[PDF] Principles of Object-Oriented Programming

8.1 Java Syntax Primer

Attributions

Principles of Object-Oriented Programming

Chapter 1 Introduction

1.1 Abstraction

1.1.1 Introduction

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1.1.2 Data Abstraction

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The product of 4.5 by 7.2 is expressed in Java as:4.5 * 7.2. In this expression, the 9F<

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The problem has not changed in that it still deals with the same geometric shape, a

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are simply the values of the width and the height. The formula to compute the area of width * height

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va to evaluate width and height andverify that their values are set to 0. Once we have deifined the width and height variables, we can solve our problem by

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calc.rectangle(4.5, 7.2) The ifirst line of code deifinescalcas a variable of typeAreaCalcand assign to it an instanceof the classAreaCalc.newis a keyword in Java. It is an example of what is ;9DD=<9;D9KKGH=J9LGJ object) of the given class. The second line of code is a call to the objectcalcto H=J>GJEL@=rectangletask wherewidthis assigned the value 4.5 andheightis

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2;12number of shapes? How can we, as programmers, buid a calculator that can

@9F1.1.2.2 Modeling a Person

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For the ifirst few years of his life, Peter did not have a clue what birthdays were, let

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1public class Person {

2/**

3* All data fields are private in order to prevent code

outside of this

4* class to access them.

5*/

6private int bDay; // birth day

7private int bMonth; // birth month; for example, 3 means

March.

8private int bYear; // birth year

9

10* Constructor: a special code used to initialize the fields

of the class.

11* The only way to instantiate a Person object is to call

new on the constructor.5

12* For example: new Person(28, 2, 1945) will create a Person

object with

13* birth date February 28, 1945.

14*/

15public Person(int day, int month, int year) {

16_bDay = day;

17_bMonth = month;

18_bYear = year;

19} 20/**

21* Uses "modulo" arithmetic to compute the number of months

till the next

22* birth day given the current month.

23* @param currentMonth an int representing the current

month. 24*/

25public int nMonthTillBD(int currentMonth) {

26return (_bMonth - currentMonth + 12) % 12;

27}
28}

Download the above code:

ZPerson.java(http://www

We now explain what the above Java code means.

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1945)will create an instance of a Person with _bDay = 28, _bMonth = 2 and _bYear

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1.2 Objects and Classes

1.2.1 Objects

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In the "real" world, objects are the entities of which the world is comprised. Everything OGJD<

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Objects are the primary units used to create abstract models. "There are a number of schools of object-oriented programming, which difffer slightly

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'=J=O=OADDL9C=9:=@9NAGJAKLGMJL=JEKL9F;= An object is characterized solely by it behaviors. "Essentially this deifines an object by the way it interacts with its world. An object that ;;=KKLGAFL=JF9DDQ ]LAK <9L9AK:=AF?KLGJ=9FGL@=JG:B=;L "7 Note:A beautiful example of a model that exhibits a particular beha vior butwithout exactly replicating the mechanics we expect to produce that beha vioris the "Dragon" optical illusion. A printout to create this simple folded paper display can be found at theweb site of the Grand Illusions T oy Shop in England(http://www.grand-illusions.com/opticalillusions/dragon%20illusion/) This does not mean however, that an object may not contain data (information) in Y=DJ=KMDLG>ALK<9L9K@GMD The existence of data in an object is an implementation technique used to

1.2.2 Classes

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Many objects difffer from each other only in thevalueof the data that they hold. For -MJG:B=;L

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methods. In addition, a class may, but not always, contain descriptions of the internal <9L9@=D<:QL@=G:B=;LK;9DD=E=L@G "8 Turning the description around, we can say that a class is a template or recipe for the ;J=9LAGFG>9H9JLA;MD9JLQH=G>G:B=;L

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instantiate") objects of the type described by the class. Be careful not to make the ;D9KKAK9 ]FL=;@FA;9DL=JEK9;D9KK<=YF=K9F=Otypein the system. Types are identiifies usedquotesdbs_dbs19.pdfusesText_25