Objects and Classes

Java MethodsJava MethodsA & ABA & AB

Object-Oriented Programmingand Data Structures

Maria Litvin ● Gary Litvin

Copyright © 2006 by Maria Litvin, Gary Litvin, and Skylight Publishing. All rights reserved.

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Objectives:

• See an example of a small program written in OOP style and discuss the types of objects used in it

• 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

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OOP

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

• An object can create other objects.

• An object can call another object’s (and its own) methods (that is, “send messages”).

• An object has data fields, which hold values that can change while the program is running.

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Objects

• Can model real-world objects

• Can represent GUI (Graphical User Interface) components

• Can represent software entities (events, files, images, etc.)

• Can represent abstract concepts (for example, rules of a game, a particular type of dance, etc.)

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Objects in the Dance Studio Program

Dancer

Foot

Dance floorControl panel

Go / Stop button

Dance selection pulldown list

Positioning button

Dance Studio window

Band

Dance group

Waltz, etc.

Dance step

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Classes and Objects

• A class is a piece of the program’s source code that describes a particular type of objects. OO programmers write class definitions.

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

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Class Object• A blueprint for

objects of a particular type

• Defines the structure (number, types) of the attributes

• Defines available behaviors of its objects

Attributes

Behaviors

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Class: Car Object: a car

Attributes: String model Color color int numPassengers double amountOfGas

Behaviors: Add/remove a passenger Get the tank filled Report when out of gas

Attributes: model = "Mustang" color = Color.YELLOW numPassengers = 0 amountOfGas = 16.5

Behaviors:

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Class vs. Object

• A piece of the program’s source code

• Written by a programmer

• An entity in a running program

• Created when the program is running (by the main method or a constructor or another method)

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Class vs. Object

• Specifies the structure (the number and types) of its objects’ attributes — the same for all of its objects

• Specifies the possible behaviors of its objects

• Holds specific values of attributes; these values can change while the program is running

• Behaves appropriately when called upon

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CRC Card

• A preliminary description of a class at the initial stage of program design

Dancer

Controls the left and right foot. Learns dance steps. Knows how to start, make the next step, stop.

Foot Dance

Collaborators

Class

Responsibilities

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Classes and Source Files

• Each class is stored in a separate file

• The name of the file must be the same as the name of the class, with the extension .java

public class Car{ ...}

Car.java By convention, the name of a class (and its source file) always starts with a capital letter.

(In Java, all names are case-sensitive.)

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Libraries

• Java programs are usually not written from scratch.

• There are hundreds of library classes for all occasions.

• Library classes are organized into packages. For example:

java.util — miscellaneous utility classes

java.awt — windowing and graphics toolkit

javax.swing — GUI development package

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import

• 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: import javax.swing.JButton;

...

JButton go = new JButton("Go");

Fully-qualified name

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import (cont’d)

• You can import names for all the classes in a package by using a wildcard .*:

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

• java.lang is imported automatically into all classes; defines System, Math, Object, String, and other commonly used classes.

Imports all classes from awt, awt.event, and swing packages

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public class SomeClass

• Fields

• Constructors

• Methods}

Attributes / variables that define the object’s state; can hold numbers, characters, strings, other objects

Procedures for constructing a new object of this class and initializing its fields

Actions that an object of this class can take (behaviors)

{

Class header

SomeClass.java

import ... import statements

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public class Foot{ private Image picture; private CoordinateSystem coordinates;

public Foot (int x, int y, Image pic) { picture = pic; coordinates = new CoordinateSystem (x, y, pic); }

public void moveForward (int distance) { coordinates.shift (distance, 0); }

public void moveSideways (int distance) { coordinates.shift (0, distance); } ...}

Fields

Constructor

Methods

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Fields

• A.k.a. instance variables

• Constitute “private memory” of an object

• Each field has a data type (int, double, String, Image, Foot, etc.)

• Each field has a name given by the programmer

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private [static] [final] datatype name;

Fields (cont’d)

Usually private

May be present: means the field is a constant

int, double, etc., or an object: String, Image, Foot

You name it!

May be present: means the field is shared by all objects in the class

private Foot leftFoot;

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Constructors

• Short procedures for creating objects of a class

• Always have the same name as the class

• Initialize the object’s fields

• May take parameters

• A class may have several constructors that differ in the number and/or types of their parameters

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Constructors (cont’d)

public class Foot{ private Image picture; private CoordinateSystem coordinates;

public Foot (int x, int y, Image pic) { picture = pic; coordinates = new CoordinateSystem(x, y, pic); } ...}

The name of a constructor is always the same as the name of the class

A constructor can take parameters

Initializes fields

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Constructors (cont’d)

public class Foot{

...

public Foot (int x, int y, Image pic) { ... } ...}

// FootTest.java ... Image leftShoe = ...; ... Foot leftFoot = new Foot (5, 20, leftShoe); ...

An object is created with the new operator

The number, order, and types of parameters must match

Constructor

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Constructors (cont’d)

JButton go = new JButton("Go");

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Methods

• Call them for a particular object:

leftFoot.moveForward(20);

amy.nextStep( );

ben.nextStep( );

go.setText("Stop");

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Methods (cont’d)

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

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

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

{

...

}

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Methods (cont’d)

• A method can return a value to the caller

• The keyword void in the method’s header indicates that the method does not return any value

public void moveSideways(int distance) { ... }

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Encapsulation and Information Hiding

• A class interacts with other classes only through constructors and public methods

• Other classes do not need to know the mechanics (implementation details) of a class to use it effectively

• Encapsulation facilitates team work and program maintenance (making changes to the code)

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Methods (cont’d)

• 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.

Class X

private field

private method

Class Y

public method public method

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Inheritance

• In OOP a programmer can create a new class by extending an existing class

Superclass(Base class)

Subclass(Derived class)

subclass extends superclass

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A Subclass...

• inherits fields and methods of its superclass

• can add new fields and methods

• can redefine (override) a method of the superclass

• must provide its own constructors, but calls superclass’s constructors

• does not have direct access to its superclass’s private fields

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public class Pacer extends Walker{ public Pacer (int x, int y, Image leftPic, Image rightPic) { super (x, y, leftPic, rightPic); }

public void turnAround () { Foot lf = getLeftFoot (); Foot rf = getRightFoot (); lf.turn (180); rf.turn (180); lf.moveSideways (-PIXELS_PER_INCH * 8); rf.moveSideways (PIXELS_PER_INCH * 8); }}

A new method

Calls Walker’s constructor using super

Constructor

Calls Walker’s accessor methods

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public class Walker{ ... public int distanceTraveled() { return stepsCount * stepLength; } ...}

Overrides Walker’s distanceTraveled method

public class Slowpoke extends Walker{ ... public int distanceTraveled() { return super.distanceTraveled ( ) / 10; } ...}

Calls superclass’s distanceTraveled method

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Review:

• Name a few objects used in Dance Studio.

• Name a few library classes that we used in Dance Studio.

• What are import statements used for?

• What is a field? A constructor? A method?

• Which operator is used to construct an object?

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Review (cont’d):

• What is the difference between private and public methods?

• Why are fields usually private?

• What is inheritance?

• Can a subclass add new fields? New methods?

• How does a subclass call its superclass’s constructors?