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:
• variable (also known as class variable)
• method (also known as class method)
• block
• nested class

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

Example of static variable
• class Student8{
• int rollno;
• String name;
• static String college ="ITS";
•
• Student8(int r,String n){
• rollno = r;
• name = n;
• }
• void display ()
{System.out.println(rollno+" "+name+" "+college)
;}

• public static void main(String args[])
• {
• Student8 s1 = new Student8(111,"Karan");
• Student8 s2 = new Student8(222,"Aryan");
• s1.display();
• s2.display();
• }
• }

Program of counter without static variable
• class Counter{
• int count=0;//will get memory when instance i
s created
•
• Counter(){
• count++;
• System.out.println(count);
• }

• {
•
• Counter c1=new Counter();
•
• }
• }

Program of counter by static variable
• class Counter2{
• static int count=0;//will get memory only onc
e and retain its value
•
• Counter2(){
• count++;
• System.out.println(count);
• }

• {
•
• Counter2 c1=new Counter2();
•
• }
• }

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

• class Calculate
• {
• static int cube(int x)
• {
• return x*x*x;
• }
•

• {
• int result=Calculate.cube(5);
• System.out.println(result);
• }
• }

Restrictions for 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.

• class A
• {
• int a=40;//non static
•
• {
• System.out.println(a);
• }
• }

why java main method is static?
• 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.

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

• class A2
• {
•   static
• {
• System.out.println("static block is invoked");
• }
• {
•    System.out.println("Hello main");
•   }
• }

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

• Yes, one of the way is static block but in
previous version of JDK not in JDK 1.7

• class A3
• {
•   static
• {
•   System.out.println("static block is invoked");

•   System.exit(0);
•   }
• }

this keyword in java
• There can be a lot of usage of java this
keyword. In java, this is a reference
variable that refers to the current object.

Usage of java this keyword
• this can be used to refer current class instance variable.
• this can be used to invoke current class method
(implicitly)
• this() can be used to invoke current class constructor.
• this can be passed as an argument in the method call.
• this can be passed as argument in the constructor call.
• this can be used to return the current class instance
from the method.

this: to refer current class instance variable
• The this keyword can be used to refer current
class instance variable. If there is ambiguity
between the instance variables and
parameters, this keyword resolves the problem
of ambiguity.

• class Student{
• int rollno;
• String name;
• float fee;
• Student(int rollno,String name,float fee){
• this.rollno=rollno;
• this.name=name;
• this.fee=fee;
• }
• void display()
{System.out.println(rollno+" "+name+" "+fee);}
• }

• class TestThis2
• {
• {
• Student s1=new Student(111,"ankit",5000f);
• Student s2=new Student(112,"sumit",6000f);
• s1.display();
• s2.display();
• }
• }

this: to invoke current class method

• class A
• {
• void m(){System.out.println("hello m");}
• void n()
• {
• System.out.println("hello n");
• //m();//same as this.m()
• this.m();
• }
• }

• class TestThis4
• {
• {
• A a=new A();
• a.n();
• }
• }

this() : to invoke current class constructor
• The this() constructor call can be used to
invoke the current class constructor. It is used
to reuse the constructor. In other words, it is
used for constructor chaining.

Calling default constructor from parameterized
constructor:
• class A{
• A(){System.out.println("hello a");}
• A(int x){
• this();
• System.out.println(x);
• }
• }
• class TestThis5{
• public static void main(String args[]){
• A a=new A(10);
• }}

Calling parameterized constructor from default
constructor:
• class A{
• A(){
• this(5);
• System.out.println("hello a");
• }
• A(int x){
• System.out.println(x);
• }
• }
• class TestThis6{
• public static void main(String args[]){
• A a=new A();
• }}

Java static keyword

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