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Is static late binding necessary to overload static variables?

A friend of mine asked me whether he can override a static variable in Java and I was shocked that he even thought about such a weird way of coding. Then he explained me that this is possible in PHP and I want to know whether there are a good reasons why a good developer should do that. In my opinion static members are characterized as class members and not related to an object and therefore they are not related to derivation of classes, but I cannot convince him as he is so naive and stubborn.
Can anyone give either a good argument against this whole thing or convince me that this is a cool feature?

The static inheritance does not make any sense. It is not that it is not possible, just that you get no benefit from it.
With normal inheritance you get the benefit of having a different implementation for the same thing and not knowing/caring which implementation will be used. With static inheritance you don't have an object to operate with and you are using a class name, so you cannot take advantage of polymorphism.
E.g. if you are calling Child.someMethod() you are tied to implementation of the child and if you actually just need the parent, you can just do Parent.someMethod() instead. If you need to add something to Parent implementation, you just make a Child.someOtherMethod() where you call the parent and do some other things after. The static inheritance is just syntactic sugar...

As far as I know, the static keyword in Java is used to define Class variables. A Class variable has one instance for all Objects of that class. So in Java you can not override a static variable, it doesn't make sense. Any changes done to a static variable in one Class propagates to another class. This is what static is used for, in JAVA.
This is the same way IT SHOULD WORK in PHP (I am not really a PHP expert), but if your friend can provide code showing that a static variable in PHP was overridden and the variable has a different value that from another class, I will be very glad.

Single instance with methods in java

I am wondering about programming decision - which I think is matter of style.
I need to have single instance of class which has only methods and no attributes.
To obtain that in java I have two options:
create an abstract class with static methods within, thus it will not be possible to create any instance of the class and that is fine,
use a singleton pattern with public methods.
I tend to go for second approach although met with 1. Which and why is better of those, or there is third option.

Would it make sense for that singleton to implement an interface, allowing you to mock out those methods for test purposes?
I know it goes against testing dogma these days, but in certain situations I think a static method is fine. If it's the kind of behaviour which you're never going to want to fake for test purposes, and which is never going to be polymorphic with other implementations, I don't see much point in making a singleton. (Singletons are also generally the enemy of testability, although if you only directly refer to them in the injection part of your code, they can implement appropriate interfaces so their singletoneity never becomes a problem.)
It's worth mentioning that C# has "static classes" for this kind of situation - not only do they prohibit other code from deriving from or instantiating the class, but you can't even use it as a parameter. Basically it signals the intent very clearly.
I would definitely suggest at least having a private constructor to prevent instantiation by the outside world.

My personal view is that the class should contain a private constructor and NOT be abstract. Abstract suggest to a reader that there is a concrete version of the class somewhere, and they may waste time searching for it. I would also make sure you comment your code effectively.
public class myClass {
/** This class should never be instantiated. */
private myClass() {
}
public static void myMethod() {
}
...
//etc
...
}

For option #1, it may not even be that important to restrict instantiation of your static utility class. Since all it has is static methods and no state, there is no point - but neither harm - instantiating it. Similarly, static methods can't be overridden so it does not make sense - nor difference - if it is subclassed.
If it had any state, though - or if there is a chance that it will get stateful one day - it may be better to implement it as a normal class. Still I would prefer not to use it as a Singleton, rather to pass its sole instance around via dependency injection. This makes unit testing so much easier in the long run.

If it holds a state I would use the singleton pattern with private constructors so you can only instantiate from within the class. If it does not hold a state, like the apache commons utility classes, I would use the static methods.

I've never seen the problem with static methods. You can think of static methods as somehow breaking OO, but they make perfect sense if you think of static as a marker that something is stateless. You find this in the java apis in places like java.Math. If you're worried about subclassing you can always make it final.
There is a danger in that a class like that can end up as a "utility method garbage can", but as long as the functionality doesn't diverge too much then there's nothing wrong with it.
It's also clearer, as there's no need to manage an object lifecycle like you would with a singleton (and since there's no state, what's the point of that anyway?).

For a single instance, I suggest you have an enum, with one instance.
However, for a class with no attributes, you don't have to have an instance. You can use a utility class. You can use an enum, with no instances and only static methods. Note: this cannot be easily mocked out.
You can still implement an interface if you ever need to mock out the implementation in testing.

Public static methods - a bad sign?

I've just read this article here: http://hamletdarcy.blogspot.com/2008/04/10-best-idea-inspections-youre-not.html, and the last bit in particular got me thinking about my code, specifically the advice:
What in the world is a public method doing on your object that has no dependency on any fields within the object? This is certainly a code smell. The problem is that the "auto-fix" for the inspection is to apply the static keyword. Nooooo. That's not what you want to do. A public method without any dependency on object state can't possibly be part of an object that has one clearly stated charter. It's just not cohesive and should be placed somewhere else. So: if the method is private, accept the auto-fix, but if the method is public then don't.
The code in question is essentially an object transformer. It takes an object of type A and converts it to a different type.
My hierarchy is like this:
Interface ObjectTransformer -> GenericObjectTransformer
and then below this, GenericObjectTransformer is extended by ObjectTransformerA and ObjectTransformerB
Now, some functionality is required by both ObjectTransformerA and ObjectTransformerB, but doesnt actually depend on any instance variables of GenericObjectTransformer, so its a protected static method in GenericObjectTransformer.
Is this a violation of the rule above? Obviously this is protected rather than public, but its still a method accessible from outside of the class that has nothing to do with the class itself?
Any thoughts?

I disagree with the excerpt you pulled.
A public method without any dependency on object state can't possibly be part of an object that has one clearly stated charter. It's just not cohesive and should be placed somewhere else. So: if the method is private, accept the auto-fix, but if the method is public then don't.
Just because a method is static and has no relation to state, doesn't mean it falls under the "low cohesion" category. Cohesion/Functionality isn't based on state.
When you are trying to determine Cohesiveness think about the role of the class as a whole, not just the instance variables. If the logic you are looking at is related to the generic concept (GenericObjectTransformer) then leave it there.
If it is a routine to calculate the orbit of the moon, or the depth of the ocean move it to a utility class (another smelly area of our field).

It feels slightly unclean, but is seem preferable to the alternatives I can think of.
I think that the original
A public method without any dependency
on object state can't possibly be part
of an object that has one clearly
stated charter.
You reference is too black and white, and your situation is even greyer.
By having your protected method you are nicely documenting that its intended for use by derived classes. If you don't put it in the base class, then presumbly it's got to go in some ObjectTransformUtility class. Is that win? More artefacts, more places to look.
One thought: if your ObjectTransormer class undergoes significant change then how likely are you to need to change these utility methods. After all if their business is to work agains the object's interface then in fact their cohesion is quite high.

Java abstract static Workaround

I understand that neither a abstract class nor an interface can contain a method that is both abstract and static because of ambiguity problems, but is there a workaround?
I want to have either an abstract class or an interface that mandates the inclusion of a static method in all of the classes that extend/implement this class/interface. Is there a way to do this in Java? If not, this may be my final straw with Java...
EDIT 1: The context of this problem is that I have a bunch of classes, call them Stick, Ball, and Toy for now, that have a bunch of entries in a database. I want to create a superclass/interface called Fetchable that requires a static method getFetchables() in each of the classes below it. The reason the methods in Stick, Ball, and Toy have to be static is because they will be talking to a database to retrieve all of the entries in the database for each class.
EDIT 2: To those who say you cannot do this in any language, that is not true. You can certainly do this in Ruby where class methods are inherited. This is not a case of someone not getting OO, this is a case of missing functionality in the Java language. You can try to argue that you should never need to inherit static (class) methods, but that is utterly wrong and I will ignore any answers that make such points.

You have a couple of options:
Use reflection to see if the method exists and then call it.
Create an annotation for the static method named something like #GetAllWidgetsMethod.
As others have said, try to not use a static method.

There are lots of answers about 'this does'nt make sense..' but indeed I met a similar problem just yesterday.
I wanted to use inheritance with my unit tests. I have an API and several its implementations. So I need only 1 set of unit tests for all implementations but with different setUp methods which are static.
Workaround: all tests are abstract classes, with some static fields with protected access modifier. In all implementations I added static methods which set these static fields. It works rather nice, and I avoided copy and paste.

I too am dealing with this problem. For those that insist that it "doesn't make sense", I would invite you to think outside of that semantic box for a moment. The program I am working with is inherently about reflection.
Reflection, as you know, can take three orders of magnitude longer than straight-up binary function calling. That is an inevitable problem, and the software needs to port to as many machines as possible, some of which will be 32 bit and slower than my development machine to begin with. Thus, the applicability of a class to the requested operation needs to be checked via a static method, and all of the reflective methods are run at once during module booting.
Everything works, first and foremost. I've built the entire thing. The only catch is that a module can be compiled in a .class without compile time checking to see if the identifying static function exists at all, resulting in an innately useless class. Without the identifier, and its included information, for security's sake the module is not loaded.
I clearly understand the issue with the complete definition of "abstract" and "static", and understand that they don't make sense together. However, the ability to have a class method that is compiler-enforced for inclusion is lacking in Java, and as much as I like the language, I miss it. Thus, this is a human constraint on every programmer that ever works on the software, which I'm sure we can all agree is a pain.

There's a lot of 'this makes no sense' or 'this can't be because' and 'why do you want it?' (or worse: 'you don't have to want it!') in all those answers. However, these answers also indirectly give reasons why it should be possible.
It must be differentiated between the concept and the implementation.
Sure, overriding a static method makes no sense. And it also isn't what the question was about.
It was asked for a way to force implementation of a certain static method (or constant or whatever) in every derived class of an abstract class. Why this is required it the matter of the one who wants to write an appllication with Jave, and no business of anyone else.
This has nothing to do with how the compiler compiles the method and how it is done at runtime.
Why shoudl it be possible? because there are things that are class specific (and not instance specific) and therefore should be static, while they NEED to be impleented in every single subclass (or class that implements an interface).
Let's say there is an abstract class 'Being'. Now there are subclasses like 'animals' and 'plants'.
Now there are only mammals and fishes allowed for animals. This information is specific to the animals class, not to any instance nor doe sit belong to any superclass or subclass. However, this information must be provided by teh class, not an instance, because it is required to properly construct an animal instance. So it MUST be there and it CANNOT be in the instance.
In fact, Java has such a thing- Every object has a class specific field 'class'. It is class-specific, not inherited, no override and it must be there. Well the compiler creates it implicitly, but obviously the compiler CAN do it. So why not allowing this for own fields too.
After all, it is just a matter of definition how the combination 'abstract static' is interpreted when the compiler checks the intheritance chain for abstract functions.
Nobody was ever demanding that there should be an inheritance of the superclass class functions (which could still make some sense, depending on what this function actually does - after all classes inherit static functions of their superclasses, even though you might get a warning that you should access it directly when you call it by the subclass))
But to summarize: the Java language offers no way to do it at compile time while there is no reason (othe rthan plain dogmatic) to not doing so.
The only way is to write a static final function to the abstract class that tries to find the static function/field of the subclass when it is loaded (or loads all existing subclasses and checks them). If properly made, it gives a runtime error on first use. Complex and dirty but better than nothing. At least it prevents bugs where you get the information from the wrong superclass.
It won't work for interfaces, though.

A type system allows you to express some constraints among types, but it's limited. That's why javadocs are littered with constraints in human language, asking people to follow rules that the compiler cannot check.
if you want to extend it beyond what language provides natively, you can write your own static analysis tool. that is not uncommon. for example: findbug. also IDEs do that too, they checking thing beyond what language dictates. you can write a plug in to enforce that a subclass must have a static method of such signature.
in your case, it's not worth it. have javadoc in the superclass urge implementors to include a static method, that's good enough.
I'll provide a convoluted way of expressing your constraint anyway, but DO NO DO IT. people get really carried away of make everything checkable at compile time, at the price of making code unreadable.
interface WidgetEnumerator
{
List getAllWidgets();
}
public class Abs<T extends WidgetEnumerator>
{
static List getAllWidgets(Class<? extends Abs> clazz){ ... }
}
public class Sub extends Abs<SubWidgetEnumerator>
{
}
public class SubWidgetEnumerator implements WidgetEnumerator
{
public List getAllWidgets() { ... }
}
How it works: for any subclass of Abs, it is forced to provide an implementation of WidgetEnumerator. subclass author cannot forget that. Now invocation Abs.getAllWidgets(Sub.class) contains sufficient information to resolve that implementation, i.e. SubWidgetEnumerator. It is done through reflection, but it is type safe, there are no string literals involved.

I think I can give you a better answer after seeing your edits--your best bet is probably a factory pattern. (Not lovely, but better than singleton).
abstract class Widget
public static Widget[] getAllWidgetsOfType(Class widgetType) {
if(widgetType instanceof ...)
}
class Ball extends Widget
class Stick extends Widget
class Toy extends Widget
This is not a very good way to do it, but it's typical. Hibernate is the tool you would normally use to solve this problem, this is exactly what it's designed for.
The big problem is that it requires editing the base class whenever you add a new class of a given type. This can't be gotten around without reflection. If you want to use reflection, then you can implement it this way (Psuedocode, I'm not going to look up the exact syntax for the reflection, but it's not much more complex than this):
public static Widget[] getAllWidgetsOfType(Class widgetType) {
Method staticMethod=widgetType.getStaticMethod("getAllInstances");
return staticMethod.invoke();
}
This would give the solution you were asking for (to be bothered by the need to modify the base class each time you add a child class is a good instinct).
You could also make it an instance method instead of a static. It's not necessary, but you could then prototype the method (abstract) in Widget.
Again, all this is unnecessary and sloppy compared to Hibernate...
Edit: If you passed in a live "Empty" instance of a ball, stick or toy instead of it's "Class" object, you could then just call an inherited method and not use reflection at all. This would also work but you have to expand the definition of a Widget to include an "Empty" instance used as a key.

Static methods are relevant to an entire class of object, not the individual instances. Allowing a static method to be overridden breaks this dictum.
The first thing I would consider is to access your database from a non-static context. This is actually the norm for Java apps.
If you absolutely must use a static method, then have it parameterised with instance specific arguments (of a generic type) to allow the different subclasses to interact with it. Then call that single static method from you polymorphic methods.

No. You can't do that. If you're willing to compromise and make the method non-static or provide an implementation of the static method in your abstract class, you'll be able to code this in Java.

Is there a way to do this in Java?
I don't think there is a way to do this in any language. There's no point to it, since static methods belong to a class and can't be called polymorphically. And enabling polymorphic calls is the only reason for interfaces and abstract classes to exist.

Create a context interface containing your method with a name that matches your problem domain. (Name it "World" if you absolutely have to, but most of the time there's a better name)
Pass around implementation instances of the context object.

Ok, maybe my question was poorly asked, it seems like most of you didn't get what I was trying to do. Nonetheless, I have a solution that is somewhat satisfactory.
In the abstract super class, I am going to have a static method getAllWidgets(Class type). In it I'll check the class you passed it and do the correct fetching based on that. Generally I like to avoid passing around classes and using switches on stuff like this, but I'll make an exception here.

static methods can't be abstract because they aren't virtual. Therefore anywhere that calls them has to have the concrete type with the implementation. If you want to enforce that all implementations of an interface have a certain static method, then that suggests a unit test is required.
abstract class A
{
public static void foo()
{
java.lang.System.out.println("A::foo");
}
public void bar()
{
java.lang.System.out.println("A::bar");
}
}
class B extends A
{
public static void foo()
{
java.lang.System.out.println("B::foo");
}
public void bar()
{
java.lang.System.out.println("B::bar");
}
}
public class Main
{
public static void main(String[] args)
{
B b = new B();
b.foo();
b.bar();
A a = b;
a.foo();
a.bar();
}
}

For what it is worth I know exactly what you are trying to do.
I found this article while searching for the reasons I can't do it either.
In my case I have HUNDREDS of classes that inherit from a central base base and I want simply to get a reference like this:
ValueImSearchingFor visf = StaticClass.someArbitraryValue()
I do NOT want to write/maintain someArbitraryValue() for each and every one of hundreds of the inherited classes -- I just want to write logic once and have it calc a Unique Class-Sepcific value for each and every future written class WITHOUT touching the base class.
Yes I completely get OO - I've been writing Java for about as long as it's been available.
These specific classes are more like "Definitions" as opposed to actual Objects and I don't want to instantiate one every time I just need to see what someArbitraryValue() actually is.
Think of it as a PUBLIC STATIC FINAL that allows you to run a Method ONCE to set it initially. (Kinda like you can do when you define an Enum actually...)

I'd make a WidgetCollection class with an abstract Widget inner class.
You can extend the WidgetCollection.Widget class for each of your types of Widget.
No static methods necessary.
Example (not compiled or tested):
class WidgetCollection<W extends Widget> {
Set<W> widgets = new HashSet<W>();
Set<W> getAll() {
return widgets;
}
abstract class Widget {
Widget() {
widgets.add(this);
}
abstract String getName();
}
public static void main(String[] args) {
WidgetCollection<AWidget> aWidgets = new WidgetCollection<AWidget>();
a.new AWidget();
Set<AWidget> widgets = aWidgets.getAll();
}
}
class AWidget extends Widget {
String getName() {
return "AWidget";
}
}

It doesn't make sense to do what you're asking:
Why can't static methods be abstract in Java

Private variables/methods in anonymous class?

I have created an anonymous class in which I declare a few variables and methods. My java teacher tells me to make these private. I don't see how changing the modifier makes any difference since these variables and methods are private to the anonymous class anyway, so I prefer to have no modifier at all. Who is right and what makes more sense? See below for example code where I choose no modifier for 'map' and 'convert' rather than making them private.
Collections.sort(list, new Comparator<String>(){
public int compare(String a, String b){
return convert(a).compareTo(convert(b));
}
Map<String, String> map = new HashMap<String, String>();
String convert(String s) {
String u = map.get(s);
if (u == null)
map.put(s, u = s.toUpperCase());
return u;
}
});

I would be tempted to make them private simply for the fact that if you refactor the code and pull the anonymous class out as a standard class (Intellij, for example, can do this at the click of a button), having private fields is what you really want. You won't have to go and rework your classes to match your standard.

Personally I would make them private (and final where possible) anyway - it's just a good habit to be in in general.
To put it another way: if you had to put an access modifier on (if, say, the keyword package was also used as an access modifier) what would you choose? Private, presumably - after all, you don't actually want to grant any other class access, do you?
Now, having decided that private is the most logically appropriate access modifier, I would make that explicit in the code.
Then again, I'd quite possibly not create an anonymous inner class with a member variable anyway - I'd be tempted to turn that into a named nested class instead.

Your professor is right.
Make all class variable private and expose them via properties (if not anonymous).
The general rule of thumb is to keep member data such as variable including your Map object private.

Default modifier is not the same as the private modifier, there're subtle differences.
However, in your case it's more a religious question whether to make convert() default or private. I don't see any advantage in making it private though.
Anyway, your code has a memory leak as the String Cache is never cleared :-P
Also, for even shorter/less code, use the Comparator String.CASE_INSENSITIVE_ORDER:
Collections.sort(list, String.CASE_INSENSITIVE_ORDER);

It really doesn't matter, but it's probably a good idea to keep your teacher happy as he/she will be grading you.

I'd say it's a matter of style. You can't access the member map outside out of the anonymous class, but it might be best to define them as private for consistency with other classes.
If this were my code, I would say that if a class is complicated enough to need data members, it might be worth pulling it out into a separate class, in which case I'd certainly make the data members private.

The key point is when you say "I don't see how changing the modifier makes any difference since these variables and methods are private to the anonymous class anyway"... you're assuming a lot about how your class is going to be used. Treat every class like it will be passed around and used in a variety of ways, in other words, use modifiers as appropriate. Besides, it makes the intent of class clear. It's not like Java is a terse language anyway, so you might as well be clear.

I don't see much benefit to marking things private just for the hell of it. It won't really gain you anything and someone reading the code might attach some significance to the choice when there really isn't any.

I would question the need for all this complexity. Take a look at: String.compareToIgnoreCase()

You want these fields to be private, so mark them private.If a member is marked neither public not private then something suspicious is going on. Also mark fields that shouldn't change final. Keeping things standardised means less thinking, or at least less thinking on the irrelevant, and less to change when modifying code.
From a language point of view, the only real difference is that if you have extended a base class in the same package, you have now hidden fields or overridden "package-private" (default access) methods. The members can also be accessed via reflection (without setAccessible) by code in the same package (this can have mobile-code security implications).

difference between default and protected.
protected:
object/method is accessible to all classes that are in the same package, and also accessible to sub/extension classes.
default:
object/method is accessible to all classes that are in the same package.
What is your intention of your object/method and code modifier accordingly.
Do not allow yourself to be confused when you come back to the code after six months because in huge projects you want to know that that object/method is or is not accessed anywhere else.
In three weeks, not just months, you would forget what the intended accessibility of those objects, 101% guaranteed. Then if you had a huge project and you had a hundred modifiers that were not specific and you desperately wanted to update the code, you would be frustrated by the compulsion to run reference check on those 100 objects/methods. May be someone took your jar and found the hidden cookies in them and used them, then you changed your code and broke someone's code.
Code your modifiers according to your intention unless you are either one or more of these:
you have no further desire to work
in large java projects.
you are a
extremely intelligent high
functioning autistic person who has
an indexed memory of every event of
your life and can write a completely functional peer-peer file sharing service
within two weeks on a lap top in a
coffee shop.
you deliberately use it
as another tool to obfuscate your
code.