Prototyping in Java instead of extending

If you are looking for extension methods, you could try Xtend. Xtend is language that compiles to java code and eliminates boilerplate code.

The following text is stolen from the Xtend Docs for extensions:

By adding the extension keyword to a field, a local variable or a parameter declaration, its instance methods become extension methods.

Imagine you want to have some layer specific functionality on a class Person. Let us say you are in a servlet-like class and want to persist a Person using some persistence mechanism. Let us assume Person implements a common interface Entity. You could have the following interface

interface EntityPersistence {
  public save(Entity e);
  public update(Entity e);
  public delete(Entity e);
}

And if you have obtained an instance of that type (through a factory or dependency injection or what ever) like this:

class MyServlet {
  extension EntityPersistence ep = Factory.get(typeof(EntityPersistence))
  ...

}

You are able to save, update and delete any entity like this:

val Person person = ...
person.save  // calls ep.save(person)
person.name = 'Horst'
person.update  // calls ep.update(person)
person.delete  // calls ep.delete(person)

I don't think you can do this in Java. You can though in Groovy, using metaclasses

String.metaClass.world = {
    return delegate + " world!"
}

println "Hello".world()