Module 5 - Creating a Class Using Encapsulation

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Java Programming Language

Module 5

Creating a Class: Using Encapsulation


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Java Programming Language Module 5, slide 2 of 20Copyright 2007 Sun Microsystems, Inc. All Rights Reserved. Sun Services, Revision G

Objectives

Understand the concept of encapsulation

Implement encapsulation in the Java language Use the statickeyword

Examine staticimports


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Examining the Concept of Encapsulation

Unencapsulated data

year:int

day:int

month:int

symbol:String

name:String

price:double


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Encapsulation Steps

Encapsulation Step 1: Group-Related Data

MyDate

day : int

month : int

year : int

Stock

symbol : String

name : String

price : double


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Encapsulation Steps

Group Data With Behavior

setDay(int) : boolean

setYear(int) : boolean

setMonth(int) : boolean

getDay() : int

getMonth() : int

getYear() : int

MyDate

day : int

month : int

year : int

Verify days in month

setName()

setPrice(double)

Stock(symbol : String)

getName() : String

getPrice() : double

Stock

symbol : String

name : String

price : double

getSymbol() : String


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Encapsulation Steps

Implement Access Control

+setDay(int) : boolean

+setYear(int) : boolean

+setMonth(int) : boolean

+getDay()

+getMonth()

+getYear()

MyDate-day : int

-month : int

-year : int

+setName()

+setPrice(double)

+Stock(symbol : String)

+getName() : String

+getPrice() : double

-symbol : String

-name : String

-price : double

+getSymbol() : String

+ symbol represents external or public access- symbol represents internal or private access


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The Benefits of Encapsulation

Protecting data integrity

Application maintainability


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Implementing Encapsulation in JavaTechnology

The package statement: A class in a package is visible and therefore

accessible to all other classes in the same package.

A class marked public is visible to classes in other

packages. A class not marked public is hidden to classes in

other packages.

The class statement encapsulates attributes,constructors, and methods into a single unit that can becompiled.


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Implementing Encapsulation in JavaTechnology

Access modifiers: Private

Default

Protected Public

Modifier Same Class Same Package Subclass Universe

private Yesdefault Yes Yes

protected Yes Yes Yes

public Yes Yes Yes Yes


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Encapsulation Examples

1 package com.abc.util;2

3 public class Date {4 private int day;56 public Date() {//... }78 public void addDays(int days) { }

9 int getDaysInMonth(int month) { }10 }


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1 package com.abc.brokerage;2

3 public class Stock {4 private String symbol;5 public Stock(String symbol, double price) { }67 public String getSymbol() { }8 public void setSymbol(String symbol) { }

9 }


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1 package com.abc.brokerage;2 importabc.util.Date;

34 class StockAnalyzer {5 private MyDate date;67 double sell(Stock stock, int quantity) { }8 public double buy(Stock stock, int quantity) { }

9 }


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Using the staticKeyword

Class attributes

Class methods


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Class Attributes

1 public class Count {2 private int serialNumber;3 publicstaticintcounter= 0;

45 public Count() {6 counter++;7 serialNumber = counter;8 }9 }

c1 : Count

Count

serialNumber=1

-serialNumber : int

+counter : int = 0

c2 : Count

serialNumber=2

instanceOfinstanceOf


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Class Attributes

Accessing a static field1 public class OtherClass {2 public void incrementNumber() {3 Count.counter++;4 }5 }


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Class Methods

Class method example1 public class Count2 {2 private int serialNumber;3 private static int counter = 0;45 publicstaticintgetTotalCount(){6 return counter;

7 }89 public Count2() {10 counter++;11 serialNumber = counter;12 }


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Class Methods

Accessing a class method1 public class TestCounter {2 public static void main(String[] args) {3 System.out.println("Number of counter is "4 +Count2.getTotalCount());5 Count2 count1 = new Count2();6 System.out.println("Number of counter is "

7 +Count2.getTotalCount());8 }9 }


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Static Initializers

Static blocks execute only once

Static blocks execute in the order of their appearance1 import com.mycompany.utilities.Database.*;2 public class Count4 {3 public static int counter;4 static {5 if (Database.tableExists(?COUNTER?)) {6 counter = Database.getSingleField('COUNTER');7 }8 }9 }10

11 public class TestStaticInit {12 public static void main(String[] args) {13 System.out.println("counter ="+ Count4.counter);14 }15 }


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Static Imports

1 package com.myproject.shapes;2 static import Math.PI:34 public class Circle {5 private double radius;67 public Circle(double radius) {8 this.radius = radius;

9 }1011 public double area() {12 return PI*radius*radius;13 }14

15 public double circumferance {16 return 2*PI*radius;17 }18 }

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Module 5 - Creating a Class Using Encapsulation