Comp 110 Style
description
Transcript of Comp 110 Style
COMP 110STYLE
Instructor: Jason Carter
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INTERFACES AND MORE STYLE Define contracts between our users and
implementers Optional – they may not be used Good style to use them Will study additional elements of style in this
chapter
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TWO WAYS OF DOING THE BMI SPREADSHEET
ABMISpreadsheet is one way to implement the spreadsheet user-interface
Let us create AnotherBMISpreadsheet to illustrate another way
Difference is in number of variables used
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BMI SPREADSHEET
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ABMISPREADSHEETABMISpreadsheet Instance
weight height
getWeight()
setWeight()
getHeight()
setHeight() getBMI()
ObjectEditor
calls
writes
weight
calls
reads
height
calls
reads writes
callsnew
weight
new height
reads
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ANOTHERBMISPREADSHEETAnotherBMISpreadsheet Instance
weight height
getWeight()
setWeight()
getHeight()
setHeight() getBMI()
ObjectEditor
calls
writes
weight
calls
reads
height
calls
readswrites
callsnew
weight
new height
reads
bmi
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METHODS THAT CHANGEABMISpreadsheet Instance
weight height
setWeight()
setHeight() getBMI()
ObjectEditor
writes
calls
writes
callsnew
weight
new height
reads
bmi
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SETWEIGHT()ABMISpreadsheet Instance
weight
setWeight()
ObjectEditor
writes
calls new
weight
bmi
public void setWeight(double newWeight) {
weight = newWeight;bmi = weight / (height*height);
}
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SETHEIGHT()ABMISpreadsheet Instance
height
setHeight()
ObjectEditor
writes
calls new
height
bmi
public void setHeight(double newHeight) {
height = newHeight;bmi = weight / (height*height);
}
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GETBMI()ABMISpreadsheet Instance
getBMI()
ObjectEditor
reads
bmi
public double getBMI() {return bmi;
}
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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; }}
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GRAPHICAL ALGORITHMABMISpreadsheet Instance
weight height
getWeight()
setWeight()
getHeight()
setHeight() getBMI()
ObjectEditor
calls
writes
weight
calls
reads
height
calls
readswrites
callsnew
weight
new height
reads
bmi
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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
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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
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STEPWISE REFINEMENT
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
public void setWeight(double newWeight) {
weight = newWeight;bmi = weight/(height*height);
}
Natural Language
Programming Language
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OBJECTEDITOR USER INTERFACE?public 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; }}
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OBJECTEDITOR USER INTERFACES
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SIMILARITIES IN THE TWO CLASSESpublic 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; }}
public class ABMISpreadsheet { double height; public double getHeight() { return height; } public void setHeight(double newHeight) { height = newHeight; } double weight; public double getWeight() { return weight; } public void setWeight(double newWeight) { weight = newWeight; } public double getBMI() { return weight/(height*height); }}
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REAL-WORLD ANALOGY
manufactures
Corvette Specification
implements
implementsmanufactures
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INTERFACE
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public class AnotherBMISpreadsheet implements BMISpreadsheet { double height, weight, bmi; public double getHeight() { return height; } public void setHeight (double newHeight) { height = newHeight; bmi = weight/(height*height); } …
IMPLEMENTING AN INTERFACE Contract
Parameter names never
matter to Java
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implements
implements
INTERFACE
ABMISpreadsheet
BMISpreadsheetABMISpreadsheet
instance
ABMISpreadsheetinstance
instance of
AnotherBMISpreadsheet
AnotherBMISpreadsheetinstance
AnotherBMISpreadsheetinstance
instance of
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implements
implements
USING INTERFACES TO CLASSIFY
ABMISpreadsheet
BMISpreadsheetBMISpreadsheet
instance
BMISpreadsheetinstance
instance of
AnotherBMISpreadsheet
BMISpreadsheetinstance
BMISpreadsheetinstance
instance of
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USING CAR SPECIFICATIONS TO CLASSIFY
manufactures
Corvette Specification
implements
implementsmanufactures
Corvette
Corvette
Corvette
Corvette
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CANNOT INSTANTIATE SPECIFICATION Cannot order a car from a specification
Must order from factory A car defined by Corvette specification ordered from
factory implementing the specification
Cannot instantiate interface Must instantiate class BMISpreadsheet instance created by instantiating
class implementing interface
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INTERFACE AS A SYNTACTIC SPECIFICATIONpublic class ABMISpreadsheet implements
BMISpreadsheet{ double height; public double getHeight() { return height; } public void setHeight(double newHeight) { height = newHeight; } double weight; public double getWeight() { return weight; } public void setWeight(double newWeight) { weight = newWeight; } public double getBMI() { return weight/(height*height); }}
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INTERFACE AS A SYNTACTIC SPECIFICATIONpublic class ABMISpreadsheet implements
BMISpreadsheet{ double height; public double getHeight() { return height; } public void setHeight(double newHeight) { height = newHeight; } double weight; public double getWeight() { return weight; } public void setWeight(double newWeight) { weight = newWeight; } public double getBMI() { return 13450; }}
Syntactic Contract
Bombay Market Index
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INTERFACE REQUIRED Define interfaces for
All classes (that are instantiated) Some are not Include all public methods
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DIFFERENCES IN THE TWO CLASSESpublic 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; }}
public class ABMISpreadsheet { double height; public double getHeight() { return height; } public void setHeight(double newHeight) { height = newHeight; } double weight; public double getWeight() { return weight; } public void setWeight(double newWeight) { weight = newWeight; } public double getBMI() { return weight/(height*height); }}
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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
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TIME-SPACE TRADEOFF
Time MiserSpace Miser
Space
Time
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TIME-SPACE TRADEOFF
Space
Time
Time MiserSpace Miser
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RELATING INTERFACE AND CLASS NAMESClass Name:
<Qualifier><Interface> - ABMISpreadsheet- ASpaceEfficientBMISpreadsheet- SpaceEfficientBMISpreadsheet
<Interface><Qualifier> Impl- BMISpreadsheetImpl- BMISpreadsheetSpaceEfficientImpl
Interface Name:
<ClassName>Interface- ABMISpreadsheetInterface
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PROGRAMMING STYLE: THE ART OF PROGRAMMING
Science of programming Does the program work correctly?
Art programming Does the program follow “good” style rules?
Good style rules make it easier to Get the program working correctly Change the program
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WRITING STYLE To the point Avoid repetition Good flow Illustrate ideasThere is more than one way to write a document
that conveys some facts: e.g. the influence of BMI on health
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PROGRAMMING STYLE: THE ART OF PROGRAMMING
Define interfaces Make programs
efficient Space vs. time
Comment program Use appropriate
identifier names Use named constants
Variables vs. names Constants vs. magic
numbers
Avoid code repetition Give least privilege
No public instance variables
Implementation independent constants in interfaces
Initialize variables Independent code in
separate method
There is more than one way to write a program that correctly meets its specification: e.g.
implement the BMI user-interface
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PROGRAMMING STYLE: THE ART OF PROGRAMMING
Use Interfaces
Efficiency
Comment Program
Avoid Code Repetition
Giving Least Privilege
Use Named Constants
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COMMENTS
double bmi; //computed by setWeight and setHeight
Single line comments
/* This version recalculates the bmi when weight or height change, not when getBMI is called*/public class AnotherBMISpreadsheet {…}
/* recompute dependent properties */bmi = weight / (height * height);
Arbitrary comments
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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 {…}
You should use this convention
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System.out.println(newHeight); /*debugging statement */
REMOVING DEBUGGING CODE
/* System.out.println(newHeight); /*debugging
statement */ */
/* System.out.println(newHeight); // debugging
statement */
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WHAT TO COMMENT? Any code fragment needing explanation
ClassTop-level algorithm, author, date modified
Variable declarationPurpose, where used, how its value is computed
Method declarationparams, return value, algorithm, author, date modified
Statement sequenceExplanation
Debugging codeSummarizing vs. Elaborating Comments?
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WHAT TO COMMENT?
double w; // weight
double weight; // weight
double weight;
double bmi; // computed by setWeight and setHeight
Bad variable name
Redundant
Self-commenting
Useful comment
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JAVADOC TAGS
/* * @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 tags
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COMMENTING INTERFACES
/* * @param newWeight the new value of the property, weight */public void setWeight (double newWeight) { }
/* * @return the value of the variable, weight */public double getWeight () { }
Implementation independent comments
Commenting both interface and implementation?
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PROGRAMMING STYLE: THE ART OF PROGRAMMING
Use Interfaces
Efficiency
Comment Program
Avoid Code Repetition
Giving Least Privilege
Use Named Constants
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IMPROVING THE STYLEpublic class AnotherBMISpreadsheet implements BMISpreadsheet{ 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; }}
Code repetition
Assuming ABMICalculator does not exist
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IMPROVING THE STYLE (EDIT)public class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; … public void setHeight(double newHeight) { height = newHeight; bmi = calculateBMI(); } … public void setWeight(double newWeight) { weight = newWeight; bmi = calculateBMI(); } public double getBMI() { return bmi; }
double calculateBMI() { return weight/(height*height); }}
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RE-USING CODEpublic class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; public double getHeight() { return height; } public void setHeight(double newHeight) { height = newHeight; bmi = calculateBMI(); } public double getWeight() { return weight; } public void setWeight(double newWeight) { weight = newWeight; bmi = calculateBMI(); } double calculateBMI() { return weight/(height*height); } ….}
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CHANGING RE-USED CODE ONCEpublic class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; … public void setHeight(double newHeight) { height = newHeight; bmi = calculateBMI(); } public double getWeight() { return weight; } public void setWeight(double newWeight) { weight = newWeight; bmi = calculateBMI(); }
double calculateBMI() { return (weight/2.2)/(height * 2.54/100*height*2.54/100); } …}
Declaring vs. calling methods?
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PROGRAMMING STYLE: THE ART OF PROGRAMMING
Use Interfaces
Efficiency
Comment Program
Avoid Code Repetition
Giving Least Privilege
Use Named Constants
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ONLY PUBLIC METHODS IN INTERFACE
public class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; public double getHeight() { return height; } public void setHeight(double newHeight) { height = newHeight; bmi = calculateBMI(); } … double calculateBMI() () { return (weight/2.2)/(height * 2.54/100*height*2.54/100); } …}
Not in interface
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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
ObjectEditor
ABMICalculator User ABMICalculator
getWeight() setWeight()
getHeight() setHeight()
getBMI() computeBMI()
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IMPROVING THE STYLE
Use Interfaces
Efficiency
Comment Program
Use Named Constants
Avoid Code Repetition
Giving Least Privilege
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IMPROVING THE STYLE
public class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; public double getHeight() { return height; } public void setHeight(double newHeight) { height = newHeight; bmi = calculateBMI(); } … double calculateBMI() () { return (weight/2.2)/(height * 2.54/100*height*2.54/100); } …}
Magic numbers
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IMPROVING THE STYLE
public class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; public double getHeight() { return height; } public void setHeight(double newHeight) { height = newHeight; bmi = calculateBMI(); } … double calculateBMI() () { return (weight/LBS_IN_KG) / (height*CMS_IN_INCH/100*height*CMS_IN_INCH/100); } …}
Named constants
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DECLARING 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); } …}
Un-initializing declarationInitializing declaration
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MORE ON 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); } …}
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ACCIDENTAL OR MALICIOUS MODIFICATION
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); } …}
Violating least privilege
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LITERALS VS. NAMED CONSTANTS VS. VARIABLES
Use constants for program values that do not change Use named constants for magic numbers
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MORE CODE REPETITION
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); } …}
Within same method and has the same value
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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 heightInMeters = height*CMS_IN_INCH/100; return (weight/LBS_IN_KG) / (heightInMeters*heightInMeters); } …}
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REMOVING CODE REPETITION
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 heightInMeters = height*CMS_IN_INCH/100; return (weight/LBS_IN_KG) / (heightInMeters*heightInMeters); } …}
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LOCAL VS. GLOBAL VARIABLE
public class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; double heightInMeters = 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) / (heightInMeters*heightInMeters); } …}
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LOCAL VS. GLOBAL VARIABLEpublic class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; double heightInMeters = height*CMS_IN_INCH/100; ... final double LBS_IN_KG = 2.2; final double CMS_IN_INCH = 2.54; … public void setHeight(double newHeight) { heightInMeters = newHeight; bmi = calculateBMI(); } … double calculateBMI() { return (weight/LBS_IN_KG) / (heightInMeters*heightInMeters); } …}
Violating least privilege
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public class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; … public void setHeight(double newHeight) { height = newHeight; bmi = calculateBMI(); } public double getWeight() { return weight; } public void setWeight(double newWeight) { weight = newWeight; bmi = weight/(height*height); } … double calculateBMI () { double heightInMetres = height*CMS_IN_INCH/100; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres); } …}
SCOPE
heightInMeters scope
Not a scope
height scope
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SCOPE OF PUBLIC ITEMS
public class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; … public double getWeight() { return weight; } …}
ObjectEditor
ABMISpreadsheet
getWeight() scope
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IDENTIFIER SCOPE Region of code where the identifier is visible Arbitrary scopes not possible Least Privilege => Make scope as small as
possible
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SCOPEpublic class AnotherBMISpreadsheet implements BMISpreadsheet{ double height, weight, bmi; … public void setHeight(double newHeight) { height = newHeight; bmi = calculateBMI(); } public double getWeight() { return weight; } public void setWeight(double newWeight) { weight = newWeight; bmi = weight/(height*height); } … double calculateBMI() () { double heightInMetres = height*CMS_IN_INCH/100; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres); } …}
heightInMeters scope
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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 = height*CMS_IN_INCH/100; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres); } …}
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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; heightInMeters = height*CMS_IN_INCH/100; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres); } …}
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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; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres); } …}
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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; return (weight/LBS_IN_KG) / (heightInMetres*heightInMetres); } …}
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INITIALIZING ALL VARIABLES (EDIT)
public class AnotherBMISpreadsheet implements BMISpreadsheet{ double height = 60, weight = 70, bmi = getBMI(); … 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); } …}
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INITIALIZING ALL VARIABLES
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); } …}
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PRINTING PROPERTIESpublic class ABMISpreadsheet implements BMISpreadsheet { double height; public double getHeight() { return height; } public void setHeight(double newHeight) { height = newHeight; } double weight; public double getWeight() { return weight; } public void setWeight(double newWeight) { weight = newWeight; } public double getBMI() { return weight/(height*height); }}
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PRINTING PROPERTIES (EDIT)
public class ABMISpreadsheet implements BMISpreadsheet { double height; … public void setHeight(double newHeight) { System.out.println(“Height: “ + newHeight); height = newHeight; } double weight; … public void setWeight(double newWeight) { weight = newWeight; } public double getBMI() { return weight/(height*height); }}
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PRINTING PROPERTIESpublic class ABMISpreadsheet implements BMISpreadsheet { double height; … public void setHeight(double newHeight) { System.out.println(“Weight: “ + weight); System.out.println(“Height: “ + height); double bmi = getBMI(); System.out.println(“BMI: “ + bmi); height = newHeight; } double weight; … public void setWeight(double newWeight) { weight = newWeight; } public double getBMI() { return weight/(height*height); }}
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LESS CLUTTERED CODE (EDIT)public class ABMISpreadsheet implements BMISpreadsheet { double height; … public void setHeight(double newHeight) { System.out.println(“Weight: “ + weight); System.out.println(“Height: “ + height); double bmi = getBMI(); System.out.println(“BMI: “ + bmi); height = newHeight; } double weight; … public void setWeight(double newWeight) { weight = newWeight; } public double getBMI() { return weight/(height*height); }}
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LESS CLUTTERED CODEpublic class ABMISpreadsheet implements BMISpreadsheet { double height; … public void setHeight(double newHeight) { System.out.println(“Weight: “ + weight); System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI()); height = newHeight; } double weight; … public void setWeight(double newWeight) { weight = newWeight; } public double getBMI() { return weight/(height*height); }}
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REMOVING DUPLICATIONpublic class ABMISpreadsheet implements BMISpreadsheet { double height; … public void setHeight(double newHeight) { System.out.println(“Weight: “ + weight); System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI()); height = newHeight; } double weight; … public void setWeight(double newWeight) { System.out.println(“Weight: “ + weight); System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI()); weight = newWeight; } public double getBMI() { return weight/(height*height); }}
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REMOVING DUPLICATION (EDIT)public class ABMISpreadsheet implements BMISpreadsheet { double height; … public void setHeight(double newHeight) { printProperties();height = newHeight; } double weight; … public void setWeight(double newWeight) { printProperties();weight = newWeight; } public double getBMI() { return weight/(height*height); } void printProperties() { System.out.println(“Weight: “ + weight); System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI()); }}
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REMOVING DUPLICATIONpublic class ABMISpreadsheet implements BMISpreadsheet { double height; … public void setHeight(double newHeight) { printProperties(); height = newHeight; } double weight; … public void setWeight(double newWeight) { printProperties(); weight = newWeight; } … void printProperties() { System.out.println(“Weight: “ + weight); System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI()); }}
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SEPARATION OF CONCERNSpublic class ABMISpreadsheet implements BMISpreadsheet { double height; … public void setHeight(double newHeight) { printProperties(); height = newHeight; } double weight; … public void setWeight(double newWeight) { printProperties(); weight = newWeight; } … void printProperties() { System.out.println(“Weight: “ + weight); System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI()); }}
Less cluttered
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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;}
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PRINTING BMI IN GETBMI()
public class ABMISpreadsheet implements BMISpreadsheet { double height; … public double getBMI() { printProperties(); return weight/(height*height); } … void printProperties() { System.out.println(“Weight: “ + weight); System.out.println(“Height: “ + height); System.out.println(“BMI: “ + getBMI()); }}
getBMI() never terminates
Recursive, calls itself indirectly
Infinite recursion
Avoid recursion for now
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NON-PUBLIC INSTANCE VARIABLES
public class ABMISpreadsheet implements BMISpreadsheet { double height, weight, bmi; …}
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MAKING INSTANCE VARIABLES PUBLIC
public class ABMISpreadsheet implements BMISpreadsheet { public double height, weight, bmi; …}
Other classes
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HARD TO CHANGE
public class ABMISpreadsheet implements BMISpreadsheet { public double height, weight; …}
Other classes
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CONSISTENCY CONSTRAINTS VIOLATED
Inconsistent values
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PRECONDITIONS CANNOT BE ENFORCED
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ENCAPSULATION PRINCIPLE Do not make instance variables public
Expose them through public methods
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public final double CMS_IN_INCH = 2.54;
PUBLIC CONSTANTS
public interface BMISpreadsheet {
public final double CMS_IN_INCH = 2.54;
}
Inconsistent value cannot be stored
Implementation independent ABMISpreadsheet
AnotherBMISpreadsheet
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PRINCIPLE Declare implementation-independent named
constants in interfaces implementing classes can access them
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EXTRA SLIDES
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STEPWISE REFINEMENT
Declare two instance variables weight and height
Define a getter method that computes the value of BMI and returns it
public double getBMI() {return weight/(height*height);
}
Natural Language
Programming Language
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REAL-WORLD ANALOGY
manufactures
Corvette Specification
implements
implementsmanufactures