The document discusses the differences between static and non-static (or dynamic) variables and methods in object-oriented programming. Static variables and methods belong to the class as a whole rather than individual objects. They can be accessed directly via the class name without needing an instance. In contrast, non-static or instance variables and methods are dependent on object instances and belong to each object separately. The document provides examples to demonstrate proper and improper uses of static, concluding that static is not always appropriate for object-oriented design goals since it does not model real-world entities as independently behaving objects.

1. Static
vs.
Non Static
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Object
Oriented
Concept
Abdul Rahman Sherzad

2. 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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3. 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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4. 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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5. 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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6. 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;
}
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7. 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;
}
}
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8. 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
}
}
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9. 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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10. 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.
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11. 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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12. END
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