Cannot cast to an Extended Class in Java

10,855

Solution 1

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);

Solution 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
}
Share:
10,855

Related videos on Youtube

Joshua Pinter
Author by

Joshua Pinter

Updated on October 30, 2022

Comments

  • Joshua Pinter
    Joshua Pinter about 1 year

    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

  • Joshua Pinter
    Joshua Pinter almost 10 years
    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
    Joshua Pinter almost 10 years
    Hmm... okay, making more sense, except why the parent.toString()?
  • Joshua Pinter
    Joshua Pinter almost 10 years
    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.
  • Szymon
    Szymon almost 10 years
    I think that's a JSONObject constructor that will be easiest to use.
  • Lai Xin Chu
    Lai Xin Chu almost 10 years
    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.
  • Joshua Pinter
    Joshua Pinter almost 10 years
    That constructor doesn't exist. How would I go about constructing a JSONObject from an existing JSONObject?
  • Joshua Pinter
    Joshua Pinter almost 10 years
    Really, that's the only way, you think?
  • Szymon
    Szymon almost 10 years
    It is in the documentation though... I used the link you provided.
  • Joshua Pinter
    Joshua Pinter almost 10 years
    Sorry, you're right. Worked like a charm. just had to fix a missing colon in your reply. Nice work Szymon, thanks.
  • Joshua Pinter
    Joshua Pinter almost 10 years