Say in my vb.net program I already have too many classes and module.
I want to normalize things a bit. So I want to create a new class called FolderStats and I want folderstats to be accessible only by another class folderStatsuser.
I do not want any other class to know that FolderStats even exist
What should I do?
If only FolderStatsUser should have any knowledge of it, then it is an implementation detail of FolderStatsUser. Having a private class might be useful
public class FolderStatsUser {
private class FolderStats {}
}
#Jim - here is the vb.net version...
Public Class FolderStatsUser
Private Class FolderStats
End Class
End Class
You can read more about nested classes at https://msdn.microsoft.com/en-us/library/twwxww86(v=vs.71).aspx
You could use an assembly and use the Friend access modifier so that only classes in the assembly can access your class. Classes that shouldn't access it would need to be in another assembly.
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 just wonder if I want to make my code more testable, should I use private variables?
If not, when should I use private variables?
Private variables should just be used for internal storage for your class not testing directly against. Set up your class how it is designed to be accessed with the proper properties and methods, then write your test to access your properties and methods just as any other part of your code would.
I have concept of static variables but what are the benefits of static methods in a class. I have worked on some projects but I did not make a method static. Whenever I need to call a method of a class, I create an object of that class and call the desired method.
Q: Static variable in a method holds it's value even when method is executed but accessible only in its containing method but what is the best definition of static method?
Q: Is calling the static method without creating object of that class is the only benefit of static method?
Q: What is the accessible range for static method?
Thanks
Your description of a static variable is more fitting to that found in C. The concept of a static variable in Object Oriented terms is conceptually different. I'm drawing from Java experience here. Static methods and fields are useful when they conceptually don't belong to an instance of something.
Consider a Math class that contains some common values like Pi or e, and some useful functions like sin and cos. It really does not make sense to create separate instances to use this kind of functionality, thus they are better as statics:
// This makes little sense
Math m = new Math();
float answer = m.sin(45);
// This would make more sense
float answer = Math.sin(45);
In OO languages (again, from a Java perspective) functions, or better known as methods, cannot have static local variables. Only classes can have static members, which as I've said, resemble little compared to the idea of static in C.
Static methods don't pass a "this" pointer to an object, so they can't reference non-static variables or methods, but may consequently be more efficient at runtime (fewer parameters and no overhead to create and destroy an object).
They can be used to group cohesive methods into a single class, or to act upon objects of their class, such as in the factory pattern.
Syntax (php) for static methods:
<?php
class Number {
public static function multiply($a, $b) {
return $a * $b;
}
}
?>
Client code:
echo Number::multiply(1, 2);
Which makes more sense than:
$number = new Number();
echo $number->multiply(1, 2);
As the multiply() method does not use any class variables and as such does not require an instance of Number.
Essentially, static methods let you write procedural code in an object oriented language. It lets you call methods without having to create an object first.
The only time you want to use a static method in a class is when a given method does not require an instance of a class to be created. This could be when trying to return a shared data source (eg a Singleton) or performing an operation that doesn't modify the internal state of the object (String.format for example).
This wikipedia entry explains static methods pretty well: http://en.wikipedia.org/wiki/Method_(computer_science)#Static_methods
Static variables and static methods are bound to the class, and not an instance of the class.
Static methods should not contain a "state". Anything related to a state, should be bound to an instantiated object, and not the class.
One common usage of static methods is in the named constructor idiom. See: http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.8.
Static Methods in PHP:
Can be called without creating a class object.
Can only call on static methods and function.
Static variable is used when you want to share some info between different objects of the class.As variable is shared each object can update it and the updated value be available for all other objects as well.
As static variable can be shared,these are often called as class variable.
static elements are accessible from any context (i.e. anywhere in your script), so you can access these methods without needing to pass an instance of the class from object to object.
Static elements are available in every instance of a class, so you can set values that you want to be available to all members of a type.
for further reading a link!