Dynamically build java/scala method body at runtime and execute it

Suppose I have the following Interface in java:

public interface DynamicMethod {
    String doit();
}

I would like to build an Object during runtime which conforms to the above interface such that I inject doit method body in it and then execute it? Is this possible with Java Reflection API, or any other way? Or probably in some way in Scala?

---

Note that doit body for my objects would be dynamic and are not known a priori. You can assume that in run-time an array CodeArray[1..10] of Strings is provided and each entry of this array holds the code for each doit method. I would appreciate if you could answer with a sample code.