Static.md

Java static keyword

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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 an instance of the class.

The static can be:

1- Variable (also known as a class variable) 2- Method (also known as a class method) 3- Block 4- Nested class

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

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

Advantages of static variable

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

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Understanding the problem without static variable

 class Student{  
         int rollno;  
         String name;  
         String college="ITS";  
    } 

Suppose there are 500 students in my college, now all instance data members will get memory each time when the object is created. All students have its unique rollno and name, so instance data member is good in such case. Here, "college" refers to the common property of all objects. If we make it static, this field will get the memory only once.

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Example Program

class Student{  
   int rollno;//instance variable  
   String name;  
   static String college ="ITS";//static variable  
   //constructor  
   Student(int r, String n){  
   rollno = r;  
   name = n;  
   }  
   //method to display the values  
   void display (){System.out.println(rollno+" "+name+" "+college);}  
}  
//Test class to show the values of objects  
public class TestStaticVariable1{  
 public static void main(String args[]){  
 Student s1 = new Student(111,"Karan");  
 Student s2 = new Student(222,"Aryan");  
 //we can change the college of all objects by the single line of code  
 //Student.college="BBDIT";  
 s1.display();  
 s2.display();  
 }  
}  

Output

111 Karan ITS
222 Aryan ITS

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Java static method

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

• A static method belongs to the class rather than the object of a class.
• A static method can be invoked without the need for creating an instance of a class.
• A static method can access static data member and can change the value of it.

Example

class Student{  
     int rollno;  
     String name;  
     static String college = "ITS";  
     //static method to change the value of static variable  
     static void change(){  
     college = "BBDIT";  
     }  
     //constructor to initialize the variable  
     Student(int r, String n){  
     rollno = r;  
     name = n;  
     }  
     //method to display values  
     void display(){System.out.println(rollno+" "+name+" "+college);}  
}  
//Test class to create and display the values of object  
public class TestStaticMethod{  
    public static void main(String args[]){  
    Student.change();//calling change method  
    //creating objects  
    Student s1 = new Student(111,"Karan");  
    Student s2 = new Student(222,"Aryan");  
    Student s3 = new Student(333,"Sonoo");  
    //calling display method  
    s1.display();  
    s2.display();  
    s3.display();  
    }  
}  

Output

       111 Karan BBDIT
       222 Aryan BBDIT
       333 Sonoo BBDIT

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Restrictions for the static method

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

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

Why is the Java main method static?

Ans) It is because the object is not required to call a static method. If it were a non-static method, 
JVM creates an object first then call main() method that will lead the problem of extra memory allocation.

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Java static block

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

Example of static block

 class A2{  
      static{System.out.println("static block is invoked");}  
      public static void main(String args[]){  
       System.out.println("Hello main");  
      }  
    }  

Output:

static block is invoked
       Hello main

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Can we execute a program without main() method?

Ans) No, one of the ways was the static block, but it was possible till JDK 1.6. Since JDK 1.7, it is not possible to execute a java class without the main method.

 class A3{  
      static{  
      System.out.println("static block is invoked");  
      System.exit(0);  
      }  
    }  

Output

static block is invoked

But Since JDK 1.7
Error: Main method not found in class A3, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

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