try to cast it, I get a java.lang.ClassCastException error:

private JSONArray    jsonClients;
        MyJSONObject clientJSONRecord;

clientJSONRecord = (MyJSONObject) jsonClients.getJSONObject(0);

The full error message is:

java.lang.ClassCastException: org.json.JSONObject cannot be cast to com.insightemissions.trak.extensions.MyJSONObject

Any help?

Cheers,

JP

If the source is not real instance of MyJSONObject, it is not possible to cast. — kosa, Dec 06 '13 at 04:41
The JSONObject comes from this library: http://www.json.org/javadoc/org/json/JSONObject.html#JSONObject() — Joshua Pinter, Dec 06 '13 at 04:46
@HarshitRathi I don't think the actual JSON matters in this case. If I simply remove the casting, the code works fine. I'm simply trying to add some additional functionality to keep the code DRY. — Joshua Pinter, Dec 06 '13 at 04:47
@Nambari Can you explain a bit more. The `getJSONObject(index)` methods results in a JSONObject instance. — Joshua Pinter, Dec 06 '13 at 04:48
http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html may help you. — kosa, Dec 06 '13 at 05:00

score 18 · Accepted Answer · edited Dec 06 '13 at 05:48

18

jsonClients.getJSONObject(0) returns an object of the type JSONObject which is your parent type.

You cannot cast it to the inherited type. It only works the other way, i.e. casting an inherited class to a parent class. This has nothing to do with your objects in particular, it just the way inheritance works.

Because you get an instance of JSONObject from the method and you cannot control how it's instantiated, you could add a constructor to your MyJSONObject class to create an object from the parent object:

public MyJSONObject(JSONObject parent) {
    super(parent.toString());
}

And use it this way:

JSONObject parent = jsonClients.getJSONObject(0);
MyJSONObject child = new MyJSONObject(parent);

edited Dec 06 '13 at 05:48

Joshua Pinter

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answered Dec 06 '13 at 04:47

Szymon

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You're correct, that's what it returns. How do I go about turning it into a MyJSONObject to unlock the additional functionality? – Joshua Pinter Dec 06 '13 at 04:48
Hmm... okay, making more sense, except why the `parent.toString()`? – Joshua Pinter Dec 06 '13 at 05:25
I think that's a JSONObject constructor that will be easiest to use. – Szymon Dec 06 '13 at 05:32
That constructor doesn't exist. How would I go about constructing a JSONObject from an existing JSONObject? – Joshua Pinter Dec 06 '13 at 05:40
It is in the documentation though... I used the link you provided. – Szymon Dec 06 '13 at 05:46
1

Sorry, you're right. Worked like a charm. just had to fix a missing colon in your reply. Nice work Szymon, thanks. – Joshua Pinter Dec 06 '13 at 05:47
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/42624/discussion-between-josh-pinter-and-szymon) – Joshua Pinter Dec 06 '13 at 05:53

score 2 · Answer 2 · answered Dec 06 '13 at 05:01

2

The problem you have is that the objects inside the JSONArray (I presume the JSONArray object is created by the library) do not contain MyJSONObject objects that are defined by you.

Your code would work only if you created the JSONArray yourself and populated it with MyJSONObject objects.

Given what you are trying to achieve with this "extended functionality", I think inheritance is much of an overkill.

Why not just use a helper method?

public Integer getIntegerUnlessNull(JSONObject, String key) throws JSONException {
    String key_value = object.getString (key);

    if ( key_value.equals("null") ) {
        return null;

    } else {
        return Integer.parseInt( key_value );
    }
}

Then you can just do this:

Integer getInteger = getIntegerUnlessNull(object, "key");
if (getInteger == null) {
    // if null do something
}

answered Dec 06 '13 at 05:01

Lai Xin Chu

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In this particular case, you're probably right with going with the helper class, however, I'm keen to find out how to extend it with more functionality in case I need to build on it more. – Joshua Pinter Dec 06 '13 at 05:29
I am afraid if you really wish to go in that direction, you have to ensure that the library builds its JSON objects using your custom class. In other words, you will have to acquire the library's source code and rewrite it yourself. – Lai Xin Chu Dec 06 '13 at 05:37
Really, that's the only way, you think? – Joshua Pinter Dec 06 '13 at 05:41

Cannot cast to an Extended Class in Java

2 Answers2