Re: Why only public methods on interfaces?

Tom Anderson <>
Sun, 10 Apr 2011 16:02:21 +0100
On Sat, 9 Apr 2011, Mike Schilling wrote:

"Patricia Shanahan" <> wrote in message

The really unfortunate decision is not the public methods only, but the
decision to make non-specification of access mean public interface
method declarations but package access in class method declarations.
That cuts off any possibility of changing interfaces now to make them
more flexible.

Actually, you could make an argument that "protected" on an interface
method would mean "only visible within the package", because it can't
mean "and also visible to derived types".

It could mean 'visible to types derived from implementors of the
interface'. It would mostly act to constrain the access modifier on the
implementing method.

If you interpreted interface-protected your way, you would have to permit
implementors to declare their implementations package-access, otherwise it
would be pointless, and that would mean that you would be writing
implementations that appeared to have a narrower access modifier than
their specification.

In code:

package x;

public interface I {
  protected void m();

public class A implements I {
  protected void m() {} // legal, looks normal, but widens access!

public class B implements I {
  /*package*/ void m() {} // is this legal?

package y;

class C extends x.A {
  void foo() {
  m(); // i probably shouldn't be able to do this
  I i = this;
  i.m(); // i won't be able to do this

Which is why ...

On Sat, 9 Apr 2011, Mike Schilling wrote:

Unless new syntax like "package-private" is introduced.

That seems like a very good idea. Indeed, a future revision of the
language could do that, and at the same time deprecate the omission of an
access modifier altogether.

What i'd really like to see is a slightly more complicated but much more
expressive system of access control. I worked this out with a friend a
while ago, and i think the model should be:

1. The universe consists of a tree of things: the leaves are features
(fields and methods), and the branches are containers (packages and
classes - classes are also features in some ways, and i don't think that
causes any difficulty), with the root being the default package. Packages
can contain packages and classes; classes can contain classes and

2. Every feature explicitly exposes itself at one scope; it is visible to
any code defined within that scope, including nested scopes. 'public'
means it is exposed to the default package. 'private' means that it is
exposed to the outermost enclosing class. This is as now. A new modifier
'package' means that it is exposed to the innermost enclosing package.
This differs from the current default in that it is visible to subpackages
(the current default is not visible in subpackages, although it is visible
in nested classes in the same package). The lack of an explicit modifier
would elicit a compiler warning, and be treated the same as 'package'
(this widens access to these features, but in a backwards-compatible way).

3. In addition to these simple modifiers, it is possible to explicitly
specify an exposure scope - this can be any scope enclosing the feature.
The syntax i would use would be 'private(name.of.scope)'. 'public' is thus
shorthand for 'private()', 'package' is shorthand for
'private(innermost.enclosing.package)', and 'private' is shorthand for
'private(outermost.enclosing.class)'. The scope name is resolved in the
usual way, so scopes which are classes would not need to be fully

4. In addition to the exposure scope, a feature can be made visible to
subclasses of its defining class, whatever package they may be in. This is
done by replacing 'private' with 'protected'; this accepts an explicit
scope in the same way as 'private'. The natural exposure scope for an
unqualified 'protected' would be the same as for an unqualified 'private',
ie the outermost enclosing class, but this would break backwards
compatibility, so sadly, it must mean
'protected(innermost.enclosing.package)'. Perhaps we should allow a
shorthand for the outermost enclosing class, perhaps 'this', so you could
write 'protected(this)' to allow access only to subclasses and inner

The main thing that this gives you is a way to make features visible
across packages in a hierarchy. This is a frequent problem in large
projects which are intended to be modules in a larger world (eg on a vast
project, or libraries for public consumption); a class in might
need to be visible to one in or, but should not be
visible to unrelated code in Similarly, methods in a class in often need to be accessible to classes in, but not At present, you end up with public methods marked 'PRIVATE API,
DO NOT USE', which is total rubbish. The proposed access system would let
you scope all these methods to, and have them visible to
everything in the project.

What it no longer lets you do is have things in that are *not*
visible throughout the project. However, i would suggest that such things
really belong in a subpackage. The parent package should be minimal, and
only contain things for general consumption.

It also lets you have features which are visible to subclasses but not the
rest of the package, which is something i have always wanted.

If we extend the explicit scope notation to accept multiple exposure
scopes, then we have essentially copied the C++ friend mechanism. That
would let me solve the classic JPA problem of how i stop unrelated classes
calling the internal-use-only innerWhatever methods in this scenario (this
is actually a slight variation in the classic pattern, but i hope it's

public class Department {
  private List<Employee> employees;

  public void addEmployee(Employee emp) {

  public void removeEmployee(Employee emp) {

  void innerAddEmployee(Employee emp) {

  void innerRemoveEmployee(Employee emp) {

public class Employee {
  private Department dept;

  public void setDepartment(Department dept) {
  if (this.dept != null) this.dept.innerRemoveEmployee(this);
  if (dept != null) dept.innerAddEmployee(this);
  this.dept = dept;

At the moment, the methods have to be at least package-access, because
Employee needs to be able to call them. But with the proposed mechanism, i
could declare them:

  private(Employee) void innerAddEmployee(Employee emp)
  private(Employee) void innerRemoveEmployee(Employee emp)

And access would be restricted to just where they were needed.

Compared to C++'s friendship system (which i don't understand well, so
correct me if i'm wrong), we have finer-grained control over what is
exposed (individual features, rather than all the features in the class),
but coarser-grained control over who we expose it to (classes, rather than
individual methods). The former is clearly better, because it means we can
expose the things we need to, whilst keeping most details of the class
encapsulated. I would argue that the latter is also better, because the
exposing class has no business knowing which particular methods of the
friend class are going to make use of the exposure; in the above example,
i want to be free to refactor the calls from Employee to Department.inner*
into a separate method (perhaps updateDepartmentEmployeeLists), without
having to modify Department (particularly because it might be a private
method - in which case in C++, Department would have to know about a
private method of Employee!).


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