Instantiating an inner class

Question

I have a utility method and when irrelevant logic is removed from it, the simplified method would look like this:

public static <A extends Foo> List<A> getFooList(Class<A> clazz) {
   List<A> returnValue = new ArrayList<A>();
   for(int i=0; i < 5; i++) {
        A object = clazz.newInstance();
        returnValue.add(object);
   }

   return returnValue;
}

The problem is, that if clazz is an inner class such as Foo.Bar.class, then the newInstance() method will not work even if Bar would be public, as it will throw a java.lang.InstantiationException.

Is there a way to dynamically instantiate inner classes?

Answer 1

If it's genuinely an inner class instead of a nested (static) class, there's an implicit constructor parameter, which is the reference to the instance of the outer class. You can't use Class.newInstance at that stage - you have to get the appropriate constructor. Here's an example:

import java.lang.reflect.*;

class Test
{
    public static void main(String[] args) throws Exception
    {
        Class<Outer.Inner> clazz = Outer.Inner.class;

        Constructor<Outer.Inner> ctor = clazz.getConstructor(Outer.class);

        Outer outer = new Outer();
        Outer.Inner instance = ctor.newInstance(outer);
    }
}

class Outer
{
    class Inner
    {
        // getConstructor only returns a public constructor. If you need
        // non-public ones, use getDeclaredConstructors
        public Inner() {}
    }
}

Answer 2

Something more generic:

    public static <T> T createInstance(final Class<T> clazz) throws SecurityException, NoSuchMethodException,
            IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException {

            T instanceToReturn = null;
            Class< ? > enclosingClass = clazz.getEnclosingClass();

            if (enclosingClass != null) {
                Object instanceOfEnclosingClass = createInstance(enclosingClass);

                Constructor<T> ctor = clazz.getConstructor(enclosingClass);

                if (ctor != null) {
                    instanceToReturn = ctor.newInstance(instanceOfEnclosingClass);
                }
            } else {
                instanceToReturn = clazz.newInstance();
            }

            return instanceToReturn;
     }

Answer 3

This exception will be thrown only if clazz represents either an abstract class or an interface. Are you sure you're passing a Class object that represents a concrete class?

Answer 4

In Jmockit 1.41, use this:

ConstructorReflection.newInstance