Java

Static Keyword in Java

Static is a keyword used for memory management.
Static means single copy storage for variable or method.
Static keyword can be applied to variables, methods, inner class and blocks.
Static members belongs to class rather than instance of class.

Static in java is a keyword that indicates that the variables or functions are shared between all the instances of a particular class since it belongs to the type, not the object. It is used when the programmer wants to share the same variable or method of a class.

The variable preceded by ‘static’ keyword is ‘static variable’

static int a=10;   // variable
static void m1(){   // method 
  }

Static variable is used to refer common property of all objects of class.

How to access static variable?
There are two ways to access static variable:
1. Static variable can be accessed by Class name
  A. a ; [A is class name]
  Where A is the class name and ‘a’ is a static variable declared in that class
2. Static variable can be accessed by object
  I have a class name called ‘Sample’. Now, we can create the object of the Sample class
```
Sample h=new Sample ();
System.out.println(h.a); //’a’ is static variable inside ‘sample’ class
                    
```
How can I access static variable in two ways?

package com.javabykiran.Static;
    /*
    * @author Java By Kiran
    */
    public class Staticvar 
    { 
        static int i=10;
        public static void main(String[] args)
        { 
           Staticvar s =new Staticvar(); 
           System.out.println(s.i); //Not Recommended
           System.out.println(Staticvar.i); //Recommended
        }
    }


    Output:
     10 
     10

In the above program, we printed the value of variable by using object name and by using class name
Static variable gets loaded into the memory at the time of class loading
So, we can access static variable by reference variables as well.
In the above program, if we create only the reference of class like
```
Staticvar s1=null;
System.out.println(s1.i); // possible
System.out.println(Staticvar.i); //possible
```
The above example compiles and executes successfully because the static variable get loaded into the memory at the time of class loading.
Static variable and method doesn't belong to Object/Instance of the class since static variables are shared across all the instances of Object

Example 1

    package com.jbk;
/*
* @author Java By Kiran
*/
public class StaticVar_Demo{ 
  int a =10;
  static int b =10;
public static void main(String[]args){ 
    StaticVar_Demo st= new StaticVar_Demo();     
    System.out.println(st.a); 
    System.out.println(st.b);
    StaticVar_Demo st1 = new StaticVar_Demo(); 
    int x = st1.a++;
    System.out.println(x); 
    int y = st1.b++; 
    System.out.println(y);
    StaticVar_Demo st2 = new StaticVar_Demo(); 
    int p = st2.a++;
    System.out.println(p); 
    int q = st2.b++; 
    System.out.println(q);
    StaticVar_Demo st3 = new StaticVar_Demo(); 
    int c = st3.a++;
    System.out.println(c); 
    int d = st3.b++; 
    System.out.println(d);
  }
}

Example 2

    package com.javabykiran.Static;
/*
* @author Java By Kiran
*/
public class Staticvar { 
    static int i=10;
    int b=20; 
  void m1(){
    System.out.println(i);
   }
    static void m2(){ 
        System.out.println(i);
   }
   public static void main(String[] args) { 
       Staticvar s =new Staticvar();
       s.m1();
       Staticvar s1=new Staticvar();
       s1.m2();
    }
 }


    Output:
     10
     10

Local variables cannot be declared as static else the compiler displays a modifier error [at compile time]
We cannot call non-static members from static members because static variables get stored into memory before object creation and non-static member get stored into memory after object creation
So, when we access a non-static member through a static member, it leads to a compile time error as they are not present in the memory.

    package com.javabykiran.Static;
/*
* @author Java By Kiran
*/
public class Staticvar {
    static int i = 10;  // static variable 
    int b = 20;         // non static variable 
  void display() {      // non static method
     static int a = 50;  //here a compile time error shows
     System.out.println(i);
  }
  static void show() {      // static method 
    System.out.println(i); // OK
    System.out.println(b);
    //if I want access non static variable in static method,
    //compiler throw error
 }
public static void main(String[] args) { 
    Staticvar s1 = new Staticvar(); 
 
    s1.show();
    s1.display();
  }
}

If you apply static keyword with any method, it will be treated as static method
Static method belongs to a class rather than object of a class
Static method can be invoked without creating instance of the class
Static method can access static data member and can change its value
Static methods also load into memory before object creation

package com.javabykiran.Static;
/*
* @author Java By Kiran
*/
 public class Staticvar {
    static int i=10; 
    int b=20;	 
   void display(){		
     System.out.println(i);
   }
   static void show(){  //static method 
    System.out.println(i); 
    System.out.println(b);//if we want access non static
    //variable in static method, compiler throw error
   }
 public static void main(String[] args){
    Staticvar s1=new Staticvar(); 
    s1.show();
    s1.display();
  }
}

Static method can be accessed by nullable reference like
```
Staticvar s1=null;
s1.show();
```
By using non-static member we can access static members, but by using static member we cannot access non-static members

Example

package com.javabykiran.Static;
/*
* @author Java By Kiran
*/
 public class StaticMethod {
    static int i=10; //static variable 
    int b=20;       //global variable 
void display() 
{  
  System.out.println("This is display method");
}
static void show() { //static method 
  System.out.println("Hello,This is JAVA BY KIRAN classes");
  }
public static void main(String[] args) 
 { 
  StaticMethod s =new StaticMethod(); 
    s.display();
    s.show();
    StaticMethod s1 = null; 
    s1.show();
 } 
}


    Output:
     This is display method
     Hello, This is JAVA BY KIRAN classes
     Hello, This is JAVA BY KIRAN classes

Java's static block is the group of statements that gets executed when the class is loaded into memory by Java ClassLoader.
It is used to initialize static variables of the class. Mostly it's used to create static resources when the class is loaded
We can't access non-static variables in static block
We can have multiple static blocks in a class, although it doesn't make much sense
Static block code is executed only once when the class is loaded into memory
Static block always get executed first because they get stored into the memory at the time of class loading or before object creation

Example

   package Static;
public class StaticBolckLab2 { 
   StaticBolckLab2()
   {
    System.out.println("This is constructor");
   }
   {
    System.out.println("This is Non static Block");
   }
   static
   {
    System.out.println("This is static Block");
   }
public static void main(String[] args) { 
   StaticBolckLab2 s= new StaticBolckLab2(); 
   StaticBolckLab2 s1=new StaticBolckLab2();
 }


    Output:
     This is static Block
     This is Non staticBlock
     This is constructor
     This is Non static Block
     This is constructor

In the above example, we can prove that static members have one copy storage
A static block cannot access non-static variables and methods

Example

   package com.javabykiran.Static;
/*
* @author Java By Kiran
*/
public class StaticBlock {
    static int i=10;  //static variable 
    int b=20;	      //global variable
    void display(){	
        System.out.println("This is display method");
    }
    static void show(){  //static method       
    System.out.println("JAVA BY KIRAN classes Pune");
    //System.out.println(b);   //if we want access variable ‘b’,non-static method is needed
    //if there is a variable in the static method, JVM throws a compile time error
   }
    {
        display();
    }
    static{
        display();   //this is a compile time error, we cannot call non-static methods
		System.out.println(i); // no error
        System.out.println(" "+b);  //this compile time error shows we cannot access non-static //variable in static block
      }
public static void main(String[] args) { 
    StaticBlock s =new StaticBlock(); 
    s.display();
    s.show();
    StaticBlock s1 = null; 
    s1.show();
   } 
 }

Outer class cannot be declared as static
But inner classes can be used as static

     package com.javabykiran.Static;
/*
* @author Java By Kiran
*/
public class StaticInnerclass { 
   static class A {  //start of static inner class
   static int a=10; 
   int b=20;
   void display(){ 
     System.out.println("This is local method");
   }
   static void show(){ 
       int c=63;
       System.out.println("This is static method");
   }
}
public static void main(String[] args){
     A a =new A(); 
     System.out.println(a.b); 
     System.out.println(a.a); 
     a.display();
     a.show();
  } 
}


    Output:
     20      
     10       
     This is local method 
     This is static method

Inner class can access their local variables and methods

Example

   package com.javabykiran.Static;
/*
* @author Java By Kiran
*/
class StaticClassDemo { 
    static class JBKway{  //start of inner class 
     static int a=10; 
     int b=20;
     void display(){
        System.out.println("This is local method");
    }
    static void show(){ 
        int c=63;
        System.out.println("This is static method");
        System.out.println("C="+c);
    }	
}
public static void main(String[]args) {
   JBKway a1 =null;
   //System.out.println(a1.b);//runtime error
   System.out.println(a1.a);
   // a1.display(); //runtime error 
      a1.show();
  System.out.println(a1.a);
  }
}


    Output:
     10
     This is static method 
     C=63
     10

Local variable cannot be static
Constructor cannot be static
Outer class cannot be static

Java

Static Keyword in Java

Things to remember:

Static Variable

How to access static variable?

How can I access static variable in two ways?

Static Method

Static Block

Static Inner Class

Note: