How to create instance of a class that extends interface?

Question

I have a method like this:

public IMyInterface factoryMethod(Class<? extends IMyInterface> concreteClass){
    IMyInterface o = concreteClass.newInstance();
    //...some stuff
    return o;
}

If method invoked I get exception: java.lang.NoClassDefFoundError: packages/IMyInterface

Seems that it tries to find class IMyInterface and then find its constructor. Of course it fails.

How to fix this method?

Are you certain IMyInterface is on your classpath? Usually a NoClassDefFoundError is a runtime error raised due to classpath issues. — Stefan Nuxoll, Jan 21 '14 at 22:14
Can you create instance of your class without reflection? `NoClassDefFoundError` usually happens when class that your code depends on was presented at class path while compiling, but is not in classpath while executing code. — Pshemo, Jan 21 '14 at 22:15
Can you show snippets of the rest of your code. For example, IMyInterface? — Jason, Jan 21 '14 at 22:17
@Jason But in generics bounds you use `extends` for interfaces as well as classes. — rgettman, Jan 21 '14 at 22:19
Class cannot extend interface!! Here you can look at very nice explanations and examples for interfaces --> http://www.tutorialspoint.com/java/java_interfaces.htm — i-bob, Jan 21 '14 at 22:22
@i-bob Read rgettman's comment (the one above your comment). Also take a look [here](http://stackoverflow.com/q/976441/1393766). — Pshemo, Jan 21 '14 at 22:24
That was issue with classpath. I work on ear app. And I had issue with links between jar packages. — Victor Mezrin, Jan 22 '14 at 13:08

score 1 · Answer 1 · answered Jan 21 '14 at 23:53

I tested your code and its working fine. But i was also able to reproduce your issue.

Assuming, you are calling

factoryMethod(MyClass.class);

Then, if the class "MyClass" is inside its OWN Classfile its working fine with just providing

concreteClass.newInstance();

When having the implementation of the class as a inner Class Type, the Class not found exception is thrown.

The reason is, that a inner class is actually a type of a nested class, and therefore creating a instance is only possible, when working with the enclosing Class, also.

In the example given bellow, the newInstance() method has been used along with an inner class.

  public class Test {

    public class MyClass implements MyInterface {
    }

    public interface MyInterface {
    }

    public static void main(String[] args){
        MyInterface ret = method(MyClass.class);

        if (ret instanceof MyClass){
            System.out.println("Is MyClass");
        }
    }

    public static MyInterface method(Class<? extends MyInterface> actualClass){
        try {
            Class<?> enclosingClass = Class.forName(actualClass.getDeclaringClass().getCanonicalName());
            Object enclosingInstance = enclosingClass.newInstance();

            Class<?> innerClass = Class.forName(actualClass.getName());
            Constructor<?> constructor = innerClass.getDeclaredConstructor(enclosingClass);

            Object innerInstance = constructor.newInstance(enclosingInstance);
            return (MyInterface) innerInstance;
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return null;    
    }
}

Thank you for investigation, but MyClass is not the inner class — Victor Mezrin, Jan 22 '14 at 10:43

How to create instance of a class that extends interface?

1 Answers1