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.
The static can be:
If you declare any variable as static, it is known as a static variable.
It makes your program memory efficient (i.e., it saves memory).
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.
//Java Program to demonstrate the use of static variable
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();
}
}
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 other objects. So each object will have the value 1 in the count variable.
//Java Program to demonstrate the use of an instance variable
//which get memory each time when we create an object of the class.
class Counter{
int count=0;//will get memory each time when the instance is created
Counter(){
count++;//incrementing value
System.out.println(count);
}
public static void main(String args[]){
//Creating objects
Counter c1=new Counter();
Counter c2=new Counter();
Counter c3=new Counter();
}
}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.
//Java Program to illustrate the use of static variable which
//is shared with all objects.
class Counter2{
static int count=0;//will get memory only once and retain its value
Counter2(){
count++;//incrementing the value of static variable
System.out.println(count);
}
public static void main(String args[]){
//creating objects
Counter2 c1=new Counter2();
Counter2 c2=new Counter2();
Counter2 c3=new Counter2();
}
}If you apply static keyword with any method, it is known as static method.
//Java Program to demonstrate the use of a static method.
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();
}
}//Java Program to get the cube of a given number using the static method
class Calculate{
static int cube(int x){
return x*x*x;
}
public static void main(String args[]){
int result=Calculate.cube(5);
System.out.println(result);
}
}There are two main restrictions for the static method. They are:
class A{
int a=40;//non static
public static void main(String args[]){
System.out.println(a);
}
}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.
class A2{
static{System.out.println("static block is invoked");}
public static void main(String args[]){
System.out.println("Hello main");
}
}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);
}
}Since JDK 1.7 and above, output would be:
