Cannot cast to an Extended Class in Java
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
}
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Joshua Pinter
Updated on October 30, 2022Comments
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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
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Joshua Pinter almost 10 yearsYou're correct, that's what it returns. How do I go about turning it into a MyJSONObject to unlock the additional functionality?
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Joshua Pinter almost 10 yearsHmm... okay, making more sense, except why the
parent.toString()
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Joshua Pinter almost 10 yearsIn 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.
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Szymon almost 10 yearsI think that's a JSONObject constructor that will be easiest to use.
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Lai Xin Chu almost 10 yearsI 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.
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Joshua Pinter almost 10 yearsThat constructor doesn't exist. How would I go about constructing a JSONObject from an existing JSONObject?
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Joshua Pinter almost 10 yearsReally, that's the only way, you think?
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Szymon almost 10 yearsIt is in the documentation though... I used the link you provided.
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Joshua Pinter almost 10 yearsSorry, you're right. Worked like a charm. just had to fix a missing colon in your reply. Nice work Szymon, thanks.
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Joshua Pinter almost 10 years