I was playing with the following question: Using Java 8's Optional with Stream::flatMap and wanted to add a method to a custom Optional<T> and then check if it worked.
More precise, I wanted to add a stream() to my CustomOptional<T> that returns an empty stream if no value is present, or a stream with a single element if it is present.
However, I came to the conclusion that Optional<T> is declared as final.
Why is this so? There are loads of classes that are not declared as final, and I personally do not see a reason here to declare Optional<T> final.
As a second question, why can not all methods be final, if the worry is that they would be overridden, and leave the class non-final?
According to this page of the Java SE 8 API docs, Optional<T> is a value based class. According to this page of the API docs, value-based classes have to be immutable.
Declaring all the methods in Optional<T> as final will prevent the methods from being overridden, but that will not prevent an extending class from adding fields and methods. Extending the class and adding a field together with a method that changes the value of that field would make that subclass mutable and hence would allow the creation of a mutable Optional<T>. The following is an example of such a subclass that could be created if Optional<T> would not be declared final.
//Example created by #assylias
public class Sub<T> extends Optional<T> {
private T t;
public void set(T t) {
this.t = t;
}
}
Declaring Optional<T> final prevents the creation of subclasses like the one above and hence guarantees Optional<T> to be always immutable.
As others have stated Optional is a value based class and since it is a value based class it should be immutable which needs it to be final.
But we missed the point for this. One of the main reason why value based classes are immutable is to guarantee thread safety. Making it immutable makes it thread safe. Take for eg String or primitive wrappers like Integer or Float. They are declared final for similar reasons.
Probably, the reason is the same as why String is final; that is, so that all users of the Optional class can be assured that the methods on the instance they receive keep to their contract of always returning the same value.
Though we could not extend the Optional class, we could create our own wrapper class.
public final class Opt {
private Opt() {
}
public static final <T> Stream<T> filledOrEmpty(T t) {
return Optional.ofNullable(t).isPresent() ? Stream.of(t) : Stream.empty();
}
}
Hope it might helps you. Glad to see the reaction!
Related
In a legacy code, I'm working with, I found the following thing:
#Autowired
final lateinit var controller: CustomController
what does this final keyword mean here?
In a Kotlin documentation I found a short description about final keyword that is blocking overriding of the methods in open classes but no information about fields. Also - the class within which I found the line is not open
A final property or a method in Kotlin prevents overriding of the field / method. That being said, Kotlin by default considers a property or a method/function to be final unless specified by the keyword open. In your case, the final keyword is redundant.
Here's a small demo test case to illustrate the same.
open class Parent {
open val someValue = 0
final val otherValue = 13 // redundant modifier 'final' warning in Android Studio
}
class Child : Parent() {
override val someValue = 5
// override val otherValue = 19 // compile error
}
There is an interesting problem called Fragile Base Class in OOP and why some languages like Kotlin prefer final by default.
What you have there is a property, not a field.
It looks just like a field, as it would in Java; but in Kotlin, it actually defines a public getter method, a public setter method, and a private backing field*.
So the final modifier applies to the accessor methods, preventing those from being overridden in a subclass. (As you say, the backing field itself can't be overridden anyway.)
As Siddharth says, final is the default in Kotlin, so you usually wouldn't need to specify it, though there are a few situations in which it would be needed — e.g. if it were already overriding something, or you were using the all-open or kotlin-spring compiler plug-ins. (The use of #Autowired suggests that this is a Spring module, which probably explains why final is needed here.) In any case, your IDE would probably indicate where it's not needed, e.g. by showing it greyed-out.
(* Only the getter is necessary; the setter isn't generated for a val, and the backing field isn't generated if you override the accessor(s) and they don't refer to it.)
As we know, every entity of ElasticsearchEntityMapper requires annotation
#Document(indexName="foo")
public class Foo {...}
, so that we can use convenient methods, like:
<T> Page<T> queryForPage(SearchQuery query, Foo.class);
But it is very common, that the indices of elasticsearch were designed time-based or language-based. i.e.
#Document(indexName="2019")
public class Foo {...}
#Document(indexName="2020")
public class Foo {...}
or
#Document(indexName="english")
public class Foo {...}
#Document(indexName="german")
public class Foo {...}
Of course, we can't create classes like this, one class with different indices.
What should we do in this case? One class each index, Foo2019, Foo2020? <- Very bad idea.
I am wondering, why does spring-data-elasticsearch design entity in this way? It's not really flexible. Or I misunderstand the usage?
Thanks for your help in advance! :)
You are right in that using this with fix Strings is not flexible.
But you can also use a SpEL Expression for the index name. And when using the ElasticsearchOperations methods and not the repository methods, you can pass in the name of the index, this is overriding the name defined in the #Documentannotation.
Say I have the base class:
struct Base
{
virtual void foo();
};
and the derived class is final struct A final : public Base. Does it make sense to make the member functions final as well? I've seen in several places e.g.
struct A final : public Base {
void foo() final;
}
I am not sure it provides any value in this case as if the class itself is final I guess all the member functions are final by default as well. Am I missing something? Are there any guidelines?
In case a struct or a class (A in your case) is final, you cannot declare another one inheriting it. Therefore, there's no need to also declare any methods as final.
Maybe this is a convention in some places to be clear that this method also cannot be overridden (just as a "reminder" for the final of the struct).
Here is my singlton class using enum:
public enum MyInstanceFactory {
INSTANCE;
private SOMEOBJECT;
private int countInitialization = 0;
private MyInstanceFactory(){
countInitialization++;
System.out.println("WOW!! This has been initialized: ["+countInitialization+"] times");
SOMEOBJECT = SOMETHING
}
public Session getSomeobject(){ return SOMEOBJECT; }
}
Now I am calling it like inside MVC controller
Session cqlSession = MyInstanceFactory.INSTANCE.getSomeobject();
In this way it calls constructer only first time and next and onwards it return the correct value for SOMEOBJECT.
My question is I want to do the same thing when a spring application start i.e. initializing contructor once and use **getSomeobject** multiple times.
I saw THIS SO ANSWER but here they are saying
If it finds a constructor with the right arguments, regardless of visibility, it will use reflection to set its constructor to be accessible.
Will reflection create problem for a singlton class?
If you need a non-subvertible singleton class (not just a singleton bean that's shared by many other beans, but actually a singleton class where the class can only ever be instantiated once), then the enum approach is a good one. Spring won't try to instantiate the enum itself, because that really makes no sense; that would be a much more extremely broken thing to do than merely calling a private constructor.
In that case, to refer to the enum instance from Spring configuration, you do the same thing as for any other static constant; for example:
<util:constant static-field="MyInstanceFactory.INSTANCE" />
As Bloch states in Item 3 ("Enforce the singleton property with a private constructor or an enum type") of Effective Java 2nd Edition, a single-element enum type is the best way to implement a singleton. Unfortunately the old private constructor pattern is still very widespread and entrenched, to the point that many developers don't understand what I'm doing when I create enum singletons.
A simple // Enum Singleton comment above the class declaration helps, but it still leaves open the possibility that another programmer could come along later and add a second constant to the enum, breaking the singleton property. For all the problems that the private constructor approach has, in my opinion it is somewhat more self-documenting than an enum singleton.
I think what I need is an annotation which both states that the enum type is a singleton and ensures at compile-time that only one constant is ever added to the enum. Something like this:
#EnumSingleton // Annotation complains if > 1 enum element on EnumSingleton
public enum EnumSingleton {
INSTANCE;
}
Has anyone run across such an annotation for standard Java in public libraries anywhere? Or is what I'm asking for impossible under Java's current annotation system?
UPDATE
One workaround I'm using, at least until I decide to actually bother with rolling my own annotations, is to put #SuppressWarnings("UnusedDeclaration") directly in front of the INSTANCE field. It does a decent job of making the code look distinct from a straightforward enum type.
You can use something like this -
public class SingletonClass {
private SingletonClass() {
// block external instantiation
}
public static enum SingletonFactory {
INSTANCE {
public SingletonClass getInstance() {
return instance;
}
};
private static SingletonClass instance = new SingletonClass();
private SingletonFactory() {
}
public abstract SingletonClass getInstance();
}
}
And you can access in some other class as -
SingletonClass.SingletonFactory.INSTANCE.getInstance();
I'm not aware of such an annotation in public java libraries, but you can define yourself such a compile time annotation to be used for your projects. Of course, you need to write an annotation processor for it and invoke somehow APT (with ant or maven) to check your #EnumSingleton annoted enums at compile time for the intended structure.
Here is a resource on how to write and use compile time annotations.