Why friend directive is missing in Java?

Question

I was wondering why Java has been designed without the frienddirective that is available in C++ to allow finer control over which methods and instance variables

Answer 1

Why not simply think that Java requires friend classes to be co-located ? The package-private visibility allows everyone from the same package to access those members. So you're not only limited to explicitly declared friends, but you allow any (existing or future) friend to alter some members that are specifically designed for this purpose (but not your private stuff). You're still able to fully rely on encapsulation.

Answer 2

In addition to the aforementioned package visibility, Java also offers inner and anonymous classes which are not only friends by default, but also automatically have a reference to the containing class. Since creating such helper classes is probably the only reasonable way to use friend in C++, Java doesn't need it since it has another mechanism for that. Iterators are a very good example of this.

Answer 3

In general i think it was because of the added cognitive complexity and low number of cases in which it creates an improvement.

I would say that the extremely huge number of lines of java in production at this moment can attest that the friend keyword is not really a big loss :).

Please see @dwb's answer for some more specific reasons.

Answer 4

Completely agree with spaceghost's statement in his answer

Contrary to popular opinion, here are many cases, in particular for infrastructure capabilities, where friend access leads to BETTER design, not worse design.

My example is simple - if a class A has to provide a special "friend" interface to class B in java we have to place them into the same package. No exceptions. In that case if A is a friend of B and B is a friend of C, A has to be a friend of C which isn't always true. This "friendship transitivity" breaks encapsulation more then any problems which C++ friendship could lead to.

Answer 5

Just to add to the other answers:

There is the default package visibility in Java. So, you could call all classes in the same package neighbors. In that case you have explicit control of what you show to the neighbors - just members with package visibility.

So, it's not really a friend but can be similar. And yes, this too leads to bad design...

Answer 6

Here are a few reasons off the top of my head:

• friend is not required. It is convenient, but not required
• friend supports bad design. If one class requires friend access to another, you're doing it wrong. (see above, convenient, not required).
• friend breaks encapsulation. Basically, all my privates are belong to me, and that guy over there (my friend).