static Keyword in Java

02 Dec, 2021
The static keyword in Java is mainly used for memory management. The static keyword in Java is used to share the same variable or method of a given class. The users can apply static keywords with variables, methods, blocks, and nested classes. The static keyword belongs to the class than an instance of the class. The static keyword is used for a constant variable or a method that is the same for every instance of a class.
The static keyword is a non-access modifier in Java that is applicable for the following: 
BlocksVariablesMethodsClasses
Blocks
Variables
Methods
Classes
Note: To create a static member(block, variable, method, nested class), precede its declaration with the keyword static. 
When a member is declared static, it can be accessed before any objects of its class are created, and without reference to any object. For example, in the below java program, we are accessing static method m1() without creating any object of the Test class. 
Java
// Java program to demonstrate that a static member// can be accessed before instantiating a class  class Test{    // static method    static void m1()    {        System.out.println("from m1");    }      public static void main(String[] args)    {          // calling m1 without creating          // any object of class Test           m1();    }}
from m1
If you need to do the computation in order to initialize your static variables, you can declare a static block that gets executed exactly once, when the class is first loaded. 
Consider the following java program demonstrating the use of static blocks. 
Java
// Java program to demonstrate use of static blocks  class Test{    // static variable    static int a = 10;    static int b;          // static block    static {        System.out.println("Static block initialized.");        b = a * 4;    }      public static void main(String[] args)    {       System.out.println("from main");       System.out.println("Value of a : "+a);       System.out.println("Value of b : "+b);    }}
Static block initialized.
from main
Value of a : 10
Value of b : 40
For a detailed article on static blocks, see static blocks
When a variable is declared as static, then a single copy of the variable is created and shared among all objects at the class level. Static variables are, essentially, global variables. All instances of the class share the same static variable.
Important points for static variables:
We can create static variables at the class level only. See here
static block and static variables are executed in the order they are present in a program.
Below is the Java program to demonstrate that static block and static variables are executed in the order they are present in a program. 
Java
// Java program to demonstrate execution// of static blocks and variables  class Test{    // static variable    static int a = m1();          // static block    static {        System.out.println("Inside static block");    }          // static method    static int m1() {        System.out.println("from m1");        return 20;    }          // static method(main !!)    public static void main(String[] args)    {       System.out.println("Value of a : "+a);       System.out.println("from main");    }}
from m1
Inside static block
Value of a : 20
from main
When a method is declared with the static keyword, it is known as the static method. The most common example of a static method is the main( ) method. As discussed above, Any static member can be accessed before any objects of its class are created, and without reference to any object. Methods declared as static have several restrictions: 
They can only directly call other static methods.
They can only directly access static data.
They cannot refer to this or super in any way.
Below is the java program to demonstrate restrictions on static methods.
Java
// Java program to demonstrate restriction on static methods  class Test{    // static variable    static int a = 10;          // instance variable    int b = 20;          // static method    static void m1()    {        a = 20;        System.out.println("from m1");                   // Cannot make a static reference to the non-static field b         b = 10; // compilation error                           // Cannot make a static reference to the                  // non-static method m2() from the type Test         m2();  // compilation error                    //  Cannot use super in a static context         System.out.println(super.a); // compiler error     }          // instance method    void m2()    {            System.out.println("from m2");    }                      public static void main(String[] args)    {        // main method     }}
Output:
prog.java:18: error: non-static variable b cannot be referenced from a static context
         b = 10; // compilation error
         ^
prog.java:22: error: non-static method m2() cannot be referenced from a static context
         m2();  // compilation error
         ^
prog.java:25: error: non-static variable super cannot be referenced from a static context
         System.out.println(super.a); // compiler error 
                            ^
prog.java:25: error: cannot find symbol
         System.out.println(super.a); // compiler error 
                                 ^
  symbol: variable a
4 errors
Use the static variable for the property that is common to all objects. For example, in class Student, all students share the same college name. Use static methods for changing static variables.
Consider the following java program, that illustrates the use of static keywords with variables and methods.
Java
// A java program to demonstrate use of// static keyword with methods and variables  // Student classclass Student {    String name;    int rollNo;      // static variable    static String cllgName;      // static counter to set unique roll no    static int counter = 0;      public Student(String name)    {        this.name = name;          this.rollNo = setRollNo();    }      // getting unique rollNo    // through static variable(counter)    static int setRollNo()    {        counter++;        return counter;    }      // static method    static void setCllg(String name) { cllgName = name; }      // instance method    void getStudentInfo()    {        System.out.println("name : " + this.name);        System.out.println("rollNo : " + this.rollNo);          // accessing static variable        System.out.println("cllgName : " + cllgName);    }}  // Driver classpublic class StaticDemo {    public static void main(String[] args)    {        // calling static method        // without instantiating Student class        Student.setCllg("XYZ");          Student s1 = new Student("Alice");        Student s2 = new Student("Bob");          s1.getStudentInfo();        s2.getStudentInfo();    }}
name : Alice
rollNo : 1
cllgName : XYZ
name : Bob
rollNo : 2
cllgName : XYZ
A class can be made static only if it is a nested class. We cannot declare a top-level class with a static modifier but can declare nested classes as static. Such types of classes are called Nested static classes. Nested static class doesn’t need a reference of Outer class. In this case, a static class cannot access non-static members of the Outer class. 
Note: For static nested class, see a static nested class in java
Implementation:
Java
// A java program to demonstrate use// of static keyword with Classes  import java.io.*;  public class GFG {      private static String str = "GeeksforGeeks";      // Static class    static class MyNestedClass {                // non-static method        public void disp(){           System.out.println(str);         }    }        public static void main(String args[])    {        GFG.MyNestedClass obj            = new GFG.MyNestedClass();        obj.disp();    }}
GeeksforGeeks
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