Object Oriented Concept

Static

vs.

Non Static

Abdul Rahman Sherzad

https://www.facebook.com/Oxus20

oxus20@gmail.com

Static vs. Non-static

» Static

˃ class variable

˃ class method

» Non-static

˃ instance variable

˃ instance method

» The absence of the keyword static before non-local variables and methods means dynamic / non-static (one per object / instance)

» Static ones are associated with class, not object. Can be called using class name directly

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2

Static Variables

» Static means "refer to to the class in general", not to

an individual object

» A variable may be declared (outside of a method)

with the static keyword:

˃ e.g. static int numberOfEmployees;

˃ There is one variable numberOfEmployees for the class not one per object!!!

» A static variable is shared by all instances (if any).

˃ All instances may be able read / write it

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Static Methods

» A method may be declared with the static keyword

» Static methods live at class level, not at object level

» Static methods may access static variables and

methods, but not dynamic ones! public class Employee {

private static int numberOfEmployees;

public static int getNumberOfEmployee() {

return numberOfEmployees;

}

}

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Logical Example

» Static is not the true intend of Object Oriented Design and

Concept.

» Based on following scenario:

» LAMP is an object. OFF / ON is its state and characteristic; the

state of one object is independent of another.

» For instance, we turn a LAMP "off" it does not suppose to

turn the LAMPS of the entire world goes "off".

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Example Demonstrates Bad Use of Static

public class BankAccount {

private String id;

private String name;

// 'static' only for demonstration purpose, in real example must not static!

private static double balance;

public BankAccount(String i, String n, double b) {

id = i;

name = n;

balance = b;

}

public String getId() {

return id;

}

public String getName() {

return name;

} 6

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Example Demonstrates Bad Use of Static (contd.)

public double getBalance() {

return balance;

}

public boolean deposit(double amount) {

if (amount > 0) {

balance += amount;

return true;

}

return false;

}

public boolean withdraw(double amount) {

// at least 100 must be kept in Balance

if (balance > amount + 100) {

balance -= amount;

return true;

}

return false;

}

} 7

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Use BankAccount Class public class BankApplication {

public static void main(String[] args) {

BankAccount absherzad = new BankAccount("20120", "Abdul Rahman Sherzad", 500);

absherzad.deposit(1500);

// print the balance for object 'absherzad'

System.out.println(absherzad.getBalance()); // 2000

BankAccount oxus20 = new BankAccount("20800", "OXUS 20", 400);

/*

* print the balance for object 'absherzad'

* why output 300? because balance variable defined 'static'

* 'static' variable is not dependent on object

* when account for 'oxus20' created with initial balance of 400

* Now all the objects of class BankAccount has balance of 400

*/

System.out.println(absherzad.getBalance()); // 400

}

} 8

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BankAccount Example Conclusion

» for demonstration purpose the balance variable for

the class BankAccount was defined 'static'

» BankApplication class demonstrates how the last

object initial balance overrides all the balance for

entire objects of class BankAccount.

» That is why, it is to be said that "Static is not the true

intend of Object Oriented Concept"

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When Use Static

» A variable should be static if:

˃ It logically describes the class as a whole

˃ There should be only one copy of it

» A method should be static if:

˃ It does not use or affect the object that receives the message (it uses

only its parameters)

» When a policy is applied for the whole class. For example, when you ask

your students in the class to be quite, stop writing, etc. you ask the entire

class rather than a particular student. 10

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Static Rules

» static variables and methods belong to the class in general, not to

individual objects

» The absence of the keyword static before non-local variables and

methods means dynamic (one per object / instance)

» A dynamic method can access all dynamic and static variables and

methods in the same class

» A static method can not access a dynamic variable (How could it choose

or which one?)

» A static method can not call a dynamic method (because it might access

an instance variable

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END

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