I am reading a book about Java and it says that you can declare the whole class as final. I cannot think of anything where I'd use this.
I am just new to programming and I am wondering if programmers actually use this on their programs. If they do, when do they use it so I can understand it better and know when to use it.
If Java is object oriented, and you declare a class final, doesn't it stop the idea of class having the characteristics of objects?
First of all, I recommend this article: Java: When to create a final class
If they do, when do they use it so I can understand it better and know when to use it.
A final class is simply a class that can't be extended.
(It does not mean that all references to objects of the class would act as if they were declared as final.)
When it's useful to declare a class as final is covered in the answers of this question:
Good reasons to prohibit inheritance in Java?
If Java is object oriented, and you declare a class final, doesn't it stop the idea of class having the characteristics of objects?
In some sense yes.
By marking a class as final you disable a powerful and flexible feature of the language for that part of the code. Some classes however, should not (and in certain cases can not) be designed to take subclassing into account in a good way. In these cases it makes sense to mark the class as final, even though it limits OOP. (Remember however that a final class can still extend another non-final class.)
In Java, items with the final modifier cannot be changed!
This includes final classes, final variables, and final methods:
A final class cannot be extended by any other class
A final variable cannot be reassigned another value
A final method cannot be overridden
One scenario where final is important, when you want to prevent inheritance of a class, for security reasons. This allows you to make sure that code you are running cannot be overridden by someone.
Another scenario is for optimization: I seem to remember that the Java compiler inlines some function calls from final classes. So, if you call a.x() and a is declared final, we know at compile-time what the code will be and can inline into the calling function. I have no idea whether this is actually done, but with final it is a possibility.
The best example is
public final class String
which is an immutable class and cannot be extended.
Of course, there is more than just making the class final to be immutable.
If you imagine the class hierarchy as a tree (as it is in Java), abstract classes can only be branches and final classes are those that can only be leafs. Classes that fall into neither of those categories can be both branches and leafs.
There's no violation of OO principles here, final is simply providing a nice symmetry.
In practice you want to use final if you want your objects to be immutable or if you're writing an API, to signal to the users of the API that the class is just not intended for extension.
Relevant reading: The Open-Closed Principle by Bob Martin.
Key quote:
Software Entities (Classes, Modules,
Functions, etc.) should be open for
Extension, but closed for
Modification.
The final keyword is the means to enforce this in Java, whether it's used on methods or on classes.
The keyword final itself means something is final and is not supposed to be modified in any way. If a class if marked final then it can not be extended or sub-classed. But the question is why do we mark a class final? IMO there are various reasons:
Standardization: Some classes perform standard functions and they are not meant to be modified e.g. classes performing various functions related to string manipulations or mathematical functions etc.
Security reasons: Sometimes we write classes which perform various authentication and password related functions and we do not want them to be altered by anyone else.
I have heard that marking class final improves efficiency but frankly I could not find this argument to carry much weight.
If Java is object oriented, and you declare a class final, doesn't it
stop the idea of class having the characteristics of objects?
Perhaps yes, but sometimes that is the intended purpose. Sometimes we do that to achieve bigger benefits of security etc. by sacrificing the ability of this class to be extended. But a final class can still extend one class if it needs to.
On a side note we should prefer composition over inheritance and final keyword actually helps in enforcing this principle.
final class can avoid breaking the public API when you add new methods
Suppose that on version 1 of your Base class you do:
public class Base {}
and a client does:
class Derived extends Base {
public int method() { return 1; }
}
Then if in version 2 you want to add a method method to Base:
class Base {
public String method() { return null; }
}
it would break the client code.
If we had used final class Base instead, the client wouldn't have been able to inherit, and the method addition wouldn't break the API.
A final class is a class that can't be extended. Also methods could be declared as final to indicate that cannot be overridden by subclasses.
Preventing the class from being subclassed could be particularly useful if you write APIs or libraries and want to avoid being extended to alter base behaviour.
In java final keyword uses for below occasions.
Final Variables
Final Methods
Final Classes
In java final variables can't reassign, final classes can't extends and final methods can't override.
Be careful when you make a class "final". Because if you want to write an unit test for a final class, you cannot subclass this final class in order to use the dependency-breaking technique "Subclass and Override Method" described in Michael C. Feathers' book "Working Effectively with Legacy Code". In this book, Feathers said, "Seriously, it is easy to believe that sealed and final are a wrong-headed mistake, that they should never have been added to programming languages. But the real fault lies with us. When we depend directly on libraries that are out of our control, we are just asking for trouble."
If the class is marked final, it means that the class' structure can't be modified by anything external. Where this is the most visible is when you're doing traditional polymorphic inheritance, basically class B extends A just won't work. It's basically a way to protect some parts of your code (to extent).
To clarify, marking class final doesn't mark its fields as final and as such doesn't protect the object properties but the actual class structure instead.
TO ADDRESS THE FINAL CLASS PROBLEM:
There are two ways to make a class final. The first is to use the keyword final in the class declaration:
public final class SomeClass {
// . . . Class contents
}
The second way to make a class final is to declare all of its constructors as private:
public class SomeClass {
public final static SOME_INSTANCE = new SomeClass(5);
private SomeClass(final int value) {
}
Marking it final saves you the trouble if finding out that it is actual a final, to demonstrate look at this Test class. looks public at first glance.
public class Test{
private Test(Class beanClass, Class stopClass, int flags)
throws Exception{
// . . . snip . . .
}
}
Unfortunately, since the only constructor of the class is private, it is impossible to extend this class. In the case of the Test class, there is no reason that the class should be final. The Test class is a good example of how implicit final classes can cause problems.
So you should mark it final when you implicitly make a class final by making it's constructor private.
One advantage of keeping a class as final :-
String class is kept final so that no one can override its methods and change the functionality. e.g no one can change functionality of length() method. It will always return length of a string.
Developer of this class wanted no one to change functionality of this class, so he kept it as final.
The other answers have focused on what final class tells the compiler: do not allow another class to declare it extends this class, and why that is desirable.
But the compiler is not the only reader of the phrase final class. Every programmer who reads the source code also reads that. It can aid rapid program comprehension.
In general, if a programmer sees Thing thing = that.someMethod(...); and the programmer wants to understand the subsequent behaviour of the object accessed through the thing object-reference, the programmer must consider the Thing class hierarchy: potentially many types, scattered over many packages. But if the programmer knows, or reads, final class Thing, they instantly know that they do not need to search for and study so many Java files, because there are no derived classes: they need study only Thing.java and, perhaps, it's base classes.
Yes, sometimes you may want this though, either for security or speed reasons. It's done also in C++. It may not be that applicable for programs, but moreso for frameworks.
http://www.glenmccl.com/perfj_025.htm
think of FINAL as the "End of the line" - that guy cannot produce offspring anymore. So when you see it this way, there are ton of real world scenarios that you will come across that requires you to flag an 'end of line' marker to the class. It is Domain Driven Design - if your domain demands that a given ENTITY (class) cannot create sub-classes, then mark it as FINAL.
I should note that there is nothing stopping you from inheriting a "should be tagged as final" class. But that is generally classified as "abuse of inheritance", and done because most often you would like to inherit some function from the base class in your class.
The best approach is to look at the domain and let it dictate your design decisions.
As above told, if you want no one can change the functionality of the method then you can declare it as final.
Example: Application server file path for download/upload, splitting string based on offset, such methods you can declare it Final so that these method functions will not be altered. And if you want such final methods in a separate class, then define that class as Final class. So Final class will have all final methods, where as Final method can be declared and defined in non-final class.
Let's say you have an Employee class that has a method greet. When the greet method is called it simply prints Hello everyone!. So that is the expected behavior of greet method
public class Employee {
void greet() {
System.out.println("Hello everyone!");
}
}
Now, let GrumpyEmployee subclass Employee and override greet method as shown below.
public class GrumpyEmployee extends Employee {
#Override
void greet() {
System.out.println("Get lost!");
}
}
Now in the below code have a look at the sayHello method. It takes Employee instance as a parameter and calls the greet method hoping that it would say Hello everyone! But what we get is Get lost!. This change in behavior is because of Employee grumpyEmployee = new GrumpyEmployee();
public class TestFinal {
static Employee grumpyEmployee = new GrumpyEmployee();
public static void main(String[] args) {
TestFinal testFinal = new TestFinal();
testFinal.sayHello(grumpyEmployee);
}
private void sayHello(Employee employee) {
employee.greet(); //Here you would expect a warm greeting, but what you get is "Get lost!"
}
}
This situation can be avoided if the Employee class was made final. Just imagine the amount of chaos a cheeky programmer could cause if String Class was not declared as final.
Final class cannot be extended further. If we do not need to make a class inheritable in java,we can use this approach.
If we just need to make particular methods in a class not to be overridden, we just can put final keyword in front of them. There the class is still inheritable.
Final classes cannot be extended. So if you want a class to behave a certain way and don't someone to override the methods (with possibly less efficient and more malicious code), you can declare the whole class as final or specific methods which you don't want to be changed.
Since declaring a class does not prevent a class from being instantiated, it does not mean it will stop the class from having the characteristics of an object. It's just that you will have to stick to the methods just the way they are declared in the class.
Android Looper class is a good practical example of this.
http://developer.android.com/reference/android/os/Looper.html
The Looper class provides certain functionality which is NOT intended to be overridden by any other class. Hence, no sub-class here.
I know only one actual use case: generated classes
Among the use cases of generated classes, I know one: dependency inject e.g. https://github.com/google/dagger
Object Orientation is not about inheritance, it is about encapsulation. And inheritance breaks encapsulation.
Declaring a class final makes perfect sense in a lot of cases. Any object representing a “value” like a color or an amount of money could be final. They stand on their own.
If you are writing libraries, make your classes final unless you explicitly indent them to be derived. Otherwise, people may derive your classes and override methods, breaking your assumptions / invariants. This may have security implications as well.
Joshua Bloch in “Effective Java” recommends designing explicitly for inheritance or prohibiting it and he notes that designing for inheritance is not that easy.
Related
By making private constructor, we can avoid instantiating class from anywhere outside. and by making class final, no other class can extend it. Why is it necessary for Util class to have private constructor and final class ?
This is not a mandate from a functional point of view or java complication or runtime. However, it's a coding standard accepted by the wider community. Even most static code review tools, like checkstyle, check that such classes have this convention followed.
Why this convention is followed is already explained in other answers and even OP covered that, but I'd like to explain it a little further.
Mostly utility classes are a collection of methods/functions which are independent of an object instance. Those are kind of like aggregate functions as they depend only on parameters for return values and are not associated with class variables of the utility class. So, these functions/methods are mostly kept static. As a result, utility classes are, ideally, classes with only static methods. Therefore, any programmer calling these methods doesn't need to instantiate the class. However, some robo-coders (maybe with less experience or interest) will tend to create the object as they believe they need to before calling its method. To avoid that, we have 3 options:
Keep educating people to not instantiate it. (No sane person can keep doing it.)
Mark the utility class as abstract: Now robo-coders will not create the object. However, reviewers and the wider java community will argue that marking the class as abstract means you want someone to extend it. So, this is also not a good option.
Private constructor: Not protected because it'll allow a child class to instantiate the object.
Now, if someone wants to add a new method for some functionality to the utility class, they don't need to extend it: they can add a new method as each method is independent and has no chance of breaking other functionalities. So, no need to override it. Also, you are not going to instantiate it, so no need to subclass it. Better to mark it final.
In summary, instantiating a utility class (new MyUtilityClass()) does not make sense. Hence the constructors should be private. And you never want to override or extend it, so mark it final.
It's not necessary, but it is convenient. A utility class is just a namespace holder of related functions and is not meant to be instantiated or subclassed. So preventing instantiation and extension sends a correct message to the user of the class.
There is an important distinction between the Java Language, and the Java Runtime.
When the java class is compiled to bytecode, there is no concept of access restriction, public, package, protected, private are equivalent. It is always possible via reflection or bytecode manipulation to invoke the private constructor, so the jvm cannot rely on that ability.
final on the other hand, is something that persists through to the bytecode, and the guarantees it provides can be used by javac to generate more efficient bytecode, and by the jvm to generate more efficient machine instructions.
Most of the optimisations this enabled are no longer relevant, as the jvm now applies the same optimisations to all classes that are monomorphic at runtime—and these were always the most important.
By default this kind of class normally is used to aggregate functions who do different this, in that case we didn't need to create a new object
I needed to do a java program for my class.
I did it and worked well.
In my code, I have some private methods. Do I need to build an Interface Class hide those private methods or they can be at the same class as the public methods?
At the moment all methods (public and private) are in the same class, but my coworker insists that I need to create an Interface to hide the private methods
No, you don't need to create an interface. An interface would hide them even more (from people's eyes, not from code) if you were to use the class only through the interface, but even without one, the private methods won't be available to other classes.
If you want to implement the "code to an interface" guideline fully, you can declare an interface for your public methods to implement.
What you can definitely not do is declare methods in an interface, then "implement" them as private in your class - that would reduce the visibility and not even compile.
Ultimately private methods are scoped to your class only.
It might help to repeat "what is the purpose of an Interface?"
It is: "a formal declaration of what, as far as anyone else should be concerned, this thing 'gives,' 'takes,' and 'does.'"
So ... if a client class wants to deal with "something that implements this Interface," and five other classes (none of which, say, are siblings or ancestors of one another ...), each in their own way, do so, then: any of the five would be compatible. Why? Because they supply all of the properties and/or methods that the interface requires. (That's what "implementing" actually means.)
None of this exposes any of the client's "private things." In fact, an Interface says absolutely nothing about how the class actually does what it has to do, nor what else it does. "Meet the requirements of the Interface, and you can get the job."
Why is it a good practice to mark a class with only private constructors as final? My guess is, it is to let other programmers know that it cannot be sub-classed.
It is often considered (e.g. by Josh Bloch and the designers of C#) good practice to mark everything as final unless you have an explicit reason not to. Assuming you mean class, you're correct that a class with only private constructors can't be subclassed. Thus, the final could be considered redundant, but as you say it has value for documentation. As Marc suggests, it may also aid optimization.
Making a class final has some (small) performance gain, because the JIT compiler can inline functionality from that class. I don't know if that qualifies as 'good practice', but I see the advantages.
You mean "a class with private constructor" do you?
A final class can't be subclassed. This may represent a design decision. Not all classes are designed to be subclassed, so if yours is not, it is best to mark it explicitly, to avoid subtle bugs later.
A class with private constructors only can't be instantiated by the outside world (neither subclassed). This may be useful for e.g. singletons, or classes where you want to control what instances of the class are created. E.g. before Java5, the typesafe enum pattern used this.
We mark the class as final class by making constructor as private, to avoid sub-classing.
This is good practice, in cases where we don’t want people to override our class methods and change the functionality or add the functions to our class.
For example, classes String and Math are final classes, which we can’t extend or subclass, this is to make sure that no one will change their behavior.
A final class with private constructor:
Class can not have sub-classes, we can not extent final class.
Class having private constructor, we can not create the object of that class.
It means other methods of the class will be static, so that class can access them.
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
We are in the process of refactoring some code. There is a feature that we have developed in one project that we would like to now use in other projects. We are extracting the foundation of this feature and making it a full-fledged project which can then be imported by its current project and others. This effort has been relatively straight-forward but we have one headache.
When the framework in question was originally developed, we chose to keep a variety of constant values defined as static fields in a single class. Over time this list of static members grew. The class is used in very many places in our code. In our current refactoring, we will be elevating some of the members of this class to our new framework, but leaving others in place. Our headache is in extracting the foundation members of this class to be used in our new project, and more specifically, how we should address those extracted members in our existing code.
We know that we can have our existing Constants class subclass this new project's Constants class and it would inherit all of the parent's static members. This would allow us to effect the change without touching the code that uses these members to change the class name on the static reference. However, the tight coupling inherent in this choice doesn't feel right.
before:
public class ConstantsA {
public static final String CONSTANT1 = "constant.1";
public static final String CONSTANT2 = "constant.2";
public static final String CONSTANT3 = "constant.3";
}
after:
public class ConstantsA extends ConstantsB {
public static final String CONSTANT1 = "constant.1";
}
public class ConstantsB {
public static final String CONSTANT2 = "constant.2";
public static final String CONSTANT3 = "constant.3";
}
In our existing code branch, all of the above would be accessible in this manner:
ConstantsA.CONSTANT2
I would like to solicit arguments about whether this is 'acceptable' and/or what the best practices are.
A class with only static fields is a code smell. It's not a class.
Some people use interfaces, so they can implement it to use the constants more easily. But an interface should be used only to model a behaviour of a class. (http://pmd.sourceforge.net/rules/design.html#AvoidConstantsInterface) Using static imports from Java 5 removes the need for simple constant usage at all.
Are your constants really Strings, or just used as Strings. If they are different options for some type (so called enumerations), you should used typesafe enumerations, using enum in Java 5 or the Enum provided by Commons Lang. Of course, converting your code to use enums might be a little work.
You should at least split the constants to groups of related constants in files with proper business name. Moving the final members is easy in IDE and will update all usages.
If you can afford it, convert them to enums then. (Think about using about a script to do that, often it's possible.) Class hierarchies are only usefull, if there is a relation between the constants/enums. You can keep the Strings if you have to but still think about them as entities, then extends might make sense for some (describing is-a relation). First enums can be simple classes made by yourself if serializing is not a problem. Enums are always favourable due to their type safe nature and the extra name showing intend or business/domain specific things.
If the constants are really String constants use a Properies or ResourceBundle, which can be configured by plain text files. Again you can script the refactoring using the constant names as resource bundle keys and generate both files automatically.
I don't like it, but it's probably the best you can do right now.
The right answer would be to break up the constants into coherent groups, fixing the code breaks as you go along. In C#, I'd use enums.
Peter Kofler has already discussed how you might wish to better organize constants. I'll share how to automate the transition:
The eclipse "Inline" refactoring can automatically replace constants by their defintion, saving you from having to hunt down and change each usage manually. So you'd simply change the code to:
public class ConstantsA {
public static final String CONSTANT1 = "constant.1";
public static final String CONSTANT2 = ConstantsB.CONSTANTFOO;
public static final String CONSTANT3 = ConstantsB.CONSTANTBAR;
}
public class ConstantsB {
public static final String CONSTANTFOO = "constant.2";
public static final String CONSTANTBAR = "constant.3";
}
... and then have eclipse inline COONSTANT2 and CONSTANT3 (while all affected projects are checked out, if you can't do that, look into refactoring scripts), and you're done.
I've seen this done by putting the static final String on an interface, so that you can 'implement' it and not have to worry about what to do when you need a different base class. It's just as accessible that way.
In general though, enums are pretty good at what you are trying to do, and may get rid of the "I'm not sure" feeling you are experiencing, as that's the intention of enums.
I think what you are doing is fine. Yes, the classes are tightly-coupled, but that is kind of the point -- you want to be able to reference only a single class to see all of your project-wide constants.
You do have to be diligent to ensure that ConstantsB contains only constants that are generalizable amongst all your projects, and ConstantsA contains only project-specific constants. If, later on, you realize that there is a constant in ConstantsB that you seem to be overriding in your subclasses a lot, then that's an indication it should've never been put in ConstantsB in the first place.
I think what you've got is a good first step. The next step is to gradually replace all references to ConstantsA.CONSTANT2 and ConstantsA.CONSTANT3 with ConstantsB.CONSTANT2 and ConstantsB.CONSTANT3 until you can remove the extends.
Most IDEs can be configured to show a warning if you refer to a superclass constant via a subclass, and I'd guess static analysis tools like FindBugs can do it, too.
One idea that might be slightly cleaner:
make all the constants classes interfaces
move all the constants out of ConstantsA and call it something like LegacyConstants
have LegacyConstants extend all the other, modular Constants interfaces
deprecate LegacyConstants
The goal would be not to have any inheritance between the Constants interfaces. LegacyConstants would be the only place there's any inheritance, it wouldn't declare any constants of its own, and when it's no longer used -- when every class that did use it instead refers to the proper Constants interface -- you've finished refactoring.
When you extract your constants, have the old class reference the constant defined in the new class. There's really no need to create an inheritance relationship here.
I could be wrong, but I don't think we need constants at all. It just means that you can't change the value of the constants and you probably should.