Loading a Class from a String

Question

I want to instantiate a class by the value of a String. I found several tutorials that show several methods for doing this. The class MUST inherit from a certain interface

Answer 1

Here's how I would do it:

ImplementMe noCastNeeded = 
    this.getClassLoader()
        .loadClass("my.package.IImplementedYou")
        .asSubclass(ImplementMe.class).newInstance();

There are some Exceptions to catch but that's ok I think. :)

Answer 2

You will need a cast, because the compiler cannot tell from the code that the object is of type ImplementMe. It thus requires the programmer to issue a cast, which will throw a ClassCastException if the object is not an ImplementMe instance.

Answer 3

No, there is no better way (by design). You are not supposed to do this, Java is designed as a type-safe language. However, I can understand that you sometimes need to do things like this, and for that purposes you can create a library function like this:

public <T> T instantiate(final String className, final Class<T> type){
    try{
        return type.cast(Class.forName(className).newInstance());
    } catch(InstantiationException
          | IllegalAccessException
          | ClassNotFoundException e){
        throw new IllegalStateException(e);
    }
}

Now your client code can at least call this method without casting:

MyInterface thingy =
    instantiate("com.foo.bar.MyInterfaceImpl", MyInterface.class);

Answer 4

You can shorten it a bit like

ImplementMe a = (ImplementMe) Class
                               .forName("my.package.IImplementedYou")
                               .newInstance();

but you can't get rid of the cast. There may be a way to avoid the cast, but only if you can avoid the subproblem of loading class by name.

Answer 5

Try Class.forName("my.package.IImplementedYou").

Answer 6

(MyInterface)Class.forName(className).newInstance()