Java: How to create a specific class instance?

Question

I have a method with signature

<T extends AbstractClass> T method(Class<T> abstractClass)

and I cannot modify the signature.

Also I have a string with class name com.test.MyClass. Is it possible to create a class by class name to pass to my method?

Something like that

Class<? extends AbstractClass> clz = (Class<? extends AbstractClass>) Class.forName(classNameStr);

You can't actually create concrete instances of an abstract class; you'll need something derived from it — Andrew Barber
– Andrew Barber, Commented Dec 10, 2014 at 21:26
My class name is a concrete class. It's a name of class which extends abstract class. — barbara
– barbara, Commented Dec 10, 2014 at 21:26
@Robert The OP isn't asking how to create an instance of a class given the class name as a String. She is asking how to call the given method so it returns an object of the class for which she only has the class name as a String. That's my understanding anyway. — GriffeyDog
– GriffeyDog, Commented Dec 10, 2014 at 21:34
@Barbara You can do something like this: AbstractClass obj = method((Class<? extends AbstractClass>) Class.forName("com.test.MyClass")); — GriffeyDog
– GriffeyDog, Commented Dec 10, 2014 at 22:00

Holger · Accepted Answer · 2014-12-12 17:10:17Z

2

If you want a type safe dynamic class loading, the correct way is:

Class<? extends AbstractClass> clz =
    Class.forName(classNameStr).asSubclass(AbstractClass.class);

This does not generate “unchecked” warnings as it is checked at runtime in the asSubclass method and afterwards, e.g. calling newInstance an that Class is guaranteed to return an instance of AbstractClass.

So afterwards you can do

AbstractClass obj = method(clz);

Of course, you can inline the construct as

AbstractClass obj=method(Class.forName(classNameStr).asSubclass(AbstractClass.class));

but it should be obvious why I wouldn’t recommend it.

answered Dec 12, 2014 at 17:10

Holger

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