Object Oriented Programming with Real World Examples


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This presentation covers most OOP features i.e. Class, Object, Inheritance, Polymorphism, Encapsulation, etc. with real world examples...

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Object Oriented Programming with Real World Examples

  1. 1. • • • • https://www.facebook.com/Oxus20 oxus20@gmail.com Classes and Objects Encapsulation Inheritance Polymorphism Object Oriented Programming Abdul Rahman Sherzad
  2. 2. Agenda » Object Oriented Programming » Definition and Demo ˃ Class ˃ Object ˃ Encapsulation (Information Hiding) ˃ Inheritance ˃ Polymorphism ˃ The instanceof Operator 2 https://www.facebook.com/Oxus20
  3. 3. Object Oriented Programming (OOP) » OOP makes it easier for programmers to structure and form software programs. » For the reason that individual objects can be modified without touching other aspects of the program. ˃ It is also easier to update and modify programs written in object-oriented languages. » As software programs have grown larger over the years, OOP has made developing these large programs more manageable. 3 https://www.facebook.com/Oxus20
  4. 4. Class Definition » A class is kind of a blueprint or template that refer to the methods and attributes that will be in each object. » BankAccount is a blueprint as follow ˃ State / Attributes + Name + Account number + Type of account + Balance ˃ Behaviors / Methods + To assign initial value + To deposit an account + To withdraw an account + To display name, account number & balance. Now you, me and others can have bank accounts (objects / instances) under the above BankAccount blueprint where each can have different account name, number and balance as well as I can deposit 500USD, you can deposit 300USD, as well as withdraw, etc. https://www.facebook.com/Oxus20 4
  5. 5. BankAccount Class Example public class BankAccount { // BankAccount attributes private String accountNumber; private String accountName; private double balance; // BankAccount methods // the constructor public BankAccount(String accNumber, String accName) { accountNumber = accNumber; accountName = accName; balance = 0; } 5 https://www.facebook.com/Oxus20
  6. 6. BankAccount Class Example (contd.) // methods to read the attributes public String getAccountName() { return accountName; } public String getAccountNumber() { return accountNumber; } public double getBalance() { return balance; } 6 https://www.facebook.com/Oxus20
  7. 7. BankAccount Class Example (contd.) // methods to deposit and withdraw money public boolean deposit(double amount) { if (amount > 0) { balance = balance + amount; return true; } else { return false; } } public boolean withdraw(double amount) { if (amount > balance) { return false; } else { balance = balance - amount; return true; } } 7 } https://www.facebook.com/Oxus20
  8. 8. Object Definition » An object holds both variables and methods; one object might represent you, a second object might represent me. » Consider the class of Man and Woman ˃ Me and You are objects » An Object is a particular instance of a given class. When you open a bank account; an object or instance of the class BankAccount will be created for you. https://www.facebook.com/Oxus20 8
  9. 9. BankAccount Object Example BankAccount absherzad = new BankAccount("20120", "Abdul Rahman Sherzad"); » absherzad is an object of BankAccount with Account Number of "20120" and Account Name of "Abdul Rahman Sherzad". » absherzad object can deposit, withdraw as well as check the balance. 9 https://www.facebook.com/Oxus20
  10. 10. BankAccount Demo public class BankAccountDemo { public static void main(String[] args) { BankAccount absherzad = new BankAccount("20120", "Abdul Rahman Sherzad"); absherzad.deposit(500); absherzad.deposit(1500); System.out.println("Balance is: " + absherzad.getBalance()); //2000 absherzad.withdraw(400); System.out.println("Balance is: " + absherzad.getBalance()); //1600 } } 10 https://www.facebook.com/Oxus20
  11. 11. Encapsulation Definition » Encapsulation is the technique of making the class attributes private and providing access to the attributes via public methods. » If attributes are declared private, it cannot be accessed by anyone outside the class. ˃ For this reason, encapsulation is also referred to as Data Hiding (Information Hiding). » Encapsulation can be described as a protective barrier that prevents the code and data corruption. » The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. https://www.facebook.com/Oxus20 11
  12. 12. Encapsulation Benefit Demo public class EncapsulationDemo { public static void main(String[] args) { BankAccount absherzad = new BankAccount("20120", absherzad.deposit(500); "Abdul Rahman Sherzad"); // Not possible because withdraw() check the withdraw amount with balance // Because current balance is 500 and less than withdraw amount is 1000 absherzad.withdraw(1000); // Encapsulation Benefit // Not possible because class balance is private // and not accessible directly outside the BankAccount class absherzad.balance = 120000; // Encapsulation Benefit } } 12 https://www.facebook.com/Oxus20
  13. 13. Inheritance Definition » inheritance is a mechanism for enhancing existing classes ˃ Inheritance is one of the other most powerful techniques of object-oriented programming ˃ Inheritance allows for large-scale code reuse » with inheritance, you can derive a new class from an existing one ˃ automatically inherit all of the attributes and methods of the existing class ˃ only need to add attributes and / or methods for new functionality 13 https://www.facebook.com/Oxus20
  14. 14. BankAccount Inheritance Example » Savings Account is a bank account with interest » Checking Account is a bank account with transaction fees 14 https://www.facebook.com/Oxus20
  15. 15. Specialty Bank Accounts » now we want to implement SavingsAccount and CheckingAccount ˃ A SavingsAccount is a bank account with an associated interest rate, interest is calculated and added to the balance periodically ˃ could copy-and-paste the code for BankAccount, then add an attribute for interest rate and a method for adding interest ˃ A CheckingAccount is a bank account with some number of free transactions, with a fee charged for subsequent transactions ˃ could copy-and-paste the code for BankAccount, then add an attribute to keep track of the number of transactions and a method for deducting fees 15 https://www.facebook.com/Oxus20
  16. 16. Disadvantages of the copy-and-paste Approach » tedious and boring work » lots of duplicate and redundant code ˃ if you change the code in one place, you have to change it everywhere or else lose consistency (e.g., add customer name to the bank account info) » limits polymorphism (will explain later) 16 https://www.facebook.com/Oxus20
  17. 17. SavingsAccount class (inheritance provides a better solution) » SavingsAccount can be defined to be a special kind of BankAccount » Automatically inherit common features (balance, account #, account name, deposit, withdraw) » Simply add the new features specific to a SavingsAccount » Need to store interest rate, provide method for adding interest to the balance » General form for inheritance: public class DERIVED_CLASS extends EXISTING_CLASS { ADDITIONAL_ATTRIBUTES ADDITIONAL_METHODS } 17 https://www.facebook.com/Oxus20
  18. 18. SavingsAccount Class public class SavingsAccount extends BankAccount { private double interestRate; public SavingsAccount(String accNumber, String accName, double rate) { super(accNumber, accName); interestRate = rate; } public void addInterest() { double interest = getBalance() * interestRate / 100; this.deposit(interest); } } 18 https://www.facebook.com/Oxus20
  19. 19. SavingsAccount Demo public class SavingsAccountDemo { public static void main(String[] args) { SavingsAccount saving = new SavingsAccount("20120", "Abdul Rahman Sherzad", 10); // deposit() is inherited from BankAccount (PARENT CLASS) saving.deposit(500); // getBalance() is also inherited from BankAccount (PARENT CLASS) System.out.println("Before Interest: " + saving.getBalance()); saving.addInterest(); System.out.println("After Interest: " + saving.getBalance()); } } 19 https://www.facebook.com/Oxus20
  20. 20. CheckingAccount Class public class CheckingAccount extends BankAccount { private int transactionCount; private static final int NUM_FREE = 3; private static final double TRANS_FEE = 2.0; public CheckingAccount(String accNumber, String accName) { super(accNumber, accName); transactionCount = 0; } public boolean deposit(double amount) { if (super.deposit(amount)) { transactionCount++; return true; } return false; } 20 https://www.facebook.com/Oxus20
  21. 21. CheckingAccount Class (contd.) public boolean withdraw(double amount) { if (super.withdraw(amount)) { transactionCount++; return true; } return false; } public void deductFees() { if (transactionCount > NUM_FREE) { double fees = TRANS_FEE * (transactionCount - NUM_FREE); if (super.withdraw(fees)) { transactionCount = 0; } } } } https://www.facebook.com/Oxus20 21
  22. 22. CheckingAccount Demo public class CheckingAccountDemo { public static void main(String[] args) { CheckingAccount checking = new CheckingAccount("20120", "Abdul Rahman Sherzad"); checking.deposit(500); checking.withdraw(200); checking.deposit(700); // No deduction fee because we had only 3 transactions checking.deductFees(); System.out.println("transactions <= 3: " + checking.getBalance()); // One more transaction checking.deposit(200); // Deduction fee occurs because we have had 4 transactions checking.deductFees(); System.out.println("transactions > 3: " + checking.getBalance()); } 22 } https://www.facebook.com/Oxus20
  23. 23. Polymorphism Definition » Polymorphism is the ability of an object to take on many forms. » The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object. 23 https://www.facebook.com/Oxus20
  24. 24. Polymorphism In BankAccount » A SavingsAccount IS_A BankAccount (with some extra functionality) » A CheckingAccount IS_A BankAccount (with some extra functionality) » Whatever you can do to a BankAccount (i.e. deposit, withdraw); you can do with a SavingsAccount or CheckingAccount » Derived classes can certainly do more (i.e. addInterest) for SavingsAccount) » Derived classes may do things differently (i.e. deposit for CheckingAccount) 24 https://www.facebook.com/Oxus20
  25. 25. Polymorphism In BankAccount Example public class BankAccountPolymorphismDemo { public static void main(String[] args) { BankAccount firstAccount = new SavingsAccount("20120", "Abdul Rahman Sherzad", 10); BankAccount secondAccount = new CheckingAccount("20120", "Abdul Rahman Sherzad"); // calls the method defined in BankAccount firstAccount.deposit(100.0); // calls the method defined in CheckingAccount // because deposit() is overridden in CheckingAccount secondAccount.deposit(100.0); } } 25 https://www.facebook.com/Oxus20
  26. 26. Instanceof Operator » if you need to determine the specific type of an object ˃ use the instanceof operator ˃ can then downcast from the general to the more specific type ˃ For example the object from BankAccount Parent Class will be casted to either SavingsAccount and/or CheckingAccount 26 https://www.facebook.com/Oxus20
  27. 27. END 27 https://www.facebook.com/Oxus20