Java Class Design Ganesh Samarthyamganesh@codeops.tech

Name the first object oriented programming

language

Simula (1965!)

OOP sometimes becomes

❖ Uh-oh

or

❖ Oops!

Guess the outputclass Base { public Base() { foo(); } public void foo() { System.out.println("In Base's foo "); } }

class Derived extends Base { public Derived() { i = new Integer(10); } public void foo() { System.out.println("In Derived's foo " + i.toString() ); } private Integer i; }

class Test { public static void main(String [] s) { new Derived(); } }

Guess the output

abstract class Printer { private Integer portNumber = getPortNumber();

abstract Integer getPortNumber(); public static void main(String[]s) {

Printer p = new LPDPrinter(); System.out.println(p.portNumber); }

}

class LPDPrinter extends Printer { /* Line Printer Deamon port no is 515 */ private Integer defaultPortNumber = 515; Integer getPortNumber() {

return defaultPortNumber; }

}

Date & time exercise

❖ Your project requires support for date and time functionality❖ Assume that the Java library or third-party library support is

not available for date and time related functionality

❖ Will you provide separate classes for date, time and timezone or a single class that supports all the three? How to support calculating differences between two date/time values?

Class design problem: Example

Discussion example

// using java.util.Date Date today = new Date(); System.out.println(today);

$ java DateUse Wed Dec 02 17:17:08 IST 2015

Why should we get the time and timezone details if I only want a date? Can

I get rid of these parts? No!

So what!Date today = new Date(); System.out.println(today); Date todayAgain = new Date(); System.out.println(todayAgain);

System.out.println(today.compareTo(todayAgain) == 0);

Thu Mar 17 13:21:55 IST 2016 Thu Mar 17 13:21:55 IST 2016 false

What is going on here?

java.time package!

SolutionLocalDate today = LocalDate.now(); System.out.println(today); LocalDate todayAgain = LocalDate.now(); System.out.println(todayAgain); System.out.println(today.compareTo(todayAgain) == 0);

2016-03-17 2016-03-17 true

Works fine now!

Revisiting the example … You can use only date, time, or even timezone, and combine them as

needed!

LocalDate today = LocalDate.now(); System.out.println(today); LocalTime now = LocalTime.now(); System.out.println(now);

ZoneId id = ZoneId.of("Asia/Tokyo"); System.out.println(id);

LocalDateTime todayAndNow = LocalDateTime.now(); System.out.println(todayAndNow);

ZonedDateTime todayAndNowInTokyo = ZonedDateTime.now(ZoneId.of("Asia/Tokyo")); System.out.println(todayAndNowInTokyo);

2016-03-17 13:28:06.927 Asia/Tokyo 2016-03-17T13:28:06.928 2016-03-17T16:58:06.929+09:00[Asia/Tokyo]

More classes in Date/Time API

Avoid switch casespublic'Insets'getBorderInsets(Component'c,'Insets'insets){'

''''''''Insets'margin'='null;'

'''''''''//'Ideally'we'd'have'an'interface'defined'for'classes'which'

''''''''//'support'margins'(to'avoid'this'hackery),'but'we've'

''''''''//'decided'against'it'for'simplicity'

''''''''//'

''''''''if'(c'instanceof'AbstractBuEon)'{'

'''''''''''''''''margin'='((AbstractBuEon)c).getMargin();'

''''''''}'else'if'(c'instanceof'JToolBar)'{'

''''''''''''''''margin'='((JToolBar)c).getMargin();'

''''''''}'else'if'(c'instanceof'JTextComponent)'{'

''''''''''''''''margin'='((JTextComponent)c).getMargin();'

''''''''}'

''''''''//'rest'of'the'code'omiEed'…'

Interface hierarchy for polymorphism

Using runtime polymorphism

!!!!!!!margin!=!c.getMargin();

!!!!!!!!if!(c!instanceof!AbstractBu8on)!{!

!!!!!!!!!!!!!!!!!margin!=!((AbstractBu8on)c).getMargin();!

!!!!!!!!}!else!if!(c!instanceof!JToolBar)!{!

!!!!!!!!!!!!!!!!margin!=!((JToolBar)c).getMargin();!

!!!!!!!!}!else!if!(c!instanceof!JTextComponent)!{!

!!!!!!!!!!!!!!!!margin!=!((JTextComponent)c).getMargin();!

!!!!!!!!}

Assign responsibilities correctlyclassGraphicsDevice

public voidsetFullScreenWindow(Window w){

if(w!=null){

if(w.getShape() !=null) {w.setShape(null); }

if(w.getOpacity() <1.0f){w.setOpacity(1.0f);}

if(!w.isOpaque()) {

Color bgColor =w.getBackground();

bgColor =newColor(bgColor.getRed(),

bgColor.getGreen(),

bgColor.getBlue(), 255);

w.setBackground(bgColor);

}

}

…

}

Keep data members private

Protect from direct external mutations

Ensure each class has a single responsibility

Adhere to IS-A relationship

Depend on abstractions

SOLID Principles

S Single Responsibility Principle

Every object should have a single responsibility and that should be encapsulated by the class

O Open Closed Principle Software should be open for extension, but closed for modification

L Liskov’s Substitution Principle

Any subclass should always be usable instead of its parent class

I Interface Segregation Principle

Many client specific interfaces are better than one general purpose interface

D Dependency Inversion Principle

Abstractions should not depend upon details. Details should depend upon abstractions

Tools to exploreJhawk(Java)

CodeCity(C++,Java,C#)

CppDepend(C++)

Sotograph(C++,Java,C#)

Imagix 4D(C,C++,Java)

Lattix(C/C++, Java,C#)

SolidSX(C++,Java,C#)

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Simian(C/C++, Java,C#,…)

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ganesh@codeops.tech @GSamarthyam

www.codeops.tech slideshare.net/sgganesh

+91 98801 64463 bit.ly/ganeshsg