Very simple Java Dynamic Casting

Question

Simple question but I have spent over an hour with this. My code is below. I need to make SomeClass sc dynamic. So you pass the class name as a string in a function and use that class in place of static someClass. How to go about it?

SomeClass sc;
if (someOtherClassObject instanceof SomeClass){
    sc=(SomeClass) someOtherClassObject;

What I want is

public void castDynamic (String strClassName){
  //cast the classname referred by strClassName to SomeClass 
  //if it is the  instance of SomeClass
}

EDIT: The above was simplification. The actual code is this

public void X(String className, RequestInterface request)
{
    //My current code is this, I need to change so that "XRequest"
    //can be any   class referred by "className", 
    //and "request.getRequest" the object belonging to "className" class
    //I don't want static XRequest xvr, it should be fetched dynamically

    XRequest xvr;
    if (request.getRequest() instanceof XRequest){
        xvr=(XRequest) request.getRequest();
        client.setRequest(xvr); 
    }
}

Another simple rephrase: I get an object using request.getRequest(). I have no clue what that object is. So I need to cast it to the classstring name provided. How to do that? That's all. – SQC 13 mins ago

Answer 1

You want to instantiate a class by it's name?

First of all, you need to make a Class<?> object:

Class<?> cls = Class.forName(strClassName);

Then instantiate this (note, this can throw various exceptions - access violation, ClassNotFound, no public constructor without arguments etc.)

Object instance = cls.newInstance();

Then you can cast it:

return (SomeClass) instance;

Please make sure you understand the differences between:

1. A class name (approximately a file name)
2. A class object (essentially a type information)
3. A class instance (an actual object of this type)

You can also cast the cls object to have type Class<? extends SomeClass>, if you want. It doesn't give you much, though. And you can inline it, to:

return (SomeClass)(Class.forName(strClassName).newInstance());

Oh, but you can do type checking with the cls object, before instantiating it. So you only instanciate it, if it satisfies your API (implements the interface you want to get).

EDIT: add further example code, towards reflection.

For example:

if (cls.isInstance(request)) {
  // ...
}

For invoking methods, you either need to know an interface that you can cast to, or use reflection (the getMethod methods of the cls object):

Method getRequest = cls.getMethod("getRequest");
getRequest.invoke(request);