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.

TNT NS Java Methods

METHODS

Object-Oriented Programming

and Data Structures

2nd AP edition with GridWorld

Objects and Classes

Copyright © 2011 by Maria Litvin, Gary Litvin, and Skylight Publishing All

rights reserved.

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

3-2

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

3-3

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.)

3-4

Objects in the Bug Runner program

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

WI n d OW World Location Help

Le on a grid location to construct or manipulate at |

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

Class

e A blueprint for objects ofa particular type

structure (number, types) of the

attributes

e Defines available behaviors of Its

Class: Car Object: a car

hs

R10 10001719 TA ,

« *, `

? aoe _—

o RB)

String model model = "Mustang"

int numPassengers numPassengers = 0

double amountOfGas amountOfGas = 16.5

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

Report when out of gas

Class vs Object

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

code

(oy the main method

or a constructor or another method)

Class vs Object

same for all of its objects

Bug

the grid Grid

Moves Location

Acts: moves If it can,

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

public class Car 4} name of a class

{ | (and its source file)

Import

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

java.awt.Color javax.swing.JButton

let you refer to library classes by their short

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

Import (cont'd)

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

Import javax.swing.*; and swing packages

import (cont'd)

¢ GridWorld has Its own library (gridworld.jar)

e You need to tell the compiler where to find GridWorld classes:

⁄ SomeClass.java \_

public class SomeClass ——\ Class header

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

\ (behaviors)

public class Actor

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

private int direction;

private Color color;

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.)

e Each field has a name given by the programmer

You name it!

means the field is

shared by all May be present:

objects in the class means the field

is a constant

private Location location;

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

3-2]

Constructors (cont'd)

public class Location <«

Ẳ

private int row;

private int col;

" y public Location (int r, int c)

Constructors (cont'd)

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

Location loc = new Location(3, 5);

JButton(String text, Icon icon)

Creates a button with initial text and an icon

Methods

e Call them for a particular object:

ActorWorld world = new ActorWorld();

Bug bob = new Bug0);

bob.move( );

bob.turn( );

Methods (cont'd)

e The number and types of parameters (a.k.a arguments) passed to a method must match method’s parameters:

public void drawString ( String msg, int x, inty )

g.drawString ("Welcome", 120, 50):

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 ( )

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

program maintenance (making changes to the code)

Methods (cont'd)

e Constructors and methods can call other

public and private methods of the same class

¢ Constructors and methods can call only public methods of another class

Inheritance

e In OOP a programmer can create a new class

by extending an existing class

Superclass (Base class)

Superclass Subclass

(Derived class)

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

3-32

public class UTurnBug extends Bug

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

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

}

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?