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

Accepted Answer

`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);
```

Cannot cast to an Extended Class in Java

Solution

Problem