Credit for this belongs more with William than me. I was talking about it recently with someone, and thought I would post for prosterity

public class FemtoContainer {
Map components = new HashMap();
public void registerComponent(Class class, Object instanceOfComponent){
   components.put(class, instance);
}

public Object instantiateObject(Class classToCreate){
   Constructor[] constructors classToCreate.getConstructors();
   // picking off first — actually probably want to assert that there
   //is only one non-default constructor or something… this is a really short hack for demonstration…. and probably won’t compile.
   Constructor constructor = constructors[0];
   Class[] constructorParameterTypes = constructor.getParameterTypes();
   Object[] constructorParameters=
      new Object[constructorParameterTypes.length];
   for (int i =0; i<constructorParameterTypes.length;i++){
      constructorParameters[i]=components.get(constructorParameterTypes[i]);
}
}