static keyword
Sometimes you want to define a member of a class that will be used independently of all other objects of that class. As a rule, access to a class member is organized through an object of this class, but at the same time, it is possible to create a class member for independent use without reference to a specific instance of the object. In order to create such a class member, it is enough to indicate at the very beginning of its declaration keyword static.
If a class member is declared static, then it becomes available before any objects of its class are created and without reference to any object. The static keyword can be used to declare both variables and methods. The most common example of a static member is the Main() method, which is declared as such because it must be called. operating system at the very beginning of the running program.
To use a member of type static outside the class, it is enough to specify the name of this class with the dot operator. But you don't need to create an object for this. In fact, a static member is not accessible by reference to an object, but by the name of its class.
Variables declared as static are essentially global. When objects are declared in their class, then a copy type variable static is not created. Instead, all instances of the class share the same variable of type static. Such a variable is initialized before it is used in the class.
An example of using the static keyword:
Using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace ConsoleApplication1 ( class myCircle ( // 2 methods that return the area and length of a circle public static double SqrCircle(int radius) ( return Math.PI * radius * radius; ) public static double LongCircle(int radius) ( return 2 * Math.PI * radius; ) ) class Program ( static void Main(string args) ( int r = 10; // Call methods from another class // without instantiating an object of this class Console.WriteLine("Area of a circle with radius (0) = (1 :#.##)",r,myCircle.SqrCircle(r)); Console.WriteLine("The length of the circle is (0:#.##)",myCircle.LongCircle(r)); Console.ReadLine(); ) ) )
There are a number of restrictions on the use of static type methods:
A method of type static must not have a this reference, since such a method is not executed on any object
In a method of type static, only other methods of type static are allowed to be called directly, not an instance method from the same class. The point is that instance methods operate on specific objects, and a static type method is not called on an object. Therefore, such a method has no objects that it could operate on.
Similar restrictions apply to data of type static. For a method of type static, only other data of type static defined in its class is directly accessible. In particular, he cannot operate on an instance variable of his class, since he does not have objects with which he could operate.
Static constructors
A constructor can also be declared as static. A static constructor is typically used to initialize components that apply to the entire class, not to a single instance of an object of that class. Therefore, class members are initialized with a static constructor before any objects of that class are created:
Using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace ConsoleApplication1 ( class MyClass ( public static int a; public int b; // Static constructor static MyClass() ( a = 10; ) // Regular constructor public MyClass() ( b = 12; ) ) class Program ( static void Main (string args) ( Console.WriteLine("Access an instance of class a: " + MyClass.a); MyClass obj = new MyClass(); Console.WriteLine("Access an instance of class b: " + obj.b); Console .ReadLine(); ) ) )
Note that the static type constructor is called automatically when the class is first loaded, and before the instance constructor. From this we can draw a more general conclusion: a static constructor must be executed before any instance constructor. Moreover, static constructors do not have access modifiers - they have default access and therefore cannot be called from the program.
Static classes
A class can be declared as static. A static class has two main properties. First, create static class objects it is forbidden. And secondly, a static class should contain only static members. A static class is created with the following class declaration form, modified with the static keyword.
static class class name ( // ...
Static classes are mainly used in two cases. First, a static class is required when creating extension method. Extension methods are mostly associated with the LINQ language. And secondly, a static class serves to store a collection of static methods related to each other.
In this article, we will explore static methods in Java and compare Static and Instance. The main thing to remember is that if you use the static keyword with any method, it is called a static method.
What are static methods in Java?
Static Methods are methods in Java that can be called without creating an object of the class. They are documented by the name (class the category).
The static keyword can be used with class, variable, method, and block. Static Members belong to the class and not to a specific instance, which means that if you make a member static, you can access it without an object. Let's take an example to understand this:
Here we have a static method myMethod(), we can call this method without any object because when we make a member static it becomes class level. If we remove the static keyword and make it non-static, we will need to create a class object to call it.
Static members are common to all instances (objects) of a class, but non-static members are separate to each class instance.
Class SimpleStaticExample ( // This is a static method static void myMethod() ( System.out.println("myMethod"); ) public static void main(String args) ( /* You can see that we are calling this * method without creating any object. */ myMethod(); ) )
Syntax
public static void geek(String name) ( // code to be executed....It is stored in the Permanent Generation because it is associated with (class the category) where they are, and not with objects of that class. However, their local variables, as well as the argument(s) passed to them, are on the stack.
Important points:
- A static method(s) associated with the class they are in, i.e. they will refer to it even if it does not instantiate the class, i.e. ClassName.methodName(args).
- They are intended for sharing all objects created from the same class.
- Static methods cannot be overridden.
An example of using static methods in Java:
Import java.io.*; class Flair( public static String FlairName = ""; public static void geek(String name) ( FlairName = name; ) ) class GFG ( public static void main (String args) ( Flair.flair("vaibhav"); System.out .println(Flair.flairName); Flair obj = new Flair(); obj.flair("shadow"); System.out.println(obj.flairName); ) )
Conclusion:
vaibhav
shadow
Static variables (static) and their values (primitives or references) are defined inside the class and stored in the PermGen memory space.
What if a static variable refers to an object?
static int i = 1;
static Object obj = new Object();
The first line is the value that will be stored in the PermGen section. In the second line, the obj reference will be stored in the PermGen section, and the object it refers to will be stored in the heap section.
When are they used?
- If you have code that can be shared by all instances of the same class, put that piece of code in a Static method.
- First of all, set up static access fields for the class.
What is a Java Instance Method?
Java Instance Method are ways in which an object of a class can be created before it is called. To call an instance method, we must make an object from the category in which it is defined.
Public void flair(String name) // code to be (executed.... ) // return type can be int, float String or user defined data type.
Parameters (the arguments passed to them), as well as their local variables and return value, are allocated on the stack.
Important points:
- Instances belong to the class object, not the class, i.e. they will still be referenced as they were after the class object was created.
- Each separate object A created from (class, category) has its own copy of that class's instance method(s).
- They can be overridden.
Instance method or static method in Java?
- The instance method will have direct access to instance methods and variables.
- The instance method will access static variables and static methods directly.
- Static methods will access static variables and methods directly.
- Static methods cannot access instance methods and instance variables directly. And a static method cannot use this since there is no instance for "this" to refer to.
Then we can access them directly through the class name and scope resolution operator. But what if static member variables are private? Consider the following code:
In this case, we can't directly access Anything::s_value from main() because that member is private. Usually, private members of a class are accessed through public methods. While we could create a regular method to access s_value , we would then have to create an object of that class to use the method! There is a better option: we can make the method static.
Like static member variables, static methods not tied to any one class object. Here is the example above, but with a static method:
class Anything ( private: static int s_value; public: static int getValue() ( return s_value; ) // static method ); int Anything::s_value = 3; // definition of a static member variable int main() ( std::cout<< Anything::getValue() << "\n"; }
Because static methods are not bound to a specific object, they can be called directly via the class name and scope resolution operator, or via class objects (but this is not recommended).
Static methods do not have a *this pointer
Static methods have two interesting features. First, because static methods are not bound to an object, they don't have ! This makes sense because the *this pointer always points to the object the method is working on. Static methods may not work through an object, so the *this pointer is not needed.
Second, static methods can directly access other static members (variables or functions), but they cannot access non-static members. This is because non-static members belong to the class object, but static methods do not!
One more example
Static methods can be defined outside the class body. This works the same way as with normal methods. For example:
#include
#include classIDGenerator private : static int s_nextID ; // declaration of a static member variable public : static int getNextID(); // static method declaration // Start generating ID from 1 int IDGenerator :: s_nextID = 1 ; int IDGenerator :: getNextID () ( return s_nextID ++ ; ) int main() for (int count = 0 ; count< 4 ; ++ count ) std::cout<< "The next ID is: " << IDGenerator :: getNextID () << "\n" ; return 0 ; |
The result of running the program above is:
The next ID is: 1
The next ID is: 2
The next ID is: 3
The next ID is: 4
Note that since all variables and functions of this class are static, we don't need to create an object of this class to work with it! A static member variable is used to store the value of the next identifier to be assigned to it, and a static method is used to return the identifier and increment it.
Warning about classes with all static members
Be careful when writing classes with all static members. While such "purely static classes" can be useful, they also have their drawbacks.
First, since all static members are created only once, there cannot be multiple copies of a “purely static class” (without cloning the class and renaming it later). For example, if we need two independent objects of the IDGenerator class, then this will not be possible through a "purely static" class.
C++ does not support static constructors
If you can initialize an ordinary member variable via , then logically you should be able to initialize static member variables via a static constructor. And while some modern languages do support static constructors for exactly this purpose, C++ is unfortunately not one of them.
If your static variable can be initialized directly, then no constructor is needed: you can define a static member variable even if it is private. We do this in the example above with s_nextID . Here is another example:
class Something ( public: static std::vector
classSomething public : static std :: vector< char >s_mychars ; std::vector< char >Something :: s_mychars = ( "o" , "a" , "u" , "i" , "e" ) ; // define a static member variable |
If initializing your static member variable requires executing code (such as a loop), then there are several different ways to do it. The following way is the best one:
#include
#include #include classSomething private : static std :: vector< char >s_mychars ; public : class_nested // define a nested class named _nested public : nested() // _nested constructor initializes our static member variable s_mychars . push_back("o") ; s_mychars . push_back("a") ; s_mychars . push_back("u") ; s_mychars . push_back("i") ; s_mychars . push_back("e") ; |
Most C++ keywords let you do one thing. You use int to declare an integer variable, or when a function returns an integer value, or takes an integer as an argument. You use the new operator to allocate memory, and the delete operator to free it. You can use const to indicate that the value of a variable cannot be changed. Ironically, the static keyword, although it means "unchanging", has several (and apparently unrelated) uses. The static keyword can be used in three main contexts:
- inside a function;
- inside the class definition;
- in front of a global variable within a file constituting a multi-file program.
Using static inside a function is the easiest. It simply means that after a variable has been initialized, it remains in memory until the end of the program. You can think of it as a variable that holds its value until the program ends. For example, you can use a static variable to record the number of times a function has been called by simply adding the lines static int count = 0; and count++; into a function. Since count is a static variable, the line static int count = 0; will be executed only once. Whenever the function is called, count will have the last value given to it.
You can also use static in this way to prevent the variable from being reinitialized inside the loop. For example, in the following code, the variable number_of_times will be equal to 100, even though the line static int number_of_times = 0; is inside the loop, where it is supposed to be executed every time the program reaches the loop. The trick is that the static keyword prevents the variable from being reinitialized. One of the things about using the static keyword is that it automatically sets the variable to zero for you - but don't rely on it (it makes your intentions unclear).
For(int ix=0; ix< 10; ix++) { for(int iy = 0; iy < 10; iy++) { static int number_of_times = 0; number_of_times++; } }
You can use static variables to store information about the last value of a returned function, for example if you want to store the maximum value calculated by a function. If you are parsing a string, you can also store the last character returned by the function so that you can call it with an argument indicating that it should return the last character.
The second use of static is within a class definition. Although most variables declared within a class may have a different value in each instance of the class, the static fields of a class will have the same value for all instances of that class, and it is not even necessary to create an instance of that class. It's useful to think of static class variables as holding the information needed to create new objects (for example, in a class factory). For example, if you want to number instances of a class, you can use a static variable to keep track of the last number used. It's important to note that it's good practice when using static class variables to use class_name::x; , not instance_of_class.x; . This helps remind the programmer that static variables do not belong to the same class instance, and that you do not have to create an instance of that class. As you've probably noticed by now, you can use the scope operator, :: , to access static when you access it through the class name.
It's important to keep in mind, when debugging or implementing a program using static , that you can't initialize it inside a class. In fact, if you decide to write all the class code in a header file, you won't even be able to initialize a static variable inside the header file; do it in .cpp file. Also, you need to initialize the static members of the class or they won't be in scope. (The syntax is a bit odd: type class_name::static_variable = value .)
You can also have static class functions. Static functions are functions that do not require an instance of a class and are called in the same way, by analogy with static variables, with the name of the class, and not with the name of the object. For example, a_class::static_function(); , not an_instance.function(); . Static functions can only operate on static members of a class, as they do not refer to specific instances of the class. Static functions can be used to change static variables, keep track of their values - for example, you can use a static function if you choose to use a counter to give each class instance a unique identifier.
For example, you can use the following code:
Class user ( private: int id; static int next_id; public: static int next_user_id() ( next_id++; return next_id; ) // other methods for the user class user() // class constructor ( id = user::next_id++; // or method call, id = user.next_user_id(); ) ); int user::next_id = 0;
Note that you must include the type of the static variable when you set it!
user a_user;
will set the ID to the next ID number not used by any other user object. Note that it is good style to declare an identifier as a constant.
The last use of static is a global variable in a code file. In this case, the use of static indicates that source code in other files that are part of the project cannot access the variable. Only code within the same file can see the variable (its scope is limited to the file). This technique can be used to model object-oriented code because it limits the visibility of variables and thus helps avoid name collisions. This way of using static is a holdover from C.