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Java static keyword

The static keyword in java is used for memory management mainly. We can apply java static keyword with variables, methods, blocks and nested class. The static keyword belongs to the class than instance of the class.

The static can be:

  1. variable (also known as class variable)
  2. method (also known as class method)
  3. block
  4. nested class

1) Java static variable

If you declare any variable as static, it is known static variable.

  1. The static variable can be used to refer the common property of all objects (that is not unique for each object) e.g. company name of employees,college name of students etc.
  2. The static variable gets memory only once in class area at the time of class loading.

Advantage of static variable

It makes your program memory efficient (i.e it saves memory).

Understanding problem without static variable

class Java64bit{
int emp_id;
String emp_name;
Float salary;
String department="IT";
}


Suppose there are 200 employee in my company, now all instance data members will get memory each time when object is created.All employee have its unique emp_id and emp_name so instance data member is good.Here, company refers to the common property of all objects.If we make it static,this field will get memory only once.


Java static property is shared to all objects.

Program of counter without static variable

In this example, we have created an instance variable named count which is incremented in the constructor. Since instance variable gets the memory at the time of object creation, each object will have the copy of the instance variable, if it is incremented, it won't reflect to other objects. So each objects will have the value 1 in the count variable.

package com.ashutosh;
public class StaticJava64bit {
int count=1; // get memory each time when the instance is created
StaticJava64bit(){
count++; //incrementing value
System.out.println(count);
}
public static void main(String args[]){
//Creating objects
StaticJava64bit sj=new StaticJava64bit();
StaticJava64bit sj1=new StaticJava64bit();
StaticJava64bit sj2=new StaticJava64bit();
StaticJava64bit sj3=new StaticJava64bit();
}
}

Output: 2 2 2 2



Program of counter static variable

As we have mentioned above, static variable will get the memory only once, if any object changes the value of the static variable, it will retain its value.

package com.ashutosh;
class StaticJava64bit{
static int count=0;//will get memory only once and retain its value
StaticJava64bit(){
count++;//incrementing the value of static variable
System.out.println(count);
}
public static void main(String args[]){
//creating objects
StaticJava64bit sj=new StaticJava64bit();
StaticJava64bit sj1=new StaticJava64bit();
StaticJava64bit sj2=new StaticJava64bit();
StaticJava64bit sj3=new StaticJava64bit();
}
}

Output: 1 2 3 4



2) Java static method

If you apply static keyword with any method, it is known as static method.

  1. A static method belongs to the class rather than object of a class.
  2. A static method can be invoked without the need for creating an instance of a class.
  3. static method can access static data member and can change the value of it.
  4. A static method cannot refer to "this" or "super" keywords in anyway
  5. A static method can be accessed directly by the class name and doesn’t need any object

Example of static method

package com.ashutosh;
class StaticJava64bit{
int emp_id;
String emp_name;
static String department = "IT";
//static method to change the value of static variable
static void change(){
department = "HR";
}
//constructor to initialize the variable
StaticJava64bit(int r, String n){
emp_id = r;
emp_name = n;
}
//method to display values
void show(){
System.out.println(emp_id+" "+emp_name+" "+department);
}
}
//Test class to create and display the values of object
public class StaticMethodJava64bit{
public static void main(String args[]){
StaticJava64bit.change();//calling change method
//creating objects
StaticJava64bit sj1 = new StaticJava64bit(200,"Anupra");
StaticJava64bit sj2 = new StaticJava64bit(201,"Santosh");
StaticJava64bit sj3 = new StaticJava64bit(202,"satya");
//calling show method
sj1.show();
sj2.show();
sj3.show();
}
}

Output: 200 Anupra HR
201 Santosh HR
202 satya HR



Another example of static method that performs normal calculation

package com.ashutosh;
class StaticMethodJava64bit{
static int sqrt(int a){
return a*a;
}
public static void main(String args[]){
int result=StaticMethodJava64bit.sqrt(4);
System.out.println(result);
}
}

Output: 16



Restrictions for static method

There are two main restrictions for the static method. They are:

  1. The static method can not use non static data member or call non-static method directly.
  2. this and super cannot be used in static context.

package com.ashutosh;
class StaticMethodJava64bit{
int a=2;//non static
public static void main(String args[]){
System.out.println(a);
}
}

Output: complie time error



Q) why java main method is static?

Ans) because object is not required to call static method if it were non-static method, jvm create object first then call main() method that will lead the problem of extra memory allocation.


3) Java static block

  1. Is used to initialize the static data member.
  2. It is executed before main method at the time of classloading.

Example of static block

package com.ashutosh;
public class StaticBlockJava64bit {
static{
System.out.println("static block is invoked");
}
public static void main(String args[]){
System.out.println("Print main");
}
}

Output: static block is invoked
Print main


Q) Can we execute a program without main() method?

Ans) Yes, one of the way is static block but in previous version of JDK not in JDK 1.8.

package com.ashutosh;
class StaticBlockJava64bit{
static{
System.out.println("static block is invoked");
System.exit(0);
}
}

in jdk 1.5

Output: static block is invoked


In JDK7 and above, output will be:

Output:Error: Main method not found in class StaticBlockJava64bit, please define the main method as: public static void main(String[] args)

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