How can I remove a class method in its subclasses?
For example, class File allows lots of methods, including write(). I want to have a class StaticFile that inherits File, but does not provide a write() method. Note that, I want write() to be unavailable, and not just overridden by noop, or returning error.
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What is the difference between sealed and internal in Kotlin? I have read Kotlin's documentation on sealed classes and visibility modifiers; however, it is still not clear to me when to use sealed vs. internal. Maybe someone could provide real-world code samples?
Sealed classes | Kotlin & Visibility modifiers | Kotlin resources.
sealed class will be visible in all modules, but extendable only in the same module. This means if you have this:
sealed class MyClass {} then you can do this in the same module:
class MyExtensionClass: MyClass() {}
But you can't do the same thing in another module. But you can still use both MyClass and MyExtensionClass in another module.
For example you can do this in another module:
val x: MyClass = MyExtensionClass()
You can't instantiate a sealed class directly neither in the same or another module. This means you can't do this nowhere:
val x = MyClass()
So sealed class is basically an abstract class which can only be implemented in the same module.
internal class can be used and extended in the same module just like a sealed class, but you can do neither in another module. So you can't even use or instantiate it in another module. Also you can directly instantiate an internal class as long as you are doing it in the same module.
So: Use sealed to better control extending something. For example you create a library and you want a class from this library to be used but not extended. Use internal if you wan't your class to be invisible to other modules (you create a library, but certain class in this library shouldn't even be directly compile time usable by libraries users)
A good use case for sealed class:
You build a library and have some abstract class or interface which has multiple different implementations, but you want to make sure the libraries user doesn't add its own implementations (you wan't to be in control of implementation details).
A good use case for internal class:
You have some interface and a factory that creates implementations, but you don't want the implementing class to be compile-time visible to libraries users. They just use the factory and don't need to worry about the implementation. They might build their own implementation though and therefor not use the factory you provided and this is OK.
These are not mutually exclusive. You can have an internal sealed class as well.
internal is about visibility, and sealed is about inheritance rules.
internal means the class type is only visible within the module. In other modules, you can't even mention the name of the type.
sealed means it is open (can be subclassed), but subclasses (or implementations if it's a sealed interface) can only be defined in the same module, and the compiler keeps track of an exhaustive list of all subclasses. Another rule is that you can't create anonymous subclasses of it (object: MySealedClass). The advantage of a sealed type is that the compiler knows when you've exhaustively checked a type in when statements, if/else chains, etc. It can also be used in a library to ensure that only known implementations of a class or interface are ever passed to it (prevent users from creating subclasses of something and passing them into the library).
Bonus:
Visibility modifier keywords: public, internal, private, protected
Inheritance modifier keywords: open, final, sealed
data and value also cause a class to be final implicitly as a side effect.
I included a library I'd like to use, but in accessing to one of its classes I get the error message,
"Cannot access '<init>': it is private in [class name]
Is there something I can do to rectify this on my side, or am I just stuck to not use the package?
The error means the constructor is private. Given your comment, I'm assuming you're using a library. If this is the case, you'll have to find a different way to initialize it. Some libraries have factories or builders for classes, so look up any applicable documentation (if it is a library or framework). Others also use the singleton pattern, or other forms of initialization where you, the developer, don't use the constructor directly.
If, however, it is your code, remove private from the constructor(s). If it's internal and you're trying to access it outside the module, remove internal. Remember, the default accessibility is public. Alternatively, you can use the builder pattern, factory pattern, or anything similar yourself if you want to keep the constructor private or internal.
I came across this issue when trying to extend a sealed class in another file. Without seeing the library code it is hard to know if that is also what you are attempting to do.
The sealed classes have the following unique features:
A sealed class can have subclasses, but all of them must be declared in the same file as the sealed class itself.
A sealed class is abstract by itself, it cannot be instantiated directly and can have abstract members.
Sealed classes are not allowed to have non-private constructors (their constructors are private by default).
Classes that extend subclasses of a sealed class (indirect inheritors) can be placed anywhere, not necessarily in the same file.
For more info, have a read at https://www.ericdecanini.com/2019/10/14/kotlins-sealed-class-enums-on-steroids/
Hopefully, this will help others new to Kotlin who are also encountering this issue.
Class constructors are package-private by default. Just add the public keyword before declaring the constructor.
By default constructor is public so need to remove internal keyword.
Hi got a class MustInherit and many instance.
I need a function shared function declared once (normally in the template).
But as a mustInherit, I cant call it. I need to use one of my instance of the class.
Is there another way?
Public MustInherit MyBaseClass
...
public shared function UnknowBaseFunction () as object
....
x = InheritsClassByMyBaseClass.UnknowBaseFunction()
There is no way to inherit a shared member so that it also is available through the derived class name. Obviously, by removing the Shared keyword, it will be available in all derived types, but only via instantiated objects, not as a shared method. If it doesn't make sense to have everything call the method via the base class name, then I would recommend breaking it out into a separate class altogether which is neither in the base class nor the derived classes.
I was reading about private constructor and found a few points that I couldn't understand. It said, if you declare a constructor as private:
That class cannot be explicitly instantiated from another class
That class cannot be inherited
Should be used in classes containing only static utility methods
My first question: Point 2 says the class cannot be inherited. Well, if you declare a class private then it would still satisfy this property. Is it because, if a class is private, it can still be explicitly instantiated from outside by another class?
My second question: I don't understand point 3. If I have a helper class which is full of static methods, I would never have to instantiate that class to use the methods. So, what is the purpose of a constructor in that class which you are never going to instantiate?
Answer for Java
Question 1 You're confusing a private class, with a class that has a private constructor. Private constructors are used mainly for static classes that are not meant to be instatiated (i.e. they just have a bunch of static methods on them).
Question 2 Exactly there is no need for a constructor so you have to explicitly create a private constructor so that it does not get a default constructer that the JVM will provide if none is defined
An empty class with no methods defined will always be given a no argument constructor by the JVM by default
I take java and c++ as an examples (not the best OO languages known, but very popular) - since you are not defining which languge do you mean.
Ad.2. In these languages you must either call superclass constructor explicitly or it is implicitly called for you. From a subclass you cannot call private methods (only public and protected) - this rule applies to constructors as well. This means if the class has only private constructors, there is no way to call one in subclass constructor. So you cannot subclass such class.
Ad. 3. It is just to avoid confusion - since this class is only a container for utility methods, there is no point in instantiating it. This way you can enforce this rule at compile time.
I'm looking for a way to get a static methods list for a certain class. I only get a list of instance methods with the runtime function class_copyMethodList().
Is there a way to list static methods?
Each Class is itself an Objective-C object, and in turn has an object which is (sort of) its class. You need to get this metaclass object (see also: "[objc explain]: Classes and Metaclasses"), and then ask that for its methods (which will be the class methods* you are after).
From the class_copyMethodList docs:
###Discussion
To get the class methods of a class, use class_copyMethodList(object_getClass(cls), &count)
*There's no such thing as static methods in Obj-C.