[2] Intro to Object-oriented programming - All about the “static” keyword

the “static” keyword in Java

The following can be static in Java:

Variable (also known as a class variable or class member)

Method (also known as a class method)

Block

Nested class

when a variable is static in a java class

In a class, static variable is independent of the object of the class i.e., it is not unique for each object.

It can be accessed before any objects of its class are created (because it is independent of the object) and without reference to any object.

Accessing a static variable that belongs to a class:


class UniStudent{  
		int studentNumber;  
		String name;  
		static long totalNumOfStudents = 100;  
}

public class TestStaticVariable1{  

 public static void main(String args[]){  
		// accessing a static variable is possible 
		// without creating an object of that class
	  System.out.println(UniStudent.totalNumOfStudents);
 } 
 
}

For example, in a university, the total number of students in the university is independent of the individual students


class UniStudent{  
		int studentNumber;  
		String name;  
		static long totalNumOfStudents = 100;  

		// constructor
		UniStudent(int studentNumber, String name){  
			this.studentNumber = studentNumber;  
			this.name = name;  
			UniStudent.totalNumOfStudents += 1;
		}

		void display ()
		{
			System.out.println(this.studentNumber+" "+this.name+" "+UniStudent.totalNumOfStudents);
		}
}


public class TestStaticVariable1{  
 public static void main(String args[]){  
 UniStudent s1 = new UniStudent(2018,"Sans");  
 UniStudent s2 = new UniStudent(2019,"Sunwoo");  

 s1.display();  
 s2.display();  
 }  
}

Output:


2018 Sans 102
2019 Sunwoo 102

when a method is static in java

A static method is a method that belongs to the class, not to any object

A static method can be invoked without the need for creating an instance (meaning object) of a class.

Example of calling a static method:


class UniStudent{  
		int studentNumber;  
		String name;  
		static long totalNumOfStudents = 100; 

		//static method to change the value of static variable  
		static void change(){  
			totalNumOfStudents = 100000;  
		} 
}

public class TestStaticVariable1{  

 public static void main(String args[]){  
		// calling the change method

		//A static method can be accessed directly
		// by the class name and doesn’t need any object
		UniStudent.change();
	  System.out.println(UniStudent.totalNumOfStudents);
 } 
 
}

Output: 100000

restriction of the static method:

The keywords 1.” this” and 2. “super” cannot be used in static context.

A static method can only access static variables, it CANNOT access non-static data without creating an instance.

Q. why is the main function in java static?

public static void main(String args[])

→ Because we don’t want to create an instance of the class to call/invoke main function

→ it must be accessible for an application to run, before any instantiation takes place.

static blocks in Java

Purpose of static block:

initialize the static data member

How it works:

executed before the main method, when the class is first loaded

Example:


class A {
static String firstName = "Sans"
static String fullName;

// static block initializes fullName variable
static {
   System.out.println("Static block initialized.");
   fullName = firstName + " Bhatia";
  }

public static void main(String args[]) {
      System.out.println(fullName);
	}
}

Output:


Static block initialized.
Sans Bhatia

Q. Can a program be executed without the main() method?

→ Not anymore after JDK 1.7


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

Output:


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

References:

OOP 2 | Packages, Static, Singleton Class, In-built Methods

This second video of the complete object oriented programming (#OOP) course in Java will clear some of the most important concepts for you. Covering packages, static elements, singleton class, in-built methods, and more in an easy to understand story based approach!

https://www.youtube.com/watch?v=_Ya6CN13t8k&list=PL9gnSGHSqcno1G3XjUbwzXHL8_EttOuKk&index=2

DSA-Bootcamp-Java/lectures/17-oop/notes at main · kunal-kushwaha/DSA-Bootcamp-Java

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https://github.com/kunal-kushwaha/DSA-Bootcamp-Java/tree/main/lectures/17-oop/notes

Static keyword in Java - Javatpoint

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

https://www.javatpoint.com/static-keyword-in-java