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Java static and this Keyword

Java static and this Keyword
There are various reserve words that Java provides to do different types of operations. Keywords are reserved words whose meanings are already known to Java compiler. There are two important keywords in Java that are used for very special purposes.
In this chapter we will learn about the two special types of keywords:
  • static
  • this

What is static in Java?

Static is a keyword that acts as a non-access modifier in Java that is used mainly to manage memory. The variable or Method that are marked static belongs to the Class rather than to any particular instance. A Static method cannot access an instance variable. If a Class contains any static blocks then that block will be executed only when the Class is loaded in JVM. Programmers can apply the Java keyword static with different programming objects like:
  • variables
  • methods
  • initialization – block
  • nested class

Static keyword is unacceptable to the following

  • Class
  • Constructor
  • Local inner classes
  • Inner class methods
  • Instance variables
  • Local Variables
  • Interfaces
Static variables or methods can be invoked without having an instance of the class. Only a class is needed to call up a static method or a static variable. If you declare any variable as static, it is known static variable. The static variable can refer to a common property of all objects (that is not unique for each object) e.g. company name of employees, college name of students etc. Memory in a static variable is reserved only once in a class area at the time of class loading. One advantage of using static is that it increases the efficiency of the memory.

Program for Static in Java

Example:
public class egStatic {
 static int x = 60;
 static void fun() {
  System.out.println("Within Static");
 }
 public static void main(String[] args) {
  egStatic.fun();
  System.out.println(egStatic.x);
  egStatic S1 = new egStatic();
  egStatic S2 = new egStatic();
  System.out.println(S1.x);
  S1.fun();
 }
}

Rules for using Static Variables

  • Variables or methods belong to class rather than to any particular instance
  • A static method cannot access a non-static variable of a class nor can directly invoke non-static method
  • Static members can be applied without creating or referencing an instance of the class
  • A static variable will be shared by all instances of that class which will result in only one copy

What is this keyword in Java?

This is another Java keyword which as a reference to the current object within an instance method or a constructor — the object whose method or constructor is being called. By the use of this keyword, programmers can refer to any member of the current object within an instance method or a constructor. This keyword can be used to refer to the current object and it always acts as a reference to an object in which method was invoked. There are the various uses of this keyword in Java. These are:
  • For referring current class instance variable, this keyword can be used
  • To invoke current class constructor, this() is used
  • this can be passed as message argument in a method call
  • In a constructor call, this can be passed as argument
  • For returning current class instance, this keyword is used

Program for this in Java

Example:
class Emp {
 int e_id;
 String name;

 Emp(int e_id, String name) {
  this.e_id = e_id;
  this.name = name;
 }
 void show() {
  System.out.println(e_id + " " + name);
 }
 public static void main(String args[]) {
  Emp e1 = new Emp(1006, "Karlos");
  Emp e2 = new Emp(1008, "Ray");
  e1.show();
  e2.show();
 }
}
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