I'm developping a server application in Java. The server need two type of server classes. These classes have some methods in common, the code in these methods is exactly the same. So i create an abstract super-class containing all the shared code, and both classes are inheriting it. But, there is some part of the code that need to be precised by subclasses. I mean that the superclass "rely" on subclasses methods.
Here is a purified example of what i mean:
public abstract class AbstractServer
{
public void loadConfig(String configPath)
{
//Load the configuration file.
//This code is exactly the same for subclasses.
}
public void startRMI(int port)
{
//Create an empty RMI registry.
//This part also need to be identical.
//Here' where the superclass "rely" on subclasses.
fillRegistry(); //Call the method overwritten by subclasses.
}
/**
Bind remote objects in the RMI registry
*/
protected abstract void fillRegistry(); //This method will be overriten by subclasses.
}
I feel that it's really bad to make it like that, but i can't find another cleaner way to do it.
So, what i want is some advice on how i could make it better.
Thanks, and sorry for my bad english.
Your approach is just fine. Stick with it buddy.
I feel your 'philosophical need' to understand it. Base class 'relying' on the subclass is fine as long as the base class is abstract. It knows that some things have to be registered at this point, but it doesn't have the faintest clue about what exactly to be registered. So the high-level process is encoded in the base class with 'holes' that can be plugged in by the derived class. The high level process and the position of the 'hole' itself is valuable and this justifies the implementation of the base class. The derived classes just follow the fundamental OO principle of 'coding by difference' and plugs the 'holes'.
Looks about right to me after your edits (assuming that you left out the Exception throwing part for readability) :)
All three methods would need to raise exceptions in a real world case.
Super class is inherited by sub-class. You can write methods in super class which you want to make common and leave it untouched. For the other part of code which you want it to be overwritten by sub classes define other set of methods in super class. write methods in sub-classes also. when u call method from sub-class u can put to call super-class method's
in short u have to write methods in sub class to over write the methods of superclass.
I would also make sure that your superclass is actually abstract. In this snippet it isn't. Overall though, looks decent.
Also consider declaring any instance variables in your superclass that classes that extend it will need as well.
First, there is nothing wrong with requiring subclasses' implementation in abstract (base) classes. It's just something that should not get abused, IMO. However, if I had to avoid it, I would make the ServerClass not abstract at all, and define every method of it. Instead, I would create RegistryFactory classes and pass them to the ServerClass :
class ServerClass {
public void startRMI(int port, RegistryFactory rf) {
// ...
rf.fillRegistry(this);
}
}
interface RegistryFactory {
/**
* Implement this method
*/
public void fillRegistry(ServerClass server);
}
public class RMIRegistryFactory implements RegistryFactory {
public void fillRegistry(ServerClass server) { /* ... */ }
}
Or something like that.
Your approach is fine, but it needs a simple improvement to make it perfect - make the startRMI() method final:
public final void startRMI(int port) {
fillRegistry();
}
This way you will prevent that someone overrides it (maybe because of not knowing that everything in startRMI() should be reused and that only fillRegistry() has to be customized).
Your solution generally matches the template method design pattern:
The template method is a method in a superclass, usually an abstract
superclass, and defines the skeleton of an operation in terms of a
number of high-level steps. These steps are themselves implemented by
additional helper methods in the same class as the template method.
The helper methods may be either abstract methods, for which case
subclasses are required to provide concrete implementations, or hook
methods, which have empty bodies in the superclass. Subclasses can
(but are not required to) customize the operation by overriding the
hook methods. The intent of the template method is to define the
overall structure of the operation, while allowing subclasses to
refine, or redefine, certain steps. (Wikipedia)
Given the above, the method startRMI() is a template method which defines the skeleton of an operation by using a number of high-level steps (in your case it's only one step but this doesn't make a difference). The method fillRegistry() in your example is a high-level step - it's defined as an abstract method in the superclass and has a concrete implementation in the superclasses.
On the other side, if you would override the method startRMI() in a subclass, this would not be OK anymore. That's why you should make it final to avoid confusion - this way someone who creates a subclass will know that he must implement fillRegistry() (since it's abstract) but should not change the implementation of startRMI (since it's final).
Since this is a commonly used design pattern, I wouldn't worry at all if this solution is OK, a lot of people are doing it like that and everyone who knows design patterns will recognize it, I think it feels very natural even for developers who don't know the design pattern.
Related
I've been reading through some questions here on SO concerning the use of #Override in Java. (e.g. this one on override and this one on default methods, and obviously the documentations) However, I am still confused.
I was taught to always use and implement an interface when all behaviour in that interface needed to be used by a class. I get that. But as I was taught, you would do something like this (partially taken from the docs):
public interface TimeClient {
void setTime(int hour, int minute, int second);
}
Which is then implemented by a class.
public class TestSimpleTimeClient implements TimeClient {
public static void main(String[] args) {
}
#Override
public void setTime(int hour, int minute, int second) {
System.out.println(hour + " " + minute + " " +second);
}
}
The thing that bugs me is the implementation of the method in the interface. It doesn't do anything, it's only declared as a method that take arguments but doesn't do anything else. Then you take that method and Override it in a class that implements that interface.
I understand that this is a way to "force" classes to implement a method but I don't see how this is useful in some specific use cases.
Let's say I have an interface that's implemented by a couple of classes. I want most of these classes to share the same implementation of the method, but not all. The logical, and character-efficient way would be to have a way to say: these classes take the default method in the interface, but these classes override the default method. How would I go about doing that? Should the one that overrides the method only implement it, whereas the ones that simply use the default method as a whole extend it? And what if you only want this behaviour for a specific method in an interface?
The thing that bugs me is the implementation of the method in the interface. It doesn't do anything, it's only declared as a method that take arguments but doesn't do anything else.
That is not an "implementation of the method in the interface". That's just an interface method declaration. In programming, terminology matters. Interfaces tend to be devoid of any implementations. (Unless you are talking about the default interface methods of Java 8, but from the rest of your question it is unclear whether you are aware of their existence.)
I understand that this is a way to "force" classes to implement a class
A class cannot implement a class. A class extends a class. But a class implements an interface. In programming, terminology matters a lot.
It is not just a way to force classes to provide an implementation. It is also a way for callers to be able to invoke an interface method without having to know anything about the class that implements it.
but I don't see how this is useful in some specific use cases.
Well, take for example the Collection<T> interface, and the contains() method, which is implemented by a myriad of classes, among which ArrayList<T>, LinkedList<T>, HashSet<T>, BoundedBlockingQueue<T>, and so on, and so forth. Your code may look like this:
boolean hasPekingese( Collection<Animal> animals )
{
return animals.contains( AllOfMyAnimals.PEKINGESE );
}
Note how the hasPekingese() method does not have to know the exact class that is implementing Collection<Animal>. Which means that you may invoke hasPekingese() from a class which keeps its animals in an ArrayList<Animal>, and you may also invoke hasPekingese() from a class which keeps its animals in a BoundedBlockingQueue<Animal>. The method hasPekingese() does not know, and does not care.
Let's say I have an interface that's shared by a couple of classes.
It is unclear what you mean by "shared". Do you mean "implemented"? In programming, terminology is of paramount importance.
I want most of these classes to share the same implementation of the method, but not all. The logical, and character-efficient way would be to have a way to say: these classes take the default method in the interface, but these classes override the default method. How would I go about doing that?
There are many ways to go about that, the most common being to have some of these classes extend some common base class, which provides the common implementation of your method, so that the derived classes inherit this method, so they do not have to implement it. The rest of the classes do not extend that common base class, so each one of them has to provide its own implementations of that method.
Should the one that overrides the method only implement it, whereas the ones that simply use the default method as a whole extend it?
That's not clear. Also, please do not call it a "default method", because as of Java 8 "default method" is a term that has a very specific meaning, and although it is related to this discussion, it is different from the sense in which you are using it.
And what if you only want this behaviour for a specific method in an interface?
If a derived class wants the method to work differently, it may re-override it. Or, you may have two different base classes, one which implements the method in a certain way, and another which implements it differently, so half of your derived classes extend the first base class, while the other half of your derived classes extend the second base class.
Interfaces are like APIs. When some provider give you interface like List you don't think about if it is an ArrayList or other implementation, you just know what you can do with this object. Why? Because when you give an interface, you can change the implementation later, and don't worry that other part of code, that is using object through interface, will need changes.
I suppose that you think about things that should plug some behaviour to current class. These things can be called Traits in other programming languages, in another you have multiple inheritance. If you want some implemented logic that is propagated to your classes, you should use abstract classes in java with proper hierarchic. Remember that you can expand classes with inheritance or composition (open-closed principle).
Default methods in Interfaces (Java 8) can be tricky, because they cannot change state of the object. They might be some stubs or mathematics equation that only work with local and static context.
Okay I know this question has been asked a few times, but I need an advice on my specific case. There are Encodable's and Decodable's, and a Message is both an Encodable and a Decodable:
interface Encodable { void encode(); }
interface Decodable { void decode(); }
class Message implements Encodable, Decodable { ... }
void processEncodable(Encodable encodable) {
...
encodable.encode();
...
}
There are other Encodable's (and Decodable's) besides Message and they need to be processed in processEncodable. So far so good, but the problem is that I want to hide encode() and decode() from outside the package, and Java interface doesn't allow protected/private methods. One might suggest abstract classes, but as you can see Message should inherit both Encodable and Decodable, so that's not the case. Any suggestions?
These days I'm very much into Scala, and Scala traits allow protected/private methods and that's more intuitive IMHO. I've gone through a few answers mentioning Java interface's design philosophy, but I don't really understand why it shouldn't permit protected methods if interface was introduced as an alternative to multiple inheritance, while abstract classes do..
Being an alternative doesn't imply that it is a full substitute. Interfaces are Service Contracts and so they expose the functionalities that a certain class provides to its clients, being a client anyone with access to the interface.
If you want to hide encode and decode from outside the package (that means that your logic should also stay in the package with the Message class) don't expose them through the interface and, instead, allow them to be protected (or package private) methods of your Message class (or a superclass, if various classes will be encodable/decodable).
This is not an isolated rule. There are mechanisms to achieve what you want without breaking the concept of an Interface. Just think about this: What good does that method to the interface if you can access it only inside a package? What's the point of having such methods in an interface if they can be class methods that are, also, available to the members of the package with the proper modifiers?
Assume, we have an abstract class A and we want to force all subclasses to have a certain field. This is not possible in Java, because we can not define abstract fields.
Workaround 1: Force subclasses to implement a method which delivers the wanted value.
abstract class A {
abstract int getA();
}
Drawback: Each subclass has to implement a method for each abstract field we want to have. This can lead to many method implementations.
Advantage: We can use the method getA in the abstract class and implement methods with it in A without implementing them in each subclass. But the value behind the method can not be overwritten by the abstract class.
Workaround 2: Simulate the abstract field by forcing the subclass to give the abstract class a value.
abstract class A {
int a;
public A(int a) {
this.a = a;
}
}
Drawback: When we have multiple fields (> 10), the super constructor call will look a bit ugly and confusing.
Advantage: We can use the field a in the abstract class and implement methods with it in A without implementing them in each subclass. Plus, the value a can be overwritten by the abstract class.
Question: Which workaround is the common way to reach the goal ? Maybe there is a better one than the above ones ?
The abstract method is probably the most object oriented.
If you have too many fields, you may want to regroup those in a POJO (if a new concept is appropriate).
I prefer the first one . i dont love to couple classes in fileds name , how they handle state and how they save it. the first one is more close to open/close principal
I recommend to avoid inheritance. inheritance is very frigle and hard to maintenance. remember effective java advice - prefer composition other inheritance
I think opt.1 is the cleaner by far. A few getters and setters is not a big deal, and I doubt that many use cases would have more than just a few abstract "fields".
About opt.2, you forget that constructors are not inherited, and thus would require all sub classes constructors to be implemented in a way that takes a into account.
Workaround 2 is very common because of 2 advantages:
1) the one you mentioned - the field does not belong to the subclass - it belongs to the parent and that is important because it was "demanded" by the parent and because the parent can use it
2) When sub-classing from the parent you are very aware of this field because when you implement the constructor you must pass it on. If I saw the first workaround I wouldn't know what to understand from it, in this way I understand that the parent class needs this field to work, so it must have a meaningful value.
note: if you have a class that has 10 fields that need to be initialized something is probably wrong in your design.
1. Actually its not about what one prefers but its about the flexibility, and the ability
to adapt changes.
2. Its always better to Encapsulate Behaviors that keeps changing, either into an Interface or Abstract class.
3. You 1st Workaround will be good in places where you need different implementation for the same Behavior in Different classes. Then at this place either an Interface or your 1st Workaround will be a good choice.
Eg:
Consider Painting as a Class with paint() Method.
Now
paint() method can have Stroking, gliding, shading etc styles of doing it.
Then its better to Encapsulate that method into an Abstract class or an Interface.
public interface Paint{
paintDoIt(String style);
}
4. Your 2nd Wordaround will be good in a place, where you want certain behaviors to be MUST implemented by the Subclass.
Eg:
Consider Car as an Abstract Class, Now to be car its very important that it must have
a Steering, 4 wheels, Engine, etc. So these features must be implemented.
where as other features like music system, LCD ,etc are optional and depends on the car type.
Sometimes when I extend one of my own classes, I want to (for the purpose of the subclass) "inject" one or two lines of code in the middle a method in the super class.
In these cases I sometimes add a call to an empty protected method for the subclass to override.
public void superClassMethod() {
// some fairly long snippet of code
doSubclassSpecificStuff();
// some other fairly long snippet of code
}
// dummy method used for overriding purposes only!
protected void doSubclassSpecificStuff() {
}
When doing this several times in the same class I must say it looks quit awkward / ugly so my questions:
Is this way of "opening up" for subclasses to "inject" code in the middle of methods considered good practice or not?
Is the pattern (anti-pattern?) called something?
Has it been used in any well known API / library? (Note that I'm talking about non-abstract classes.)
Are there any better alternatives?
The only alternative I can come up with is to use something like the command pattern and have a setMiddleOfMethodHandler(SomeRunnableHandler), and call handler.doSubclassSpecificStuff() instead of the dummy-method. It has a few drawbacks as I see it though, such as for instance not being able to touch protected data.
You've just discovered the Template method design pattern. Note though that normally the methods that comprise the individual steps are abstract (rather than empty and protected) so that subclasses must override them.
There is the Template method pattern. The idea there is that much of the work is common, except for a few bits, which are handled by a subclass implemented method.
Yes, this is a legitimate way to do things; I've used it myself.
The only problem I can see is not the specific technique, but the fact that you are using subclasses of concrete (read: non-abstract) classes at all. Subclassing concrete classes has many subtle problems, so I would recommend to avoid it altogether. See e.g. http://en.wikipedia.org/wiki/Liskov_substitution_principle for an explanation of what you must do to properly subclass a class, and the problems involved. Also, in "Effective Java" Block recommends using composition (Item 16).
Another approach (that avoids subclassing) would be to use Dependency Injection. Your method would accept a parameter of a type that implements the interface ISpecificStuff, which specifies a method doSubclassSpecificStuff():
public void superClassMethod(ISpecificStuff specificStuff) {
....
specificStuff.doSubclassSpecificStuff();
....
}
That way, any caller can decide what the method should do. This avoids the need for subclassing. Of course, you could inject via a constructor, if you need it in more than one method.
It looks fishy to me. I think the reason you're ending up having to do this is a design flaw. Your method that needs to be "split" probably does too much. The solution would be to break it up in steps, and give that "doSubclassSpecificStuff" step a specific meaning.
For ex.:
void Live()
{
BeBorn();
DoCrazyStuff(); // this can be made protected virtual
Die();
}
Yes, it's perfectly fine. This is an example of the Template Method pattern, where you use inheritance to define a method that maintains a known "skeleton", but can have custom logic.
public abstract class Ancestor
{
protected virtual void CanOverrideThisStep(){...}
protected abstract void MustDefineThisStep();
protected sealed void MustDoExactlyThis(){...}
private void HideThisStepFromEveryone(){...}
public sealed void TemplateMethod()
{
...
CanOverrideThisStep();
...
MustDoExactlyThis();
...
MustDefineThisStep();
...
HideThisStepFromEveryone();
}
}
Inheritors of Ancestor above must define a body for MustDefineThisStep(), and may at their option override CanOverrideThisStep(), but cannot touch MustDoExactlyThis(), HideThisStepFromEveryone, or the TemplateMethod driving function itself. However, except for HideThisStepFromEveryone, all the submethods are available to child classes, so a child may use MustDoExactlyThis() in the implementation of MustDefineThisStep().
This is very common; such constructions are the reason OO languages have such access modifiers such as these at their disposal. The pattern is very useful for workflows, file processing, and other tasks that are generally the same but have slightly different implementation details.
I routinely use this technique as a way to handle special cases. I'll write things like this:
public void foo()
{
theData=getTheData();
preprocessDataHook(theData);
putTheData(theData);
}
protected void preprocessDataHook(SomeObject theData)
{
// Nop. Available for subclasses to override.
}
A subclass that does not need to preprocess the data can then just not override this function. A subclass that does need to preprocess can override the function.
If we expected that all or most subclasses would need to preprocess, then this should be an abstract function to force the programmer to implement it, or make a conscious decision to do nothing. But if it's just an occassional subclass that needs to do something here, I think this is a perfectly valid approach.
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