Java abstract static Workaround

Question

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.

Answer 1

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.

Answer 2

You have a couple of options:

1. Use reflection to see if the method exists and then call it.

2. Create an annotation for the static method named something like @GetAllWidgetsMethod.

3. As others have said, try to not use a static method.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

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.

Answer 8

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.

Answer 9

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.

Answer 10

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.

Answer 11

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.

Answer 12

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();
    }
}

Answer 13

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...)

Answer 14

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";
    }
}

Answer 15

It doesn't make sense to do what you're asking:

Why can't static methods be abstract in Java