1. CS2011 Introduction to Programming I Objects and Classes
Chengyu Sun
California State University, Los Angeles

2. Represent Data
To process data using computers, we first need to represent the data in a programming language 10
int
3.1415
double
"hello"
String
90, 100, 95
int[]
90, 100, 95
85, 90, 100
79, 80, 90
int[][]

3. Example: Banking System
Create a banking system that helps a bank to manage customers and accounts, e.g. open an account, get the balance of an account, deposit, withdraw, transfer, and so on
How do we represent customers and accounts??

4. What Information Do We Need to Keep?
Customer
Account
name
String id
int
address
String
balance
double
owner ??

5. One Way to Implement a Bank
String[] customerNames;
String[] customerAddresses;
int[] accountIds;
double[] accountBalances;
String[] accountOwnerNames;

6. A Better Way to Implement A Bank
class Customer {
String name;
String address; }
class Account {
int id;
double balance;
Customer owner; }
Customer[] customer;
Account[] accounts;

7. The Object-Oriented Paradigm …
Computer programs are usually created to facilitate real-world tasks
A good programming language should make it easier to mimic (i.e. model) real-world matters

8. … The Object-Oriented Paradigm
The world consists of objects
Each object has some attributes, and is often associated with some operations
The same type of objects share the same set of attributes and/or operations

9. From Concept to Code Type Class Attributes Variables Operations
Methods
Objects in
the real world
Objects in
programs

10. Class in Java Class name Access modifier Fields, a.k.a.
instance variables
public class Customer {
String name;
String address;
public Customer(String cname, String caddress) {
name = cname;
address = caddress; }
Constructor

11. About Constructor
A constructor is a special method: no return type, and same name as class
Constructors are used to create new objects (often called instances) of a class
A class must have at least one constructor
If a class doesn’t have a constructor, JVM will automatically creates one for the class, known as the default constructor

12. Create and Access An Object
Customer c = new Customer("John", "123 Main St");
Use the new keyword like
when creating an array
Call one of the constructors
System.out.println( c.name );
Use the . operator to access object fields and methods

13. Class As Type A class can be considered a user-defined type Type
Customer c = new Customer("John", "123 Main St");
Type
Customer[] customers = new Customer[100];

14. Banking System: Data A bank manages a number of customers and accounts
Use arrays to store customers and accounts
Use counts to keep track the actual number of customers and accounts

15. Classes Are More Than Just Data
Example: add account operations like deposit, withdraw, and get balance

16. static vs Non-static
public class Foo {
static int a = 0;
int b;
Foo() { b = 0; }
public void inc() {
++a; ++b; }
public void print()
System.out.println(a);
System.out.println(b);
A static member of a class is shared by all objects of the class
Foo f1 = new Foo();
Foo f2 = new Foo();
f1.print(); f2.print(); // ??
f1.inc(); f1.print(); // ??
f2.inc(); f2.print(); // ??

17. Return An Object Example: open an account in a bank
(name, address, initial deposit)
Just like arrays, objects are created on the heap
Returning an object means returning the reference (i.e. the address) of the object

18. Pass Object to Method
Just like arrays, object parameters are pass-by-reference
Example: transfer money from one account to another

19. Keyword this A reference to an object itself
De-shadowing
A reference to a constructor
public class Foo {
int x;
public Foo( int x ) {
this.x = x; }
public Foo() {
this(-1);
The parameter
x shadows the
field x, meaning
x inside the
method refers to
the parameter
instead of the field

20. More About OO Programming
Encapsulation
Inheritance
Polymorphism

21. Readings
Chapter 9 of the textbook