Why only public methods in interfaces?

Question

Howdy all,
I'm wondering if someone can shed some light on why Java doesn't allow interfaces to declare non-public methods.
Ideally, I'd like to do something like this:
public Interface Foo {
  protected void setFooProperty(int);
  public int getFooProperty();
public Interface Goo {
  protected void makeGooey(boolean);
  public boolean isGooey();
public class A implements Foo
public class B implements Goo
public class C implements Foo, GooI don't see any technical reason why interfaces must be restricted to declaring public methods only, but it seems too arbitrary to have been done without reason. Any ideas?

Answer 1

I don't think that is a good idea to put non-public methods in public interfaces. But I beleive that's a good idea to have protected (or friendly) interfaces to be used only in package scope. It could help a lot some projets defining a second level of data exposure of an object so that developers of that package could have more information that others users from outside the package have that are interesting to develop better algorithm that work with that data. In this way, we can defien thre levels of that access: The lower is the package user, that can see the minimum that's possible, above him we have the package developer that don't work directly with the data, but needs some deep access to it to do cool stuff, and finally we have the guy that is doing the object that deals directly with the data (and encapsulate it).
In this way we can decouple a little the data from the algorithms that work on it, put between them an interface that standarize the access to the data in an intermediate level.
Hope you are able to understand what I wrote (my english isn't so good!).
RGB