This question already has answers here:
Alternatives to static methods on interfaces for enforcing consistency
(7 answers)
Closed 8 years ago.
I have several Java classes which contain static methods. I want to ensure that these static methods are implemented by each of the classes. How can I enforce that, since declaring static methods is not allowed in interfaces in Java?
If you have code that calls these methods, it won't pass compilation if they are not implemented. If you don't, you don't really need them.
Sort answer: You can't enforce that.
Static methods are not inherited the same way as instance methods, so you wouldn't be able to use it for anything meaningful anyway: You can't call MySuperClass.staticMethod() and expect some subclass to handle the call. This means that you have to call it using MySubClass.staticMethod() in which case you'll get a compilation error if MySubClass doesn't implement staticMethod.
I would suggest you look into solving it using a singleton or factory pattern and use instance methods:
MySuperClass.getInstance(parameter).yourMethod()
static methods are allowed on interfaces in Java8.
However, there's no way to enforce a class to implement a static method.
The only thing you can do, is make them non-static and abstract (either in an interface or in an abstract class).
Related
This question already has answers here:
How are Anonymous inner classes used in Java?
(18 answers)
Closed 5 years ago.
I have found a this example with the following piece of code:
CompletableFuture<T> completable = new CompletableFuture<T>() {
#Override
public boolean cancel(boolean mayInterruptIfRunning) {
// propagate cancel to the listenable future
boolean result = listenableFuture.cancel(mayInterruptIfRunning);
super.cancel(mayInterruptIfRunning);
return result;
}
};
What does this syntax means? Is this method overriding a method in the class without extending it?
This block of code creates an anonymous subclass of CompletableFuture. This anonymous subclass overrides the cancel(bool) method with a new implementation. An anonymous subclass is equivalent to creating a new class that extends CompleteableFuture with the caveat that it isn't straightforward to re-use this new subclass somewhere else.
#Override is not strictly necessary in this case but it provides you several benefits, as described by the answer to this question:
Use it every time you override a method for two benefits. Do it so that you can take advantage of the compiler checking to make sure you actually are overriding a method when you think you are. This way, if you make a common mistake of misspelling a method name or not correctly matching the parameters, you will be warned that you method does not actually override as you think it does. Secondly, it makes your code easier to understand because it is more obvious when methods are overwritten.
Additionally, in Java 1.6 you can use it to mark when a method implements an interface for the same benefits. I think it would be better to have a separate annotation (like #Implements), but it's better than nothing.
It's an anonymous class (one of the nested class types). Please also refer to the Java tutorial - Classes and Objects for further class types, if you are interested in them.
This question already has answers here:
What is the point of "final class" in Java?
(24 answers)
Closed 9 years ago.
I was asked about the reason of why a final class cannot be overridden. I tried to explain that it is more of design approach where by you don't want your class to be extended or methods to be overridden and a final class also helps to make an object immutable (though there are more steps to be done rather than just declaring the class as final).
However I would like to know if there is more to thought process behind declaring a class as final or the fact that it cannot be overridden?
Because them's the rules; It's a part of the design of the language.
http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.1.1.2
As a designer you say "my class is not suitable for inheritence"; Java honours this by explicitly preventing someone from doing so.
Final class cannot be extended because Java compiler will not allow it, because Java compiler follows the rules defined by Java Language Specification which does not allow it.
final methods can't be overriden, because that's what final is designed to do: it's a sign saying "do not override this".
From Wiki
A final method cannot be overridden or hidden by subclasses. This is used to prevent unexpected behavior from a subclass altering a method that may be crucial to the function or consistency of the class.
A common misconception is that declaring a class or method as final improves efficiency by allowing the compiler to directly insert the method wherever it is called (see inline expansion). But because the method is loaded at runtime, compilers are unable to do this. Only the runtime environment and JIT compiler know exactly which classes have been loaded, and so only they are able to make decisions about when to inline, whether or not the method is final.
A final class is a class that can't be extended, thats the reason u cannot override the final class
it is also used to make the class immutable
and one good advantage of defining class as final that it improves JVM performance
see this
This question already has answers here:
Why doesn't Java allow overriding of static methods?
(22 answers)
Closed 9 years ago.
I am trying to understand why we can't override static and final methods. I do not get the purpose behind it.
final methods can't be overriden, because that's what final is designed to do: it's a sign saying "do not override this".
static methods can't be overriden, because they are never called in a polymorphic way: when you call SomeClass.foo() it will always be the foo method of SomeClass, no matter if there is another ExtendedSomeClass that has a much groovier foo method.
final is used to avoid overriding. And a static method is not associated with any instance of a class so the concept is not applicable.
The reason for not overriding static method is that Static methods are treated as global by the JVM so there are not bound to an object instance at all. Similarly final methods cant be overriddent because when you say a method as final then it mean you are saying to the JVM that this method cannot be overridden.
The wiki has a very important misconception about final. Do read that!
A final method cannot be overridden or hidden by subclasses.[2] This
is used to prevent unexpected behavior from a subclass altering a
method that may be crucial to the function or consistency of the
class.[3]
A common misconception is that declaring a class or method as final
improves efficiency by allowing the compiler to directly insert the
method wherever it is called (see inline expansion). But because the
method is loaded at runtime, compilers are unable to do this. Only the
runtime environment and JIT compiler know exactly which classes have
been loaded, and so only they are able to make decisions about when to
inline, whether or not the method is final.[4]
Static methods are covered here.
Final methods cannot be overridden because the purpose of the "final" keyword is to prevent overriding.
Final cannot be overridden because that is the purpose of the keyword, something that cannot be changed or overridden.
The purpose of inheritance and polymorphism is to have objects of same class implementing methods (not the names but the code in the methods) in different ways. And static methods cannot be accessed by objects because they are a part of the class not the instance. So there is no purpose to overriding the Static methods. And you can have a static method in subclass by the same name but that won’t be an overridden method.
If you haven't yet read about Inheritance and Polymorphism which are both features of Java, you should and also try to write the code in the question so that SO users can answer with an example.
This question already has answers here:
In what situations is static method a good practice?
(11 answers)
Closed 9 years ago.
what exactly is the use of java static methods? Yes i understand that they can be called without creating instance if in the same class. They can be called directly.
In case of main method, as it will be the first method to be called and has to be called without initializing it, static word justifies.
But in which other scenarios do we need to make a method static?
Simply
If your method not related/depend on any of the instances they can better be static methods.
You points are on top and moreover,To avoid unnecessary object creation
An example from the java API is Math, all the variables and methods are static. Does it make sense to have to create a Math object just to call a single method.
Often people use this to write some Utility methods.
You gave the answer in your question: static methods are useful whenever you don't need an instance to do something. The typical example is:
public static int min(int a, int b);
This method just gives the min of two numbers and does not need a context (instance) to work.
Now you can use it as:
int min = YourClass.min(1, 2);
Had you declared it as an instance method, you would need to unnecessarily create a YourClass object.
Static methods are used when you don't want to make an object of a class, such as in the Math class. If the math methods are static you don't have to create an object of the Math class. Other uses include for universal counter variables(making them static) and variables that everyone should know but are themselves are unable to be edited. Some more advanced uses include in threading and design patterns.
This question already has answers here:
Closed 12 years ago.
Possible Duplicates:
Why can’t static methods be abstract in Java
Static methods and their overriding
Why doesn’t Java allow overriding of static methods ?
Can we override static method in Java?
No. Static methods are tied to the class they're defined in. They're invoked through the class, not through an object, and there is no dynamic dispatch where overriding could happen.
You're probably confused because Java allows you to invoke static methods through an object reference. That's generally considered a design error, and it does not work like invoking instance methods, because the compile-time type of the reference decides which method gets invoked.
No, but you can shadow it.