Java static keyword

All about "static"
The static keyword can be used in 3 scenarios
1) static variables2) static methods3) static blocks of code.
Lets look at static variables and static methods first.
Contents
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1static variable
2static method
3Assignment
4static block
static variable

It is a variable which belongs to the class and not to object(instance)

Static variables are initialized only once , at the start of the execution . These variables will be initialized first, before the initialization of any instance variables

A single copy to be shared by all instances of the class

A static variable can be accessed directly by the class name and doesn’t need any object

Syntax : .

static method
It is a method which belongs to the class and not to the object(instance)

A static method can access only static data. It can not access non-static data (instance variables)

A static method can call only other static methods and can not call a non-static method from it.

A static method can be accessed directly by the class name and doesn’t need any object

Syntax : .

A static method cannot refer to "this" or "super" keywords in anyway

Important Note
main method is static , since it must be be accessible for an application to run , before any instantiation takes place.
Lets learn the nuances of the static keywords by doing some excercises!
Assignment
The goal of this assignment is to learn working of static variables & method
Step 1) Copy the following code into a editor

class Student {

int a; //initialized to zero

static int b; //initialized to zero only when class is loaded not for each object created.

  Student(){

   //Constructor incrementing static variable b

   b++;

  }

   public void showData(){

      System.out.println("Value of a = "+a);

      System.out.println("Value of b = "+b);

   }

//public static void increment(){

//a++;

//}

}

class Demo{

   public static void main(String args[]){

     Student s1 = new Student();

     s1.showData();

     Student s2 = new Student();

     s2.showData();

     //Student.b++;

     //s1.showData();

  }

}

Step 2) Save & Compile the code. Run the code as, java Demo.
Step 3) Expected output show below
Following diagram shows , how reference variables & objects are created and static variables are accessed by the different instances.
Step 4) It is possible to access a static variable from outside the class using the syntax ClassName.Variable_Name. Uncomment line # 27 & 28 . Save , Compile & Run . Observe the output.
Step 5) Uncomment line 15,16 & 17 . Save , Compile & Run.
Step 5) Error = ? This is because it is not possible to access instance variable "a" from static method "increment".
static block
The static block, is a block of statement inside a Java class that will be executed when a class is first loaded in to the JVM

class Test{

  static {

     //Code goes here

  }

}

A static block helps to initialize the static data members, just like constructors help to initialize instance members
전재 하 다http://www.javatutorialhub.com/wiki/All_about_%22static%22
Question：
Quick question -
When are static fields initialized? If I never instantiate a class, but I access a static field, are ALL the static blocks and private static methods used to instantiate private static fields called (in order) at that instant?
What if I call a static method? Does it also run all the static blocks? Before the method?
Answer：
A classes static initialization normally happens immediately before the first time one of the following events occurs:

an instance of the class is created,

a static method of the class is invoked,

a static field of the class is assigned,

a non-constant static field is used, or

for a top-level class, an assert statement lexically nested within the class is executed.

See JLS 12.4.1. It is also possible to force a class to initialize (if it hasn't already initialized) by using Class.forName(String, boolean, ClassLoader)
전재 하 다http://stackoverflow.com/questions/3499214/java-static-class-initialization