Java static keyword
- Static variable
- Program of
counter without static variable
- Program of
counter with static variable
- Static method
- Restrictions for
static method
- Why main method
is static ?
- Static block
- Can we execute a
program without main method ?
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
1) 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).
Understanding
problem without static variable
1.
class Student{
2. int rollno;
3. String name;
4. String college="ITS";
5. }
Suppose
there are 500 students in my college, now all instance data members will get
memory each time when object is created.All student have its unique rollno and
name so instance data member is good.Here, college refers to the common
property of all objects.If we make it static,this field will get memory only
once.
Java
static property is shared to all objects.
Example of static variable
1.
//Program of static variable
2.
3.
class Student8{
4. int rollno;
5. String name;
6. static String college ="ITS";
7.
8. Student8(int r,String n){
9. rollno = r;
10. name = n;
11. }
12. void display (){System.out.println(rollno+" "+name+" "+college);}
13.
14. public static void main(String args[]){
15. Student8 s1 = new Student8(111,"Karan");
16. Student8 s2 = new Student8(222,"Aryan");
17.
18. s1.display();
19. s2.display();
20. }
21. }
Output:111 Karan ITS
222 Aryan ITS
Program of
counter without static variable
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 to other objects. So each objects will have
the value 1 in the count variable.
1.
class Counter{
2.
int count=0;//will get memory when instance is created
3.
4. Counter(){
5. count++;
6. System.out.println(count);
7. }
8.
9.
public static void main(String args[]){
10.
11. Counter c1=new Counter();
12. Counter c2=new Counter();
13. Counter c3=new Counter();
14.
15. }
16. }
Output:1
1
1
Program
of counter by static variable
|
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.
|
1.
class Counter2{
2.
static int count=0;//will get memory only once and retain its value
3.
4. Counter2(){
5. count++;
6. System.out.println(count);
7. }
8.
9.
public static void main(String args[]){
10.
11. Counter2 c1=new Counter2();
12. Counter2 c2=new Counter2();
13. Counter2 c3=new Counter2();
14.
15. }
16. }
Output:1
2
3
2) 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.
Example of static method
1.
//Program of changing the common property of all objects(static field).
2.
3.
class Student9{
4. int rollno;
5. String name;
6. static String college = "YMCA";
7.
8. static void change(){
9. college = "DAV";
10. }
11.
12. Student9(int r, String n){
13. rollno = r;
14. name = n;
15. }
16.
17. void display (){System.out.println(rollno+" "+name+" "+college);}
18.
19. public static void main(String args[]){
20. Student9.change();
21.
22. Student9 s1 = new Student9 (111,"Karan");
23. Student9 s2 = new Student9 (222,"Aryan");
24. Student9 s3 = new Student9 (333,"Sonoo");
25.
26. s1.display();
27. s2.display();
28. s3.display();
29. }
30. }
Output:111 Karan DAV
222 Aryan DAV
333 Sonoo DAV
Another example of static method that
performs normal calculation
1.
//Program to get cube of a given number by static method
2.
3.
class Calculate{
4. static int cube(int x){
5. return x*x*x;
6. }
7.
8. public static void main(String args[]){
9. int result=Calculate.cube(5);
10. System.out.println(result);
11. }
12. }
Output:125
Restrictions for static method
|
There
are two main restrictions for the static method. They are:
|
|
1. The
static method can not use non static data member or call non-static method
directly.
2. this
and super cannot be used in static context.
|
1.
class A{
2. int a=40;//non static
3.
4. public static void main(String args[]){
5. System.out.println(a);
6. }
7. }
Output:Compile Time Error
Q) why
java main method is static?
|
Ans)
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.
|
3) Java
static block
- Is used to initialize the static data
member.
- It is executed before main method at
the time of classloading.
Example of static block
1.
class A2{
2. static{System.out.println("static block is invoked");}
3. public static void main(String args[]){
4. System.out.println("Hello main");
5. }
6. }
Output:static block is invoked
Hello main
Q) Can we execute a program without main() method?
Ans)
Yes, one of the way is static block but in previous version of JDK not in JDK
1.7.
1. class A3{
2.
static{
3.
System.out.println("static block is invoked");
4.
System.exit(0);
5.
}
6.
}
Output:static block is invoked (if not JDK7)
In JDK7
and above, output will be:
Output:Error: Main method not found in class A3, please define the main method as:
public static void main(String[] args)
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