program style identifier names comments named constants avoiding code repetition giving least...

Program Style

• Identifier Names

• Comments

• Named Constants

• Avoiding Code Repetition

• Giving Least Privilege

• Efficiency

• Interfaces

Writing Style

• To the point.

• Avoid repetition.

• Good flow.

• Illustrate ideas.

Assumes more than one way to write a document.

BMI Spreadsheet

ABMISpreadsheet

ObjectEditor

ABMISpreadsheet Instance

weight height

setWeight()

new Weight

calls

writes

setHeight()

new Height

calls

writes

height

getHeight()

calls

reads

getWeight()

reads

weight calls

reads

getBMI()

AnotherBMISpreadsheet

ObjectEditor


weight height

setWeight()

new Weight

calls

writes

setHeight()

new Height

calls

writes

height

getHeight()

calls

reads

getWeight()

reads

weight calls

getBMI()

bmi

reads

Methods that Changes

ObjectEditor


weight height

setWeight()

new Weight

calls

writes

setHeight()

new Height

calls

writes

getBMI()

bmi

reads

setWeight()

ObjectEditor


weight

setWeight()

new Weight

calls

writes

bmi

public void setWeight(double newWeight) {weight = newWeight;bmi = weight / (height*height);

}

setHeight()

ObjectEditor


height

setHeight()

new Height

calls

writes

bmi

public void setHeight(double newHeight) {height = newHeight;bmi = weight / (height*height);

}

getBMI()

ObjectEditor


getBMI()

bmi

reads

public double getBMI() {return bmi;

}

Complete Codepublic class AnotherBMISpreadsheet {

double height, weight, bmi;public double getHeight() {

return height;}public void setHeight(double newHeight) {

height = newHeight;bmi = weight/(height*height);

}public double getWeight() {

return weight;}public void setWeight(double newWeight) {

weight = newWeight;bmi = weight/(height*height);

}public double getBMI() {

return bmi;}

}

Graphical Algorithm

ObjectEditor


weight height

setWeight()

new Weight

calls

writes

setHeight()

new Height

calls

writes

height

getHeight()

calls

reads

getWeight()

reads

weight calls

getBMI()

bmi

reads

English Algorithm

• Declare three instance variables, weight, height and, bmi.• Define three getter methods return values of the three instance variables.• Define two setter methods to change weight and height.•The setter methods assign new weight and height values and compute and assign new BMI value to bmi.

Algorithm

• Description of solution to a problem.• Can be in any “language”

• graphical• natural or programming language• natural + programming language (pseudo code)

• Can describe solution to various levels of detail.

Real-World Algorithm

• Enter Class • Distribute handouts • Set up laptop projection. • Revise topics learnt in the last class. • Teach today’s topics.• Leave Class

Stepwise Refinement

• Declare three instance variables, weight, height and, bmi.• The three getter methods return values of the three instance variables.• The setter methods assign new weight and height values and compute and assign new BMI value to bmi.

Natual Language

public void setWeight(double newWeight) {weight = newWeight;bmi = weight/(height*height);

}

Programming Language

Graphical Algorithmweight

setWeight()

writes

getWeight()

reads

BMI

Object Editor User-Interface?public class AnotherBMISpreadsheet {








return bmi;}

}

Object Editor User Interface

Similarities in the two Classespublic class ABMISpreadsheet {

double height;public double getHeight() {


height = newHeight;}double weight;public double getWeight() {


weight = newWeight;}public double getBMI() {

return weight/(height*height);}

}

Similarities in the two Classespublic class AnotherBMISpreadsheet {








return bmi;}

}

Interface

Implementing an Interfacepublic class AnotherBMISpreadsheet implements BMISpreadhsheet {


return height;}public void setHeight (double newHeight) {

height = newHeight;bmi = calculateBMI();



weight = newWeight;bmi = calculateBMI();


return bmi;}

}

parameter names never matter to Java

contract

manufactures

Real-World Analogy

AccordSpecification

implements

Interface


Instance

AnotherBMISpreadsheet instance of


Instance

ABMISpreadsheet instance of

ABMISpreadsheet

Instance

ABMISpreadsheet

InstanceBMISpreadsheet

implements

Using Interface to Classify

BMISpreadsheet

Instance

AnotherBMISpreadsheet instance of

BMISpreadsheet

Instance

ABMISpreadsheet instance of

BMISpreadsheet

Instance

BMISpreadsheet

InstanceBMISpreadsheet

implements

Using Car Specification to Classify

manufactures

AccordSpecification

implements

AccordAccord

AccordAccord

Cannot Instantiate Specification

Cannot order car from a specification

•Must order from factory.

•A car defined by Accord specification ordered from factory implementing the specification.

Cannot instantiate interface

•Must instantiate class.

•BMISpreadsheet instance created by instantiating class implementing interface.

public class ABMISpreadsheet implements BMISpreadsheet{double height, weight;public double getHeight() {


height = newHeight; }public double getWeight() {


weight = newWeight; }public double getBMI() {

return height/(weight*weight);}

}

Interface as Syntactic Specification

public class ABMISpreadsheet implements BMISpreadsheet{double height, weight;public double getHeight() {


height = newHeight; }public double getWeight() {


weight = newWeight; }public double getBMI() {

return 3245.4}

}

Interface as Syntactic Specification

Syntactic Contract

Bombay Market Index?

Define interface for:– All classes (that are instantiated.)– Some are not.– Include all public instance methods

Interface Required

Differences in the Two Classespublic class ABMISpreadsheet implements BMISpreadsheet {







}

Differences in the Two Classespublic class AnotherBMISpreadsheet implements BMISpreadsheet {








return bmi;}

}

ABMISpreadsheet Vs AnotherBMISpreadSheet

• ABMISpreadsheet uses less space (variables)• Getter methods of AnotherBMISpreadhseet are faster.• Setter methods of ABMISpreadsheet are faster.• Usually getter methods are called more often that setter methods - e.g when ObjectEditor refresh command is executed.•Typically AnotherBMISpreadsheet will be faster, overall.

Time-Space Tradeoff

Time MiserSpace Miser

Space

Time

Space

Time

Time MiserSpace Miser

Time-Space Tradeoff

Relating Interface and Class Names

Class Name:•<Qualifier><Interface> (ABMISpreadsheet, ASpaceEfficientBMISpreadsheet, SpaceEfficientBMISpreadsheet)•<Interface><Qualifier>Impl (BMISpreadsheetImpl, BMISpreadsheetSpaceEfficientImpl)

Interface Name:•<ClassName>Interface (ABMISpreadsheetInterface)

Comments

double bmi; //computed by setWeight and setHeight

Single-line comment

/* recompute dependent properties */bmi = weight / (height*height);

Arbitrary comment

/* This version recalculates the bmi when weight or height change, not when getBMI is called*/public class AnotherBMISpreadsheet {…}

Removing Debugging Code

/*System.out.println(newHeight); // debugging

statement*/

System.out.println(newHeight); /*debugging statement */

/*System.out.println(newHeight); /*debugging

statement */*/

Javadoc Conventions

/* This version recalculates the bmi * when weight or height change, not when * getBMI is called */public class AnotherBMISpreadsheet {…}

/* This version recalculates the bmi when weight or height change, not when getBMI is called*/public class AnotherBMISpreadsheet {…}

What to Comment?Any code fragment needing explanation:

•class

•top-level algorithm, author, date modified

•variable declaration

•purpose, where used

•method declaration

•params, return value, algorithm, author, date modified

•statement sequence

•explanation

•Debugging code

What to Comment?

double w; // weight

double weight; // weight

double bmi; // computed by setWeight() and setHeight()

double weight; Self Commenting

Redundant

Bad Variable Name

Useful Comment

/* * @author Prasun Dewan * @param newWeight the new value of the property, weight. * sets new values of the variables, weight and bmi */public void setWeight (double newWeight) {

…}

/* * @author Prasun Dewan * @return the value of the variable, weight */public double getWeight () {

…}

Javadoc TagsJavadoc tags

Improving the Stylepublic class AnotherBMISpreadsheet implements BMISpreadsheet {








return bmi;}

}

Code Repetition

Why Avoid Code Duplication?

• Less Typing

• Changes (Inches, LB)

•can forget change all repetitions

Example Changes: LB, Inches

public class AnotherBMISpreadsheet implements BMISpreadsheet {double height, weight, bmi;public double getHeight() {






}}

bmi = (weight/2.2)/(height * 2.54/100*height*2.54/100);

bmi = (weight/2.2)/(height * 2.54/100*height*2.54/100);

How to avoid code duplication?



height = newHeight;bmi = (weight/2.2)/(height * 2.54/100*height*2.54/100);



weight = newWeight;bmi = (weight/2.2)/(height * 2.54/100*height*2.54/100);

}}

How to avoid code duplication? (edit)public class AnotherBMISpreadsheet implements BMISpreadsheet {



height = newHeight;bmi = computeBMI();



weight = newWeight;bmi = computeBMI();

}public double computeBMI() {

return (weight/2.2)/(height * 2.54/100*height*2.54/100); }

}

How to avoid code duplication?







}void calculateBMI() {


Principle of Least Privilege

• Do not give a user of some code more rights than it needs.

•Code is easier to change.

•Need to learn less to use code.

•Less likelihood of accidental or malicious damage to program.

• Like hiding engine details from car driver.

ABMICalculator

ObjectEditor

setWeight()

setHeight()getHeight()

getWeight()

getBMI() calculateBMI()computeBMI()

ABMICalculator User

Only Public Methods in Interfacepublic class AnotherBMISpreadsheet implements BMISpreadhsheet {


return height;}public void setHeight (double newHeight) {






return bmi;}double calculateBMI() {

return weight/ (height*height);}

}

not in interface

Method Invocation Syntax

System.out.println(“setWeight called”);

Actual Parameter

Method Name

Target Object

bmi = calculateBMI()

parameterless method

Internal Call

External Call

Function Return Value

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;public double getHeight() {









}

Improving the Style

Magic numbers?









return (weight/LBS_IN_KG) / (height*CMS_IN_INCH/100*height*CMS_IN_INCH/100);

}}

Improving the Style

Named Constants

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() {

return (weight/LBS_IN_KG) / (height*CMS_IN_INCH/100*height*CMS_IN_INCH/100);

}

}

Declaring Named Constants

Initializing Declaration

Un-initializing Declaration

All Caps by Conventions

Cannot Change Initial Value

Variables Vs Named Constants

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...double lbsInKg = 2.2;double cmsInInch = 2.54;double calculateBMI() { return (weight/lbsInKg) / (height*cmsInInch/100*height*cmsInInch/100);}

}

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...double lbsInKg = 2.2;double cmsInInch = 2.54;double calculateBMI() { lbsInKg = 22; return (weight/lbsInKg) / (height*cmsInInch/100*height*cmsInInch/100);}

}

Accidental or Malicious Modification

Violating Least Privilege

Constant

return (weight/lbsInKg) / (height*cmsInInch/100*height*cmsInInch/100);

return (weight/2.2) / (height*2.54/100*height*2.54/100);

Literals, Named Constants, Constants, Variables

Variable

Literal

return (weight/LBS_IN_KG) / (height*CMS_IN_INCH/100*height*CMS_IN_INCH/100)

Named Constant

Literals Vs Named Constants Vs Variables

Use constants for program values that do not change.– Use named constants for magic numbers

return (weight/2.2) / (height*2.54/100*height*2.54/100);

What is a Magic Number?

return (weight/LBS_IN_KG) / (height*CMS_IN_INCH/100*height*CMS_IN_INCH/100)

Natural Constants

return hoursWorked*hourlyWage + 50;

return hoursWorked*hourlyWage + BONUS;

Human-Created Constant

System.out.println (“Bonus: “ + 50);

• Human-created constant is a magic number.

• Natural constant may be a magic number to some.

What is a magic number?

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { return (weight/LBS_IN_KG) /

(height*CMS_IN_INCH/100*height*CMS_IN_INCH/100) ;}

}

More Code Repetition

Within Same Method and Has Same Value

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { return (weight/LBS_IN_KG) /

(height*CMS_IN_INCH/100*height*CMS_IN_INCH/100) ;}

}

Removing Code Repetition (edit)

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { double heightInMetres = height*CMS_IN_INCH/100; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres) ;}

}

Removing Code Repetition

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;double heightInMetres = height*CMS_IN_INCH/100; ...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres) ;}

}

Local Vs Global Variable

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;double heightInMetres = height*CMS_IN_INCH/100;public void setHeight(double newHeight) {

height = heightInMetres;bmi = calculateBMI();

}

...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres) ;}

}

Local Vs Global Variable

Violating least privilege









double heightInMetres = height*CMS_IN_INCH/100;return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres);

}}

Scope

heightInMetres Scope

height Scope

Not a Scope


return height;}...

}

Scope of Public Items getHeight() Scope

ObjectEditor

ABMISpreadsheet

Identifier Scope

• Region of code where the identifier is visible.

• Arbitrary scopes not possible

• Least Privilege => Make scope as small as possible

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;public void setHeight(double newHeight) {

double height = heightInMetres;bmi = calculateBMI();

}

...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { double heightInMetres = height*CMS_IN_INCH/100; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres) ;}

}

Scope

heightInMetres Scope

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { double heightInMetres = height*CMS_IN_INCH/100; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres) ;}

}

Initializing Declaration

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { double heightInMetres; heightInMetres = height*CMS_IN_INCH/100; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres) ;}

}

Un-initializing Declaration

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { double heightInMetres; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres) ;}

}

Un-initialized Variable

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { double heightInMetres = height*CMS_IN_INCH/100;; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres) ;}

}

Initializing all Variables

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height, weight, bmi;...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { double heightInMetres = height*CMS_IN_INCH/100;; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres) ;}

}

Initializing all Variables (edit)

public class AnotherBMISpreadsheet implements BMISpreadsheet{double height = 70, weight = 160, bmi = calculateBMI();...final double LBS_IN_KG = 2.2;final double CMS_IN_INCH = 2.54;double calculateBMI() { double heightInMetres = height*CMS_IN_INCH/100;; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres) ;}

}

Initializing all Variables

Printing Propertiespublic class ABMISpreadsheet implements BMISpreadsheet {







}

Printing Properties (edit)public class ABMISpreadsheet implements BMISpreadsheet {



System.out.println(“bmi:” + getBMI());height = newHeight;

}double weight;public double getWeight() {




}

Printing Propertiespublic class ABMISpreadsheet implements BMISpreadsheet {



System.out.println(“Weight: “ + weight);System.out.println(“Height: “ + height);double bmi = getBMI();System.out.println(“BMI: “ + bmi);height = newHeight;





}

Less Cluttered Code (edit)public class ABMISpreadsheet implements BMISpreadsheet {



System.out.println(“Weight: “ + weight);System.out.println(“Height: “ + height);double bmi = getBMI();System.out.println(“BMI: “ + bmi);height = newHeight;





}

Less Cluttered Codepublic class ABMISpreadsheet implements BMISpreadsheet {



System.out.println(“Weight: “ + weight);System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI());height = newHeight;





}

Removing Duplicationpublic class ABMISpreadsheet implements BMISpreadsheet {






System.out.println(“Weight: “ + weight);System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI());weight = newWeight;



}

Removing Duplication (edit)public class ABMISpreadsheet implements BMISpreadsheet {






System.out.println(“Weight: “ + weight);System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI());weight = newWeight;



}

Removing Duplicationpublic class ABMISpreadsheet implements BMISpreadsheet {



printProperties();height = newHeight;



printProperties();weight = newWeight;


return weight/(height*height);}void printProperties() {

System.out.println(“Weight: “ + weight);System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI());

}}

Separation of Concernspublic class ABMISpreadsheet implements BMISpreadsheet {







return weight/(height*height);}void printProperties() {


}}

Less Cluttered

Independent Code = Method

Separate Method for:

• any independent piece of code.

• even if it is not duplicated.

• if it is more than one line.

public void setWeight(double newWeight) {System.out.println(“Weight: “ + weight);weight = newWeight;

}

public void setWeight(double newWeight) {printWeight();weight = newWeight;

}

Printing BMI in getBMI()public class ABMISpreadsheet implements BMISpreadsheet {







printProperties();return weight/(height*height);

}void printProperties() {


}}

getBMI() never terminates

Recursive (calls itself, indirectly)

Infinite Recursion

Avoid recursion for now

public class ABMISpreadsheet implements BMISpreadsheet {

public double height, weight, bmi;

...

}

Non Public Instance Variables

public class ABMISpreadsheetWithPublicVariables {

public double height, weight, bmi;

...

}

Making Instance Variables Public

Other Classes

public class ABMISpreadsheetWithPublicVariables {

public double height, weight;

...

}

Hard to Change

Other Classes

Consistency Constraints Violated

Inconsistent Values

Do not make instance variables public– Expose them through public methods

Encapsulation Principle

public final double CMS_IN_INCH = 2.54;

Public ConstantsInconsistent value cannot be stored

Implementation Independent

public interface BMISpreadsheet {

}

ABMISpreadsheet AnotherBMISpreadsheet

Declare implementation-independent named constants in interfaces– implementing classes can access them.

Principle

program style identifier names comments named constants avoiding code repetition giving least...

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