Java - ClassCastException on downcast

Question

i would try to write an JSONparser for gwt.

For this i use the lib org.json on server side and the com.google.gwt.json.client on client side.

I build some interface's to call this two lib with the same methods.

I use:

public class JSONObject extends com.google.gwt.json.client.JSONObject implements IJSONObject
{
  ...
}

and

public class JSONObject extends org.json.JSONObject implements IJSONObject
{
  ...
}

So it works like a sharm, but there is one thing i wouldn't get it.

Client-side:

JSONParser.parseLenient( sJson ).isObject()

Return ans Object from Type: "com.google.gwt.json.client.JSONObject"

And i try to cast this to my custom JSONObject and get an ClassCastException.

My custom JSONObject extends from com.google.gwt.json.client.JSONObject and implements my IJSONobject.

So if i can't cast the object, i can't user the interface.

Thank you

Just because that method returns a reference to the parent class does mean it's of your subclass type as well. — Michael Markidis
– Michael Markidis, Commented Jun 10, 2016 at 6:46
What part of this code: JSONParser.parseLenient( sJson ).isObject() uses your custom class? — Michael Markidis
– Michael Markidis, Commented Jun 10, 2016 at 6:53
Then there's your answer. Can't expect it to reference your object if it's not even using your code. Right? — Michael Markidis
– Michael Markidis, Commented Jun 10, 2016 at 7:01

Clement Amarnath · Accepted Answer · 2016-06-10 06:58:56Z

2

You will get ClassCastException for sure.

Reason:

You are trying to cast an object "com.google.gwt.json.client.JSONObject" into an object "public class JSONObject extends com.google.gwt.json.client.JSONObject", which is not possible.

Example:

public class TestClassCast {

public static void main(String[] args) {

    A a = new A();
    B b = new B();

    A b1 = b; // This is possible since class B extends class A

    // B a1 = a; // This is not possible, which you are trying to do in your scenario.
}


}

class A {
    A(){};
}

class B extends A {
    B(){};
}

answered Jun 10, 2016 at 6:58

Clement Amarnath

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5 Comments

Flo Over a year ago

But they are exactly the same, without some methods. Can i not force this?

Clement Amarnath Over a year ago

No we cannot force this.

Flo Over a year ago

Have some alternate ideas how can i implement something like that?

Clement Amarnath Over a year ago

It is not possible, since child class can be cast into parent, whereas parent class cannot be cast to child. Referring to my example code, B is child of A, so instance of B can be cast into A, whereas vice versa is not allowed. ie., A b1 = b; is allowed and B a1 = a; is not allowed.

Flo Over a year ago

Got it. But away form downcast have you any idea to solve my problem... it seems like the way with my interface wouldn't work

Necromancer · Accepted Answer · 2016-06-10 06:55:14Z

0

ClassCastException on downcast

I think you should extend from super class. Please try this :

public class YourCustomJSON extends com.google.gwt.json.client.JSONValue implement IJSONobject
{
   ...
}

answered Jun 10, 2016 at 6:55

Necromancer

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1 Comment

Flo Over a year ago

But i don't want an JSONValue?!

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