ACM/JETT Workshop - August 4-5, 2005 1 03.Object-Oriented Basics & Design.

1 ACM/JETT Workshop - August 4-5, 2005

03.Object-Oriented Basics & Design


TopicsIn this Lecture, we will discuss the some of the

fundamental notions in Object-Oriented systems:– Encapsulation of data and functions as

objects.– Class vs object– Information hiding using public and private– Creating instances /objects from classes.– Calling methods on objects– Instance vs class-level data members.– Java code segments to illustrate these

concepts.


What to expect• After this lecture, you should be able to

use the given code in BankAccount.java file and do the following:– Add a new method to class BankAccount.– Create instances from class BankAccount.– Use the methods defined in BankAccount– Compile and test your code.

ACM/JETT Workshop - August 4-5, 2005

An Object-Oriented System

An object-oriented system consists of a network of objects which collaborate to solve a problem.

Objects store data and perform actions.

bankaccountbankaccountbankbank

customer

showBalance (acctId)

getBalance()


An Object-Oriented (OO) System

To understand the object-oriented concepts and OO design, we will study a small part of an application area that we are all familiar with: banking.

A simple specification of banking:A bank maintains a collection of bank accounts, where a bank account can be a checking or a savings account.

Each bank account is associated with a customer. A customer queries,deposits and/or withdraws money into a bank account.


An Object-Oriented System

The figure below shows some of the objects in the specification and a scenario where a customer calls a showBalance() function on a bank object; in turn, a bank object calls getBalance() function on a bank account object.

bankaccountbankaccountbankbank

customer

showBalance (acctId)

getBalance()


What are Objects?Objects are “ things” that encapsulate data

and behavior (actions called methods).Objects can be “physical” things (eg:

teacher, book, fish). Objects can be “conceptual” things (eg:

registration, payment, bank account).Objects correspond to “nouns” in problem

description.


What are objects?Objects encapsulate data and behavior.

A Bank Account object has – acctId– balance– calcInterest ()– deposit ()– withdraw ()– getBalance()

data

methods (functions)(behavior)


What are objects?

• Objects may have an internal structure (private).

– Eg: A bank account may store an acctId, balance etc.

• Objects reveal themselves by their external behavior (public methods).

– Eg: A back account object may be used with its public methods, like deposit and withdraw.


What are objects?• Objects reveal their “public behavior” but

keep their "inside (or implementation)" hidden.

• The “public behavior” of objects is through a set of methods (functions) that are offered to clients of objects.

• The methods (functions) can be invoked by clients.

• A client is an object that invokes the methods of an object.


Objects = data + methods

An object of type BankAccount

acctId

deposit()

withdraw()

private public

calcInterest()

balance

getBalance()

Client

A client can call any public methods

data

methods


What is a class• A class is a software blueprint that

defines the variables and the methods common to all objects of a certain kind.

• A class can be used and reused many times to create many objects.

• A class is one mechanism to define new types in Java.

• Objects are created as instances of a class.


Information hiding• In a class, • public features can be seen and

manipulated by anybody -- they are the external (interface) view.

• private features can be manipulated by only the other members of that class. They are the internal (implementation) view.

• Good OO design recommends that – methods are public and – data fields are private.


Class vs Object

acctId

deposit()

withdraw()

private public

calcInterest()

balance

getBalance()

acctId=A12

deposit()

withdraw()

private public

calcInterest()

Balance=100.00

getBalance()

BankAccount – a classmyAccount – an object

of type BankAccount


Class vs Object• How do you define a class in Java?

Show class BankAccountExample1_3



Creating objects from a class• Objects are created by instantiating a

class.

• An object is also called an instance.

• In Java, you create an object from a class using new().


Creating objects from a class

// Declare a variable // of type BankAccount

BankAccount myAcct;myAcct

// Create an instance of type BankAccount// Store the reference in the variable// myAcct

myAcct = new BankAccount();acctId = “Not set”

Balance = 0.0

myAcct


Calling Methods of an objects

The public methods of an object can be invoked with a . (dot) operator.

Example:

BankAccount myAcct;

myAcct = new BankAccount();

// deposit $100.00 by invoking the // deposit() method on myAcct

myAcct.deposit(100.00);


Instance data members of an object

// Declare a variable of type BankAccount

BankAccount myAcct;myAcct = new BankAccount();

BankAccount acct2 = new BankAccount();

// deposit 100.00 into myAcct.

myAcct.deposit(100.00);

// deposit 500.00 into acct2.

acct2.deposit(500.00);

myAcct

acctId = “Not set”balance = 0.0

acct2acctId = “Not set”

balance = 0.0

The instances myAcct and acct2 have their own copy of the data members, balance and acctId (instance data members)


Class-level Data members• Assume that we want to include the fact

that the current interest rate for all bank accounts is 5%.

• If the rate changes, all bank account instances should have the same rate.

• In this case, we can define the interest rate as a class level data member.

• In Java, class-level members are called static members.

• Both data and methods can be static.


Class-level Data members• Methods that are static can only access

static data members.

• Static methods can be called on a class without creating an instance.

• Example:

BankAccount myAcct = new BankAccount();// Calling an instance method on an instance

myAcct.deposit (100.00); // Calling a static method on a class

double rate = BankAccount.getRate();


Class-level Data members




State of an object• The state of an object is described by

the values of (some or all) of its data members.

• Example: The state of a bank account object would include the current balance.


When do objects change their state?

• Objects change their state as a result of executing a method.

• Example:

• The state of a BankAccount object (with a current balance of $0) will change when a deposit() or withdraw() method is invoked on that object.


The topics that you should be comfortable with at this time

Class vs object

How to define a class in Java

private vs public members of a class

Creating instances/objects from classes in Java.

Calling instance methods on objects and

Calling class methods on class/object using the Java code segments given.


To test your understanding

• Do Lab 1

ACM/JETT Workshop - August 4-5, 2005 1 03.Object-Oriented Basics & Design.

Documents

Transcript of ACM/JETT Workshop - August 4-5, 2005 1 03.Object-Oriented Basics & Design.