[Java] Static_2

Static_2

- The static keyword in java is used for memory management mainly

- It makes your program memory efficient

1) Static variable

- The static variable can be used to refer the common property of all objects 

- The static variable gets memory only once in class area at the time of class loading

1_1. Understanding problem without static variable

class Student{ int number; String name; String college = "HV"; }

- Suppose there are 500 students in my college

- All instance data members will get memory each time when object is created

- If we make it static, college field will get memory only once

1_2. Example

package statics; public class Student { int number; String name; static String college = "HV"; Student(int r, String n){ this.name = n; this.number = r; } void display(){ System.out.println(number+" "+name+" "+college); } public static void main(String[]args){ Student aa = new Student(111,"Kyle"); } }  

1_3. Program Counter

1) Without static variable

package statics; public class Counter { int count = 0; Counter(){ count++; System.out.println(count); } public static void main(String[] args){ Counter c1 = new Counter(); Counter c2 = new Counter(); } }  

- 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

2) static variable

package statics; public class Counter { static int count = 0; Counter(){ count++; System.out.println(count); } public static void main(String[] args){ Counter c1 = new Counter(); Counter c2 = new Counter(); } }  

2) Static method

- A static method belongs to the class rather than object of 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

2_1. Example

1) change

package statics; public class Student { int number; String name; static String college = "HV"; static void change(){ college = "BB"; } Student(int r, String n){ this.name = n; this.number = r; } void display(){ System.out.println(number+" "+name+" "+college); } public static void main(String[]args){ Student.change(); Student aa = new Student(111,"Kyle"); Student bb = new Student(222,"Tom"); aa.display(); bb.display(); } }  

2) Calculate

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); } }  

3) Restriction

- 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

package statics; public class Student { int a = 40; public static void main(String[] args){ System.out.println(a); } }  

 4) Why java main method is static?

- Object is not required to call static method

- If it were non-static method, jvm create object first then call main() that will lead the problem of extra memory allocation

5) Java static block

- Is used to initialize the static data member

- It it executed before main method at the time of classloading

class A2{ static{System.out.println("static block is invoked");} public static void main(String args[]){ System.out.println("Hello main"); } }  

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

- One of the way is static block but not in JDK 1.7

class A3{ static{ System.out.println("static block is invoked"); System.exit(0); } }