Lecture Java methods: Object-oriented programming and data structures (2nd AP edition): Chapter 3 - Maria Litvin, Gary Litvin

Chapter 3 - Objects and classes. This chapter gives an introduction to the main Java and OOP concepts but does not require their full understanding. In terms of grasping the big picture, each student proceeds at his or her own pace. The same concepts are covered later in more depth. Here students get a general idea of how things are put together, get a feel for OOP, and work on exercises that require rearranging small pieces of code.

Trang 1

TNT NS Java Methods

METHODS

Object-Oriented Programming and Data Structures

2nd AP edition with GridWorld Maria Litvin e Gary Litvin | pit Cì ¡ @@I)| ‹ A we or <

Objects and Classes

Trang 2

Objectives:

See an example of a GridWorld program,

written in OOP style, and discuss the types of objects used init

Learn about the general structure of a class, its fields, constructors, and methods

Get a feel for how objects are created and how to call their methods

Learn a little about inheritance in OOP

Trang 3

OOP

e An OO program models the application as a world of interacting objects

e An object can create other objects

e An object can call another object’s (and Its own) methods (that Is, “send messages’) e An object has data fields, which hold values

that can change while the program Is running

Trang 4

Objects

e Can model real-world objects

e Can represent GUI (Graphical User Interface) components

e Can represent software entities (events, files, Images, etc.)

e Can represent abstract concepts (for

example, rules of agame, a location ona grid, etc.)

Trang 5

Objects in the Bug Runner program

GridWorld = | ~~ @ Griaworta [.JIp]k

WI n d OW World Location Help

Trang 6

Classes and Objects

° A class IS a plece of the program’s source code that describes a particular type of

objects OO programmers write class definitions

e An object is called an instance of a class A program can create and use more than one object (instance) of the same class

Trang 8

Class: Car Object: a car hs R10 10001719 TA , « *, ` ? eS aoe _— o RB) ©) Attributes: Attributes:

String model model = "Mustang"

Color color color = Color YELLOW

int numPassengers numPassengers = 0

double amountOfGas amountOfGas = 16.5

Behaviors: Behaviors:

Add/remove a passenger | <« e Get the tank filled

Report when out of gas

Trang 9

Class vs Object

e A piece of the program's source e Anentity ina running program code

e Written by a e Created when the

programmer program Is running

(oy the main method or a constructor or another method)

Trang 10

Class vs Object

° Specifies the ¢ Holds specific

Structure (the values of attributes;

number and types) these values can

of its objects’ change while the

attributes — the program Is running

same for all of its objects

s Specifies the e Behaves

possible behaviors appropriately when

of its objects called upon

Trang 11

CRC Card

e A preliminary description of a class at the Initial stage of program design Class ` Responsibilit ` les Bug

Occupies a location in Actor

Trang 12

Classes and Source Files

e Each class Is stored in a separate file

e The name of the file must be the same as the

name of the class, with the extension java

/ Car java By convention, the

public class Car 4} name of a class

{ | (and its source file)

Trang 13

Libraries e Java programs are usually not written from scratch e There are hundreds of library classes for all occasions e Library classes are organized into packages For example:

Trang 14

Import

e Full library class names include the package name For example:

java.awt.Color

javax.swing.JButton

° Import statements at the top of the source file

let you refer to library classes by their short

NaMes: Fully-qualified

import javax.swing.JButton; name JButton go = new JButton("Go");

Trang 15

Import (cont'd)

e You can import names for all the classes ina package by using a wildcard *:

Import Java.awI.*; Imports all classes

import java.awt.event.*: from awt, awt.event,

Import javax.swing.*; and swing packages

° java.lang is imported automatically into all classes; defines System, Math, Object,

String, and other commonly used classes

Trang 16

import (cont'd)

Trang 17

⁄ SomeClass.java \_

Import import statements

public class SomeClass ——\ Class header

Ẳ Attributes / variables that define the

se Fields ——— object’s state; can hold numbers,

characters, strings, other objects Procedures for constructing

° Constructors a new object of this class

and initializing its fields Actions that an object

se Methods of this class can take

\ (behaviors)

Trang 18

public class Actor

Ẳ

private Grid<Actor> grid; Fields private Location location;

Trang 19

Fields

e A.k.a instance variables

° Constitute “private memory” of an object

e Each field has a data type (int, double, String, Color, Location, etc.)

Trang 20

You name it! Fields (cont'd) \ private [static] [final] datatype name; Usually - private int, double, etc., or an object: String,

May be present: Location, Color

means the field is

shared by all May be present:

objects in the class means the field

is a constant

private Location location;

Trang 21

Constructors e Short procedures for creating objects of a class

e Always have the same name as the class e Initialize the object’s fields

e May take parameters

e Aclass may have several constructors that differ in the number and/or types of their

parameters

Trang 22

Constructors (cont'd)

public class Location <«

Ẳ

Trang 23

Constructors (cont'd)

An object is created with // BugR geet Java [ i the new operator

Location loc = new Location(3, 5);

| The number, order, and

Trang 24

lu fi |] Java™ Platform —' c— | Constructors (cont'd) Naeclaaac —— saul (>) JAAD ReESUN "—— JAXBSource ^ JButton JCheckBox IMherkRayvMieniiltam Constructor Summary JButton () Creates a button with no set text or icon JButton(Action a) Creates a button where properties are taken from the Action supplied JButton(Icon icon) Creates a button with an icon — > | TButton(sString text) Creates a button with text

JButton(String text, Icon icon)

Trang 25

Methods

e Call them for a particular object:

ActorWorld world = new ActorWorld();

Bug bob = new Bug0);

world.add (new Location(3, 5), bob);

bob.move( ); bob.turn( );

Trang 26

Methods (cont'd)

Trang 27

Methods (cont'd)

e A method can return a value to the caller

e The keyword void in the method’s header

indicates that the method does not return any value

public void drawString ( )

Trang 28

Encapsulation and Information Hiding

e Aclass interacts with other classes only through constructors and public methods

e Other classes do not need to know the

mechanics (implementation details) of a class to use It effectively

e Encapsulation facilitates team work and

program maintenance (making changes to the code)

Trang 29

Methods (cont'd)

e Constructors and methods can call other

public and private methods of the same class ¢ Constructors and methods can call only

Trang 30

Inheritance

Trang 32

A Subclass e inherits fields and methods of its Superclass

s can add new fields and methods

s can redefine (override) a method of the Superclass

° musf provide Ifs own constructors, Dbut calls superclass's constructors

e does not have direct access to Its

Superclass’s private fields

Trang 33

public class UTurnBug extends Bug { public UTurnBug (Color bugColor) Constructor { setColor (bugColor); } public void turnAround () A new { method

turn(); turn(); turn(); turn();

// Or: setDirection (getDirection() + 180);

}

Trang 34

Review: e Name a few objects used tin GridWorld’s BugRunner e Name a few library classes used In GridWorld

e What are import statements used for?

e What Is a field? A constructor? A method? e Which operator is used to construct an

object?