static keyword in Java

The static keyword is used in java mainly for memory management. We may apply 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:

1. variable (also known as class variable)
2. method (also known as class method)
3. block
4. nested class

---

1) 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.

static property is shared to all objects.

Example of static variable

1. //Program of static variable  
2.   
3. class Student{  
4.    int rollno;  
5.    String name;  
6.    static String college ="ITS";  
7.      
8.    Student(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.  Student s1 = new Student (111,"Karan");  
16.  Student s2 = new Student (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. }}  

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 Counter{  
2. static int count=0;//will get memory only once and retain its value  
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. }}  

Output:1
       2
       3

---

2) 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 Student{  
4.      int rollno;  
5.      String name;  
6.      static String college = "ITS";  
7.        
8.      static void change(){  
9.      college = "BBDIT";  
10.      }  
11.   
12.      Student(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.     Student.change();  
21.   
22.     Student s1 = new Student (111,"Karan");  
23.     Student s2 = new Student (222,"Aryan");  
24.     Student s3 = new Student (333,"Sonoo");  
25.   
26.     s1.display();  
27.     s2.display();  
28.     s3.display();  
29.     }  
30. }  

Output:111 Karan BBDIT
       222 Aryan BBDIT
       333 Sonoo BBDIT

---

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:

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.

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

---

Que)why 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)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 A{  
2.   
3.   static{System.out.println("static block is invoked");}  
4.   
5.   public static void main(String args[]){  
6.    System.out.println("Hello main");  
7.   }  
8. }  

Output:static block is invoked
       Hello main

---

Que)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 A{  
2.   static{  
3.   System.out.println("static block is invoked");  
4.   System.exit(0);  
5.   }  
6. }  

Output:static block is invoked (if not JDK7)