We Keep Coding

Visibility of Methods and their cost

I recently read this article by Jake Wharton. This is targeted at Android, but also perfectly valid for java I think.
Consider the following code:
public class A {
private String someField = "abc";
class B {
public void doSomething() {
System.out.println(someField);
}
}
}
We have a simple class A and an inner class B (This has a reference to A and can acccess its members). Class B is accessing the field someField in A even though it is private. According to the article, this is done by the compiler generating synthetic accessor methods which allow the inner class to access the field.
Now my way more basic question: Why does the compiler even care about visiblities when the code is compiled? As we have seen in this example, a new method is generated, which basically just introduces overhead.
Visibilities are a great thing for architecting good software, but if the compiler is done checking that everything is correct according to the declared visibilies, why not optimize those methods away (E.g. just taking everything for being public and allow calls to it)?
First I thought this was due to security reasons, but reflection allows the access of all fields, not caring for visibility as far as I know.
This might be a basic misunderstanding of me, if so, I would be happy if someone could explain it to me.

Why does the compiler even care about visiblities when the code is compiled?
The JVM doesn't allows access to private methods/constructors/fields outside a class. It has no special rule for nested classes which were added after this rule was designed. Instead the compiler adds accessor methods so the language can support a means of access the JVM doesn't.
As we have seen in this example, a new method is generated, which basically just introduces overhead.
Only if the method isn't called very much and isn't optimised.
Adding any simple method (On Hotspot, any method of 35 bytes or less) will be inlined very quickly and has no impact on performance (except if the maximum inline level is reached)
why not optimize those methods away
It does this at runtime so that previous rules continue to be enforced.
reflection allows the access of all fields, not caring for visibility as far as I know.
Though not by default, you have to explicitly want this as an option and not have a SecurityManager which prevents it.

If compiler converts all private fields to public fields in compile time, one problem will be happened when your project is compiled into library and reused by others. In that case, all your private fields will become public.
There are some tools for optimizing this. In Android, there is a tool named ProGuard that will convert all getter/setter to direct field access.

Java extends programmatically

I am wondering about replacing Java's 'extends' keyword somehow for dynamically extending a class based on a parameter(file, environment variable, db...basically anything). Is this even possible because playing with class loaders or calling constructors does not achieve this. I am not asking "should I use interface or superclass hierarchy" rather what is extending really mean under the hood in JAVA because there aren't any good description about it just the good old inheritance jargon:
https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html

The only way to "replace the extends keyword" is to dynamically create classes at runtime, which is entirely possible but non-trivial. Vert.x is a good example of a project that makes extensive use of dynamically-generated classes.
Java wasn't designed as a dynamic language in that sense. There are several dynamic languages out there (some of which can run on the JVM), such as JavaScript.
rather what is extending really mean under the hood...
Without getting into a long treatise on OOP, when you say Derived extends Base, it means that Derived inherits both the public and protected API of Base (which it can then add to) and also the implementation of that API. It means that code expecting to see a Base instance can accept a Derived instance, because Derived "is a" Base. This link is created a compile-time. At runtime, instantiating an instance of Derived involves all of the plumbing that instantiating a Base instance involves, plus then the added plumbing for Derived.

To achieve this you need to maintain various versions of a class based on the condition and you have to customise class loader as well because at a point when you find that you have to load a particular instance, you need to load that class which is not loaded by default class loader on JVM startup.
Its better to maintain multiple versions of the class and let JVM do its job which it does perfectly.

You can't do that with a language like Java. The information about "inheritance" is not only used by the compiler, it is also "hard-baked" into the compiled byte code.
If you really want to such kind of "dynamic" meta programming; you are better of using languages that allow you to do so; instead of "violating" a language that was never intended for such kind of usage.
To use some stupid comparison: just because you happen to know "screws" and "hammer" ... you wouldn't start to use a hammer to get those screws into the wall, would you? Instead, you would be looking for a tool that works better with "screws" than a hammer.
If you still want your code to run within a JVM; you might consider languages like jython or jruby.

Why is "final" not allowed in Java 8 interface methods?

One of the most useful features of Java 8 are the new default methods on interfaces. There are essentially two reasons (there may be others) why they have been introduced:
Providing actual default implementations. Example: Iterator.remove()
Allowing for JDK API evolution. Example: Iterable.forEach()
From an API designer's perspective, I would have liked to be able to use other modifiers on interface methods, e.g. final. This would be useful when adding convenience methods, preventing "accidental" overrides in implementing classes:
interface Sender {
// Convenience method to send an empty message
default final void send() {
send(null);
}
// Implementations should only implement this method
void send(String message);
}
The above is already common practice if Sender were a class:
abstract class Sender {
// Convenience method to send an empty message
final void send() {
send(null);
}
// Implementations should only implement this method
abstract void send(String message);
}
Now, default and final are obviously contradicting keywords, but the default keyword itself would not have been strictly required, so I'm assuming that this contradiction is deliberate, to reflect the subtle differences between "class methods with body" (just methods) and "interface methods with body" (default methods), i.e. differences which I have not yet understood.
At some point of time, support for modifiers like static and final on interface methods was not yet fully explored, citing Brian Goetz:
The other part is how far we're going to go to support class-building
tools in interfaces, such as final methods, private methods, protected
methods, static methods, etc. The answer is: we don't know yet
Since that time in late 2011, obviously, support for static methods in interfaces was added. Clearly, this added a lot of value to the JDK libraries themselves, such as with Comparator.comparing().
Question:
What is the reason final (and also static final) never made it to Java 8 interfaces?

This question is, to some degree, related to What is the reason why “synchronized” is not allowed in Java 8 interface methods?
The key thing to understand about default methods is that the primary design goal is interface evolution, not "turn interfaces into (mediocre) traits". While there's some overlap between the two, and we tried to be accommodating to the latter where it didn't get in the way of the former, these questions are best understood when viewed in this light. (Note too that class methods are going to be different from interface methods, no matter what the intent, by virtue of the fact that interface methods can be multiply inherited.)
The basic idea of a default method is: it is an interface method with a default implementation, and a derived class can provide a more specific implementation. And because the design center was interface evolution, it was a critical design goal that default methods be able to be added to interfaces after the fact in a source-compatible and binary-compatible manner.
The too-simple answer to "why not final default methods" is that then the body would then not simply be the default implementation, it would be the only implementation. While that's a little too simple an answer, it gives us a clue that the question is already heading in a questionable direction.
Another reason why final interface methods are questionable is that they create impossible problems for implementors. For example, suppose you have:
interface A {
default void foo() { ... }
}
interface B {
}
class C implements A, B {
}
Here, everything is good; C inherits foo() from A. Now supposing B is changed to have a foo method, with a default:
interface B {
default void foo() { ... }
}
Now, when we go to recompile C, the compiler will tell us that it doesn't know what behavior to inherit for foo(), so C has to override it (and could choose to delegate to A.super.foo() if it wanted to retain the same behavior.) But what if B had made its default final, and A is not under the control of the author of C? Now C is irretrievably broken; it can't compile without overriding foo(), but it can't override foo() if it was final in B.
This is just one example, but the point is that finality for methods is really a tool that makes more sense in the world of single-inheritance classes (generally which couple state to behavior), than to interfaces which merely contribute behavior and can be multiply inherited. It's too hard to reason about "what other interfaces might be mixed into the eventual implementor", and allowing an interface method to be final would likely cause these problems (and they would blow up not on the person who wrote the interface, but on the poor user who tries to implement it.)
Another reason to disallow them is that they wouldn't mean what you think they mean. A default implementation is only considered if the class (or its superclasses) don't provide a declaration (concrete or abstract) of the method. If a default method were final, but a superclass already implemented the method, the default would be ignored, which is probably not what the default author was expecting when declaring it final. (This inheritance behavior is a reflection of the design center for default methods -- interface evolution. It should be possible to add a default method (or a default implementation to an existing interface method) to existing interfaces that already have implementations, without changing the behavior of existing classes that implement the interface, guaranteeing that classes that already worked before default methods were added will work the same way in the presence of default methods.)

In the lambda mailing list there are plenty of discussions about it. One of those that seems to contain a lot of discussion about all that stuff is the following: On Varied interface method visibility (was Final defenders).
In this discussion, Talden, the author of the original question asks something very similar to your question:
The decision to make all interface members public was indeed an
unfortunate decision. That any use of interface in internal design
exposes implementation private details is a big one.
It's a tough one to fix without adding some obscure or compatibility
breaking nuances to the language. A compatibility break of that
magnitude and potential subtlety would seen unconscionable so a
solution has to exist that doesn't break existing code.
Could reintroducing the 'package' keyword as an access-specifier be
viable. It's absence of a specifier in an interface would imply
public-access and the absence of a specifier in a class implies
package-access. Which specifiers make sense in an interface is unclear
- especially if, to minimise the knowledge burden on developers, we have to ensure that access-specifiers mean the same thing in both
class and interface if they're present.
In the absence of default methods I'd have speculated that the
specifier of a member in an interface has to be at least as visible as
the interface itself (so the interface can actually be implemented in
all visible contexts) - with default methods that's not so certain.
Has there been any clear communication as to whether this is even a
possible in-scope discussion? If not, should it be held elsewhere.
Eventually Brian Goetz's answer was:
Yes, this is already being explored.
However, let me set some realistic expectations -- language / VM
features have a long lead time, even trivial-seeming ones like this.
The time for proposing new language feature ideas for Java SE 8 has
pretty much passed.
So, most likely it was never implemented because it was never part of the scope. It was never proposed in time to be considered.
In another heated discussion about final defender methods on the subject, Brian said again:
And you have gotten exactly what you wished for. That's exactly what
this feature adds -- multiple inheritance of behavior. Of course we
understand that people will use them as traits. And we've worked hard
to ensure that the the model of inheritance they offer is simple and
clean enough that people can get good results doing so in a broad
variety of situations. We have, at the same time, chosen not to push
them beyond the boundary of what works simply and cleanly, and that
leads to "aw, you didn't go far enough" reactions in some case. But
really, most of this thread seems to be grumbling that the glass is
merely 98% full. I'll take that 98% and get on with it!
So this reinforces my theory that it simply was not part of the scope or part of their design. What they did was to provide enough functionality to deal with the issues of API evolution.

It will be hard to find and identify "THE" answer, for the resons mentioned in the comments from #EJP : There are roughly 2 (+/- 2) people in the world who can give the definite answer at all. And in doubt, the answer might just be something like "Supporting final default methods did not seem to be worth the effort of restructuring the internal call resolution mechanisms". This is speculation, of course, but it is at least backed by subtle evidences, like this Statement (by one of the two persons) in the OpenJDK mailing list:
"I suppose if "final default" methods were allowed, they might need rewriting from internal invokespecial to user-visible invokeinterface."
and trivial facts like that a method is simply not considered to be a (really) final method when it is a default method, as currently implemented in the Method::is_final_method method in the OpenJDK.
Further really "authorative" information is indeed hard to find, even with excessive websearches and by reading commit logs. I thought that it might be related to potential ambiguities during the resolution of interface method calls with the invokeinterface instruction and and class method calls, corresponding to the invokevirtual instruction: For the invokevirtual instruction, there may be a simple vtable lookup, because the method must either be inherited from a superclass, or implemented by the class directly. In contrast to that, an invokeinterface call must examine the respective call site to find out which interface this call actually refers to (this is explained in more detail in the InterfaceCalls page of the HotSpot Wiki). However, final methods do either not get inserted into the vtable at all, or replace existing entries in the vtable (see klassVtable.cpp. Line 333), and similarly, default methods are replacing existing entries in the vtable (see klassVtable.cpp, Line 202). So the actual reason (and thus, the answer) must be hidden deeper inside the (rather complex) method call resolution mechanisms, but maybe these references will nevertheless be considered as being helpful, be it only for others that manage to derive the actual answer from that.

I wouldn't think it is neccessary to specify final on a convienience interface method, I can agree though that it may be helpful, but seemingly the costs have outweight the benefits.
What you are supposed to do, either way, is to write proper javadoc for the default method, showing exactly what the method is and is not allowed to do. In that way the classes implementing the interface "are not allowed" to change the implementation, though there are no guarantees.
Anyone could write a Collection that adheres to the interface and then does things in the methods that are absolutely counter intuitive, there is no way to shield yourself from that, other than writing extensive unit tests.

We add default keyword to our method inside an interface when we know that the class extending the interface may or may not override our implementation. But what if we want to add a method that we don't want any implementing class to override? Well, two options were available to us:
Add a default final method.
Add a static method.
Now, Java says that if we have a class implementing two or more interfaces such that they have a default method with exactly same method name and signature i.e. they are duplicate, then we need to provide an implementation of that method in our class. Now in case of default final methods, we can't provide an implementation and we are stuck. And that's why final keyword isn't used in interfaces.

What is the purpose of access modifiers?

I know this applies to many languages, and not just Java, but that is the language I'm most familiar with.
I understand what the modifiers do, and how to use them. I just want to know, why do we need them? Why can't every object be accessible, whether or not it needs to be?

The reason becomes more apparent when you have to maintain a larger project. When a method or variable is public, you have to be careful when you make changes to it, because you never know which parts of the codebase rely on its exact behavior.
But when a variable or method is private, you know that it is not used outside of the class. That means there is a lot less code you have to pay attention to when you make changes.
By making class features private and public, you clearly separate the interface to the outside world from the internals. The less you exposes to the outside world, the more freedom you have with what the internal implementation does.
When you, for example, always make variables private and accessed them through getters and setters, you can later change them from a variable to a computed value, and then even later add caching to the computation for performance reasons. When it would be a public variable, you would have to change code everywhere the variable is used. But when you expose it to the outside world through getters and setters, all other code can keep using the class as if nothing had changed.

Making fields and methods private keeps other classes from improperly depending on the specific details of how a class works. The public interface (and the best case of all, an actual interface) describes how client code should interact with a library based on the semantics of the work being done. The implementer is then free to use whatever appropriate techniques to implement that interface and can make significant behind-the-scenes changes knowing that the client code will keep working.
An everyday example is the Collections group of interfaces. Most of the time, it's not important logically for code to know what particular kind of Set is in use, just that it's a collection that supports certain operations and doesn't have duplicates. This means that a method that accepts a Set<Integer> will work with any Set, including HashSet and ImmutableSet, because the person who wrote the interface wasn't poking around in the implementation's internals.
An example where this breaks down is the unfortunate tendency of some programmers to use packages in the com.sun namespace, especially when using cryptography. Upgrading to a new version of the JRE routinely breaks this code, which would have worked fine if the programmer had used the proper javax.crypto interfaces and factory methods instead of poking around in the JVM internals.

More or less they are used to control who can access your member variables and functions. It's the broader concept of encapsulation at work in Java(http://en.wikipedia.org/wiki/Encapsulation_(object-oriented_programming)).
From the Oracle Docs:
Access level modifiers determine whether other classes can use a
particular field or invoke a particular method. There are two levels
of access control:
At the top level—public, or package-private (no explicit modifier).
At the member level—public, private, protected, or package-private (no
explicit modifier).
http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html
As to why you should do this:
It has to do with intent of use. It would probably be best described as a design choice that helps guide usage through-out the code-base. By marking something private you are telling other developers that this field or method should not be used outside it's current purpose. It really becomes important on large projects that shuffle developers over time. It helps communicate the purpose & intended uses of classes.

To avoid other classes having direct access to internal members of the class.
This is most useful for avoiding that member variables are mutated in an uncontrolled way (e.g. without proper validation, without notifying listeners, ...).
Another reason to avoid this is that the internal implementation may change at any time but you don't want to break code that uses it.
As others have noted, the concept is called Encapsulation.

Access modifiers are there to set access levels for classes, variables, methods and constructors. This provides an individual with the chance of controlling better the privacy of the application. There are 4 access modifiers.
Modifier | Class | Package | Subclass | World
no modifier:--|----yes----|------yes--------|--------no--------|-----no----|
private:-------|----yes----|-------no--------|--------no--------|-----no----|
public:--------|----yes----|------yes--------|-------yes-------|----yes----|
protected:---|----yes----|------yes--------|-------yes-------|-----no-----|
Regarding your question, we do need and use access modifiers because we need to restrict whom can call our program and in what way.
Also, when it comes to variables if you make something public, that means that I have direct access to it. Therefore, I am allowed to do whatever I want without following your guidelines through your methods.
For example:
public int maxUsers;
public void setMaxUsers(int users) throws IllegalArgumentException{
if(users > 0 && users <= 1000){
maxUsers = users;
}else{
throw new IllegalArgumentException("The users can not be less than 0 or greater than 1000")"
}
}
Imagine your whole program being based on its maxUsers. Since, you give me the right to access that variable directly, I could do this: maxUsers = -15; and not use the setMaxUsers method, which will simply make your program behave in an abnormal way (in the best case).

Explanations
A private member is only accessible within the same class as it is declared.
A member with no access modifier is only accessible within classes in the same package.
or
If a variable is set to protected inside a Class, it will be accessible from its sub classes defined in the same classes or different package only via Inheritance.
A protected member is accessible within all classes in the same package and within subclasses in other packages.
A public member is accessible to all classes (unless it resides in a module that does not export the package it is declared in
Here's a better version of the table. (Future proof with a column for modules.)

Why is subclassing not allowed for many of the SWT Controls?

I often find myself wanting to do it. It can be very useful when you want to store some useful information or extra states.
So my question is, is there a very good/strong reason why this is forbidden?
Thanks
EDIT:
Thanks a lot for all these answers. So it sounds like there's no right-or-wrong answer to this.
Assuming I accept the fact that these classes are not to be subclassed, what's the point of not marking a Control class final, but prohibiting subclassing - effectively demoting the exception/error from compile-time to run-time?
EDIT^2:
See my own answer to this: apparently, these classes are overrideable, but requires explicit acknowledgement by the overrider.
Thanks

It doesn't look like anybody mentioned this in any of the answers, but SWT does provide an overrideable checkSubclass() method; precisely where the Unextendable exception is thrown. Override the method to a no-op and effectively make extending legal. I guess to leave this option open is ultimately the reason that the class is not made final and the extension error not made compile-time instead of run-time.
Example:
#Override
protected void checkSubclass() {
// allow subclass
System.out.println("info : checking menu subclass");
}

Designing components for inheritance is hard, and can limit future implementation changes (certainly if you leave some methods overridable, and call them from other methods). Prohibiting subclassing restricts users, but means it's easier to write robust code.
This follows Josh Bloch's suggestion of "design for inheritance or prohibit it". This is a bit of a religious topic in the dev community - I agree with the sentiment, but others prefer everything to be as open to extension as possible.

It is very hard to create class that can be safely subclassed. You have to think about endless use cases and protect you class very well. I believe that this is a general reason to mark API class as final.

As for your follow-up question:
what's the point of not marking a
Control class final, but prohibiting
subclassing - effectively demoting the
exception/error from compile-time to
run-time?
It's not possible for SWT to subclass the Control class, if they mark it final. But they have to internally. So they defer the checking to runtime.
BTW, if you want an insane hack, you can still subclass Control or any other SWT class, by putting your subclass into the org.eclipse.swt.widgets package. But I never really had to do that.

The method description of org.eclipse.swt.widgets.Widget.checkSubclass() says:
Checks that this class can be subclassed.
The SWT class library is intended to be subclassed only at specific,
controlled points (most notably, Composite and Canvas when
implementing new widgets). This method enforces this rule unless it is
overridden.
IMPORTANT: By providing an implementation of this method that allows a
subclass of a class which does not normally allow subclassing to be
created, the implementer agrees to be fully responsible for the fact
that any such subclass will likely fail between SWT releases and will
be strongly platform specific. No support is provided for user-written
classes which are implemented in this fashion.
The ability to subclass outside of the allowed SWT classes is intended
purely to enable those not on the SWT development team to implement
patches in order to get around specific limitations in advance of when
those limitations can be addressed by the team. Subclassing should not
be attempted without an intimate and detailed understanding of the
hierarchy.
Throws: SWTException
ERROR_INVALID_SUBCLASS - if this class is not an allowed subclass