STREAM STREAM A software process The Mañana Principle The Mañana Principle Don’t try to...

STREAMSTREAMA software process

The Mañana PrincipleThe Mañana PrincipleDon’t try to everything at

once!

Static MethodsStatic MethodsStateless Utility Functions

S T R E A MS T R E A M

S S tubstubs

T T estsests

R R epresentationepresentation

E E valuationvaluation

A A ttributesttributes

M M ethodsethods

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

Example

• Wanted: a class to represent a Wanted: a class to represent a DateDate..

• Must have the following two methods:Must have the following two methods:

- - setToNextDaysetToNextDay : no parameters: no parameters

- - toStringtoString : no parameters, returns String : no parameters, returns String withwith format format “<year>-<month>-“<year>-<month>-<day>”<day>”


1. Stubsclass Date {class Date {

/** /** * Advance the date to the next day * Advance the date to the next day */ */ public void setToNextDay () { public void setToNextDay () { } }

/** /** * @return A representation of this date * @return A representation of this date * in the format * in the format <year>-<month>-<day><year>-<month>-<day> */ */ public String toString () { public String toString () { return null; return null; } }

}}

class Date {class Date {

/** /** * Advance the date to the next day * Advance the date to the next day */ */ public void setToNextDay () { public void setToNextDay () { } }

/** /** * @return A representation of this date * @return A representation of this date * in the format * in the format <year>-<month>-<day><year>-<month>-<day> */ */ public String toString () { public String toString () { return null; return null; } }

}}

This will compile!

This will compile!

This will compile!

This will compile!


2. Tests

The The DateDate class can already be compiled. Using class can already be compiled. Using BlueJBlueJ, we can immediately create an instance and , we can immediately create an instance and start testing its methods. Of course, they won’t do start testing its methods. Of course, they won’t do much just yet!much just yet!

The The DateDate class can already be compiled. Using class can already be compiled. Using BlueJBlueJ, we can immediately create an instance and , we can immediately create an instance and start testing its methods. Of course, they won’t do start testing its methods. Of course, they won’t do much just yet!much just yet!

Make a list of some simple tests we would like to Make a list of some simple tests we would like to make.make.Make a list of some simple tests we would like to Make a list of some simple tests we would like to make.make.

To carry out those tests, what else will we need in To carry out those tests, what else will we need in the the DateDate class? class?To carry out those tests, what else will we need in To carry out those tests, what else will we need in the the DateDate class? class?

We’ll need a way We’ll need a way to set a particular dateto set a particular date – only then – only then can we tell if can we tell if setToNextDaysetToNextDay and and toStringtoString work. work.We’ll need a way We’ll need a way to set a particular dateto set a particular date – only then – only then can we tell if can we tell if setToNextDaysetToNextDay and and toStringtoString work. work.


2. TestsSo, So, just by thinking about testingjust by thinking about testing, we see the need , we see the need for another method. This is cohesion in action!for another method. This is cohesion in action!So, So, just by thinking about testingjust by thinking about testing, we see the need , we see the need for another method. This is cohesion in action!for another method. This is cohesion in action!


/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) { (int day, int month, int year) { } }

... setToNextDay method (as before)... setToNextDay method (as before)

... toString method (as before) ... toString method (as before) }}






3. Representations

R1: R1: use three integer fieldsuse three integer fields

- day (1..31), month (1..12), year (1..)- day (1..31), month (1..12), year (1..)



R2: R2: use one integer fielduse one integer field

- the number of days since 1- the number of days since 1stst. January, 1970. January, 1970



Many others are possible …Many others are possible …Many others are possible …Many others are possible …


3. EvaluateR1: R1: use three integer fieldsuse three integer fields








Implementation Implementation EffortEffort R1R1 R2R2

setToNextDaysetToNextDay

toString


The TEACHTEACH Scale

difficultydifficulty

TrivialTrivialTrivialTrivial

EasyEasyEasyEasy

AverageAverageAverageAverage

ChallengingChallengingChallengingChallenging

HardHardHardHard


3. EvaluateR1: R1: use three integer fieldsuse three integer fields








Implementation Implementation EffortEffort R1R1 R2R2

setToNextDaysetToNextDay

toString

Challenging

HardTrivial

Trivial


5. Attributes (we call them fields)


private int day; private int day; // 1 ≤ day ≤ daysInMonth// 1 ≤ day ≤ daysInMonth private int month; private int month; // 1 ≤ month ≤ 12// 1 ≤ month ≤ 12 private int year; private int year; // 1 ≤ year// 1 ≤ year ... setDate (as before)... setDate (as before)




private int day; private int day; // 1 ≤ day ≤ daysInMonth// 1 ≤ day ≤ daysInMonth private int month; private int month; // 1 ≤ month ≤ 12// 1 ≤ month ≤ 12 private int year; private int year; // 1 ≤ year// 1 ≤ year ... setDate (as before)... setDate (as before)










private int date; private int date; // days since 1970-01-01// days since 1970-01-01

... setDate (as before)... setDate (as before)














private int day; private int day; // 1 ≤ day ≤ daysInMonth// 1 ≤ day ≤ daysInMonth private int month; private int month; // 1 ≤ month ≤ 12// 1 ≤ month ≤ 12 private int year; private int year; // 1 ≤ year// 1 ≤ year

... setDate and setToNextDay methods... setDate and setToNextDay methods

/** /** * * @return A representation of this date@return A representation of this date * in the format * in the format <year>-<month>-<day><year>-<month>-<day> */ */ public String toString () { public String toString () { } } }}






private int day; private int day; // 1 ≤ day ≤ daysInMonth// 1 ≤ day ≤ daysInMonth private int month; private int month; // 1 ≤ month ≤ 12// 1 ≤ month ≤ 12 private int year; private int year; // 1 // 1 ≤≤ year year


/** /** * * @return A representation of this date@return A representation of this date * in the format * in the format <year>-<month>-<day><year>-<month>-<day> */ */ public String toString () { public String toString () { return year + "-" + month + "-" + day;return year + "-" + month + "-" + day; } } }}




/** /** * * @return A representation of this date@return A representation of this date * in the format * in the format <year>-<month>-<day><year>-<month>-<day> */ */ public String toString () { public String toString () { return year + "-" + month + "-" + day;return year + "-" + month + "-" + day; } } }}

6. Method ImplementationR1R1

Trivial

Trivial

Trivial

Trivial



private int date; private int date; // days since 1970-01-01 // days since 1970-01-01










/** /** * * @return A representation of this date@return A representation of this date * in the format * in the format <year>-<month>-<day><year>-<month>-<day> */ */ public String toString () { public String toString () { ... tricky !!!... tricky !!! } } }}






HardHardHardHard




... setDate and toString methods... setDate and toString methods

/** /** * Advance the date to the next day * Advance the date to the next day */ */ public void setToNextDay () { public void setToNextDay () { } } }}








/** /** * Advance the date to the next day * Advance the date to the next day */ */ public void setToNextDay () { public void setToNextDay () { date = date + 1;date = date + 1; } } }}






Trivial

Trivial

Trivial

Trivial













/** /** * Advance the date to the next day * Advance the date to the next day */ */ public void setToNextDay () { public void setToNextDay () { day = day + 1;day = day + 1; ... more stuff needed... more stuff needed } } }}






Challenging

Challenging

Challenging

Challenging


The Mañana Principle

When – during When – during implementation of a method implementation of a method – we wish we had a certain – we wish we had a certain support method,support method, write our write our

code as if we had itcode as if we had it. . Implement it later.Implement it later.




Special Case Rule:Special Case Rule:Often, we need to identify Often, we need to identify

and deal with a special case.and deal with a special case. Write the code for this in a Write the code for this in a

separate methodseparate method. . Implement it later.Implement it later.














/** /** * Advance the date to the next day * Advance the date to the next day */ */ public void setToNextDay () { public void setToNextDay () { day = day + 1;day = day + 1; checkDayOverflow ()checkDayOverflow ();; } } }}




/** /** * Advance the date to the next day * Advance the date to the next day */ */ public void setToNextDay () { public void setToNextDay () { day = day + 1;day = day + 1; checkDayOverflow ()checkDayOverflow ();; } } }}


Challenging

Challenging

Challenging

Challenging


/** /** * Advance the date to the next day * Advance the date to the next day */ */ public void setToNextDay () { public void setToNextDay () { day = day + 1; day = day + 1; checkDayOverflow ()checkDayOverflow ();; } }



/** /** * * Spot and deal with special day caseSpot and deal with special day case */ */ private void checkDayOverflow () { private void checkDayOverflow () { if (day > if (day > 3030) {) { day = 1; day = 1; month = month + 1; month = month + 1; } } } }




Challenging

Challenging

Challenging

Challenging

Incomplete

Incomplete

Incomplete

Incomplete




/** /** * * Spot and deal with special day caseSpot and deal with special day case */ */ private void checkDayOverflow () { private void checkDayOverflow () { if (day > if (day > 3030) {) { day = 1; day = 1; month = month + 1; month = month + 1; checkMonthOverflow ()checkMonthOverflow ();; } } } }

/** /** * * Spot and deal with special month caseSpot and deal with special month case */ */ private void checkMonthOverflow () { private void checkMonthOverflow () { if (month > 12) {if (month > 12) { month = 1; month = 1; year = year + 1; year = year + 1; } } } }


/** /** * * Spot and deal with special month caseSpot and deal with special month case */ */ private void checkMonthOverflow () { private void checkMonthOverflow () { if (month > 12) {if (month > 12) { month = 1; month = 1; year = year + 1; year = year + 1; } } } }

R1R1

Incomplete

Incomplete

Incomplete

Incomplete

Complete

Complete

Complete

Complete



Hard Problem Rule:Hard Problem Rule:when we when we need the answerneed the answer

to a problem we cannot to a problem we cannot immediately solve, immediately solve, defer it a defer it a

separate methodseparate method.. Implement it later.Implement it later.




/** /** * * Spot and deal with special day caseSpot and deal with special day case */ */ private void checkDayOverflow () { private void checkDayOverflow () { if (day > if (day > daysInMonth ()daysInMonth ()) {) { day = 1; day = 1; month = month + 1; month = month + 1; checkMonthOverflow (); checkMonthOverflow (); } } } }



/** /** * * @return The number of days in the month@return The number of days in the month */ */ private int daysInMonth () { private int daysInMonth () { return return 3030;; } }



R1R1

Incomplete

Incomplete

Incomplete

Incomplete

Complete

Complete

Complete

Complete




... other methods... other methods

/** /** * * @return The number of days in the month@return The number of days in the month */ */ private int daysInMonth () { private int daysInMonth () { return return 3030;; } } }}




/** /** * * @return The number of days in the month@return The number of days in the month */ */ private int daysInMonth () { private int daysInMonth () { return return 3030;; } } }}

class Date {class Date { private int day; private int day; // 1 ≤ day ≤ daysInMonth// 1 ≤ day ≤ daysInMonth private int month; private int month; // 1 ≤ month ≤ 12// 1 ≤ month ≤ 12 private int year; private int year; // 1 ≤ year// 1 ≤ year

private static final int[] private static final int[] monthDays = monthDays = {31, 28, 31, 30, 31, 30, {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; 31, 31, 30, 31, 30, 31}; ... other methods... other methods


}}


private static final int[] private static final int[] monthDays = monthDays = {31, 28, 31, 30, 31, 30, {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; 31, 31, 30, 31, 30, 31}; ... other methods... other methods


}}


private static final int[] private static final int[] monthDays = monthDays = {31, 28, 31, 30, 31, 30, {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; 31, 31, 30, 31, 30, 31};


/** /** * * @return The number of days in the month@return The number of days in the month */ */ private int daysInMonth () { private int daysInMonth () { return return monthDays[month – 1]monthDays[month – 1];; } } }}




/** /** * * @return The number of days in the month@return The number of days in the month */ */ private int daysInMonth () { private int daysInMonth () { return return monthDays[month – 1]monthDays[month – 1];; } } }}

Incomplete

Incomplete

Incomplete

Incomplete

R1R1


class Date {class Date { ... day, month, year fields... day, month, year fields



/** /** * * @return The number of days in the month@return The number of days in the month */ */ private int daysInMonth () { private int daysInMonth () { return return monthDays[month – 1]monthDays[month – 1];; } }

}}





}}




/** /** * * @return The number of days in the month@return The number of days in the month */ */ private int daysInMonth () { private int daysInMonth () { int int answeranswer = = monthDays[month – 1]monthDays[month – 1];; if (( if ((monthmonth == 2) && == 2) && isLeapYear ()isLeapYear ()) {) { answeranswer = = answeranswer + 1; + 1; } } return return answeranswer;; } } }}





R1R1C

ompl

ete

Com

plet

e

Com

plet

e

Com

plet

e






Heavy Functionality Rule:Heavy Functionality Rule:when when some statements orsome statements orexpressions become too expressions become too

long or complex, long or complex, move them move them into a separate methodinto a separate method..

Implement it later.Implement it later.


/** /** * * @return The number of days in the month@return The number of days in the month */ */ private int daysInMonth () { private int daysInMonth () { int int answeranswer = = monthDays[month – 1]monthDays[month – 1];; if (( if ((monthmonth == 2) && == 2) && isLeapYear ()isLeapYear ()) {) { answeranswer = = answeranswer + 1; + 1; } } return return answeranswer;; } }



/** /** * * @return Whether the year is a leap year@return Whether the year is a leap year */ */ private b private boolean isLeapYear () {oolean isLeapYear () { returnreturn ((divides divides (4, year) && !(4, year) && !divides divides (100, year))(100, year)) || || divides divides (400, year)(400, year);; } }



R1R1C

ompl

ete

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

e





/**/** * @param d The divisor * @param d The divisor * @param n The number being divided * @param n The number being divided * * @return Whether d divides n exactly@return Whether d divides n exactly */ */ private private boolean divides (int d, int n) {boolean divides (int d, int n) { return (n % d) == 0;return (n % d) == 0; } }


/**/** * @param d The divisor * @param d The divisor * @param n The number being divided * @param n The number being divided * * @return Whether d divides n exactly@return Whether d divides n exactly */ */ private private boolean divides (int d, int n) {boolean divides (int d, int n) { return (n % d) == 0;return (n % d) == 0; } }

R1R1C

ompl

ete

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

e


2. TestsSo, So, just by thinking about testingjust by thinking about testing, we see the need , we see the need for another method. This is cohesion in action!for another method. This is cohesion in action!So, So, just by thinking about testingjust by thinking about testing, we see the need , we see the need for another method. This is cohesion in action!for another method. This is cohesion in action!









Recall

Recall

Recall

Recall




/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { {

} }

... setToNextDay and toString methods... setToNextDay and toString methods}}




} }

... setToNextDay and toString methods... setToNextDay and toString methods}}



/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { this.day = day;this.day = day; this.month = month; this.month = month; this.year = year; this.year = year; } }

... setToNextDay and toString methods ... setToNextDay and toString methods }}



/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { this.day = day;this.day = day; this.month = month; this.month = month; this.year = year; this.year = year; } }

... setToNextDay and toString methods ... setToNextDay and toString methods }}

R1R1

But what if

bad

But what if

bad

values a

re given?

values a

re given?

But what if

bad

But what if

bad

values a

re given?

values a

re given?


/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { this.day = day;this.day = day; this.month = month; this.month = month; this.year = year; this.year = year;

} }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { this.day = day;this.day = day; this.month = month; this.month = month; this.year = year; this.year = year;

} }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { this.day = day;this.day = day; this.month = month; this.month = month; this.year = year; this.year = year; if (! if (!legalDate ()legalDate ()) {) {

}} } }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { this.day = day;this.day = day; this.month = month; this.month = month; this.year = year; this.year = year; if (! if (!legalDate ()legalDate ()) {) {

}} } }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { this.day = day;this.day = day; this.month = month; this.month = month; this.year = year; this.year = year; if (! if (!legalDate ()legalDate ()) {) { this.day = 1;this.day = 1; this.month = 1; this.month = 1; this.year = 1; this.year = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 0001-01-01"); " 0001-01-01"); }} } }


defer defer decision as decision as to what is to what is

legal !!legal !!

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

eE

asy

Eas

yE

asy

Eas

yR1R1


/**/** * * @return Whether the date fields are legal@return Whether the date fields are legal */ */ private private boolean legalDate () {boolean legalDate () { return ((1 <= day) &&return ((1 <= day) && (day <= (day <= 3030)) &&)) && ((1 <= month) && ((1 <= month) && (month <= 12)) && (month <= 12)) && (1 <= year); (1 <= year); } }

/**/** * * @return Whether the date fields are legal@return Whether the date fields are legal */ */ private private boolean legalDate () {boolean legalDate () { return ((1 <= day) &&return ((1 <= day) && (day <= (day <= 3030)) &&)) && ((1 <= month) && ((1 <= month) && (month <= 12)) && (month <= 12)) && (1 <= year); (1 <= year); } }

/**/** * * @return Whether the date fields are legal@return Whether the date fields are legal */ */ private private boolean legalDate () {boolean legalDate () { return ((1 <= day) &&return ((1 <= day) && (day <= (day <= daysInMonth ()daysInMonth ())) &&)) && ((1 <= month) && ((1 <= month) && (month <= 12)) && (month <= 12)) && (1 <= year); (1 <= year); } }


BUG:BUG: ifif monthmonth is not in the range 1..12,is not in the range 1..12, daysInMonthdaysInMonth()() will crash.will crash.


R1R1






}}





}}









Rec

all …

Rec

all …

Rec

all …

Rec

all …

Assumes:Assumes: monthmonth is is inin

the range 1..12.the range 1..12.



R1R1




R1R1

/**/** * * @return Whether the date fields are legal@return Whether the date fields are legal */ */ private private boolean legalDate () {boolean legalDate () { return (1 <= year) &&return (1 <= year) && ((1 <= month) && ((1 <= month) && (month <= 12)) && (month <= 12)) && ((1 <= day) && ((1 <= day) && (day <= (day <= daysInMonth ()daysInMonth ()));)); } }

/**/** * * @return Whether the date fields are legal@return Whether the date fields are legal */ */ private private boolean legalDate () {boolean legalDate () { return (1 <= year) &&return (1 <= year) && ((1 <= month) && ((1 <= month) && (month <= 12)) && (month <= 12)) && ((1 <= day) && ((1 <= day) && (day <= (day <= daysInMonth ()daysInMonth ()));)); } }



FIXED:FIXED: operands ofoperands of &&&& are evaluated are evaluated left-to-right. If any operand isleft-to-right. If any operand is falsefalse, ,

the result isthe result is falsefalse and remaining and remaining operands (if any) are not evaluatedoperands (if any) are not evaluated..

FIXED:FIXED: operands ofoperands of &&&& are evaluated are evaluated left-to-right. If any operand isleft-to-right. If any operand is falsefalse, ,

the result isthe result is falsefalse and remaining and remaining operands (if any) are not evaluatedoperands (if any) are not evaluated..


S T R E A M

• S S tubstubs• T T estsests• R R epresentationepresentation• E E valuationvaluation• A A ttributesttributes• M M ethodsethods

Our choice of Our choice of representation representation can have great can have great impact on the impact on the complexity of complexity of

the logic.the logic.

Our choice of Our choice of representation representation can have great can have great impact on the impact on the complexity of complexity of

the logic.the logic.

















See

n be

fore

See

n be

fore

See

n be

fore

See

n be

fore











HardHardHardHard

See

n be

fore

See

n be

fore

See

n be

fore

See

n be

fore











Trivial

Trivial

Trivial

Trivial

See

n be

fore

See

n be

fore

See

n be

fore

See

n be

fore




... setToNextDay and toString methods ... setToNextDay and toString methods


} } }}





} } }}




/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { ... tricky !!!... tricky !!! } } }}




/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { ... tricky !!!... tricky !!! } } }}


Hard

Hard

Hard

Hard




R1R1S

een

befo

re

See

n be

fore

See

n be

fore

See

n be

fore

But theBut the R2R2 DateDate has has nono dayday,, monthmonth oror yearyear

fields to set …fields to set …

But theBut the R2R2 DateDate has has nono dayday,, monthmonth oror yearyear

fields to set …fields to set …

… … andand legalDatelegalDate has has no fields on which itno fields on which it

to operateto operate. .

… … andand legalDatelegalDate has has no fields on which itno fields on which it

to operateto operate. .



} }


} }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { if ( if (legalDate (day, month, year)legalDate (day, month, year)) {) { ... still tricky !!!... still tricky !!! } else { } else { date = 1; date = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 1970-01-01"); " 1970-01-01"); }} } }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { if ( if (legalDate (day, month, year)legalDate (day, month, year)) {) { ... still tricky !!!... still tricky !!! } else { } else { date = 1; date = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 1970-01-01"); " 1970-01-01"); }} } }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { if ( if (legalDate (day, month, year)legalDate (day, month, year)) {) { setLegalDate (day, month, year)setLegalDate (day, month, year) } else { } else { date = 1; date = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 1970-01-01"); " 1970-01-01"); }} } }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { if ( if (legalDate (day, month, year)legalDate (day, month, year)) {) { setLegalDate (day, month, year)setLegalDate (day, month, year) } else { } else { date = 1; date = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 1970-01-01"); " 1970-01-01"); }} } } defer defer

decision as decision as to what is to what is

legal !!legal !!

defer hard defer hard logic !!logic !!C

ompl

ete

Com

plet

e

Com

plet

e

Com

plet

e

R2R2


/** /** * Set the date to a particular day * Set the date to a particular day * * * @param day (assume: * @param day (assume: 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month (assume: * @param month (assume: 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year (assume: 1 ≤ year) * @param year (assume: 1 ≤ year) * * */ */ private void setLegalDate private void setLegalDate (int day, int month, int year) (int day, int month, int year) { { } }

/** /** * Set the date to a particular day * Set the date to a particular day * * * @param day (assume: * @param day (assume: 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month (assume: * @param month (assume: 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year (assume: 1 ≤ year) * @param year (assume: 1 ≤ year) * * */ */ private void setLegalDate private void setLegalDate (int day, int month, int year) (int day, int month, int year) { { } }

/** /** * Set the date to a particular day * Set the date to a particular day * * * @param day (assume: * @param day (assume: 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month (assume: * @param month (assume: 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year (assume: 1 ≤ year) * @param year (assume: 1 ≤ year) * * */ */ private void setLegalDate private void setLegalDate (int day, int month, int year) (int day, int month, int year) { { ... still tricky !!!... still tricky !!! } }

/** /** * Set the date to a particular day * Set the date to a particular day * * * @param day (assume: * @param day (assume: 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month (assume: * @param month (assume: 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year (assume: 1 ≤ year) * @param year (assume: 1 ≤ year) * * */ */ private void setLegalDate private void setLegalDate (int day, int month, int year) (int day, int month, int year) { { ... still tricky !!!... still tricky !!! } } Har

dHar

dHar

dHar

d

R2R2



} }


} }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { if ( if (legalDate (day, month, year)legalDate (day, month, year)) {) { ... still tricky !!!... still tricky !!! } else { } else { date = 1; date = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to " + " ... setting date to " + " 1970-01-01"); " 1970-01-01"); }} } }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { if ( if (legalDate (day, month, year)legalDate (day, month, year)) {) { ... still tricky !!!... still tricky !!! } else { } else { date = 1; date = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to " + " ... setting date to " + " 1970-01-01"); " 1970-01-01"); }} } }


/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { if ( if (legalDate (day, month, year)legalDate (day, month, year)) {) { setLegalDate (day, month, year)setLegalDate (day, month, year) } else { } else { date = 1; date = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 1970-01-01"); " 1970-01-01"); }} } } defer defer

decision as decision as to what is to what is

legal !!legal !!

defer hard defer hard logic !!logic !!C

ompl

ete

Com

plet

e

Com

plet

e

Com

plet

e

R2R2


/**/** * @param day ( * @param day (legallegal: : 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month ( * @param month (legallegal: : 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year ( * @param year (legallegal: 1 ≤ year): 1 ≤ year) * * * * @return Whether the parameters are legal@return Whether the parameters are legal * * */ */ private private boolean legalDateboolean legalDate (int day, int month, int year) (int day, int month, int year) { {

} }

/**/** * @param day ( * @param day (legallegal: : 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month ( * @param month (legallegal: : 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year ( * @param year (legallegal: 1 ≤ year): 1 ≤ year) * * * * @return Whether the parameters are legal@return Whether the parameters are legal * * */ */ private private boolean legalDateboolean legalDate (int day, int month, int year) (int day, int month, int year) { {

} }

/**/** * @param day ( * @param day (legallegal: : 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month ( * @param month (legallegal: : 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year ( * @param year (legallegal: 1 ≤ year): 1 ≤ year) * * * * @return Whether the parameters are legal@return Whether the parameters are legal * * */ */ private private boolean legalDateboolean legalDate (int day, int month, int year) (int day, int month, int year) { { return (1 <= year) &&return (1 <= year) && ((1 <= month) && ((1 <= month) && (month <= 12)) && (month <= 12)) && ((1 <= day) && ((1 <= day) && (day <= (day <= daysInMonth ()daysInMonth ()));)); } }


… … but but daysInMonthdaysInMonth has has no no monthmonth or or yearyear fields fields

on which to operate. on which to operate.

… … but but daysInMonthdaysInMonth has has no no monthmonth or or yearyear fields fields

on which to operate. on which to operate.

R2R2

XX




/**/** * @param day ( * @param day (legallegal: : 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month ( * @param month (legallegal: : 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year ( * @param year (legallegal: 1 ≤ year): 1 ≤ year) * * * * @return Whether the parameters are legal@return Whether the parameters are legal * * */ */ private private boolean legalDateboolean legalDate (int day, int month, int year) (int day, int month, int year) { { return (1 <= year) &&return (1 <= year) && ((1 <= month) && ((1 <= month) && (month <= 12)) && (month <= 12)) && ((1 <= day) && ((1 <= day) && (day <= (day <= daysInMonth (month, year)daysInMonth (month, year)));));

} }

/**/** * @param day ( * @param day (legallegal: : 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month ( * @param month (legallegal: : 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year ( * @param year (legallegal: 1 ≤ year): 1 ≤ year) * * * * @return Whether the parameters are legal@return Whether the parameters are legal * * */ */ private private boolean legalDateboolean legalDate (int day, int month, int year) (int day, int month, int year) { { return (1 <= year) &&return (1 <= year) && ((1 <= month) && ((1 <= month) && (month <= 12)) && (month <= 12)) && ((1 <= day) && ((1 <= day) && (day <= (day <= daysInMonth (month, year)daysInMonth (month, year)));));

} }

… … so we must so we must supply them.supply them.

… … so we must so we must supply them.supply them.

R2R2














R1R1S

een

befo

re

See

n be

fore

See

n be

fore

See

n be

fore


no no yearyear field … so field … so we must supply.we must supply.

no no yearyear field … so field … so we must supply.we must supply.



private static final int[] private static final int[] monthDays = monthDays = {31, 28, 31, 30, 31, 30, 31, ... , 31}; {31, 28, 31, 30, 31, 30, 31, ... , 31};

/**/** * @param month ( * @param month (assumeassume: : 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year ( * @param year (assumeassume: : 1 ≤ year1 ≤ year) ) * * @return The number of days in the month@return The number of days in the month */ */ private int daysInMonth (int month, int year) private int daysInMonth (int month, int year) { { int int answeranswer = = monthDays[month – 1]monthDays[month – 1];; if (( if ((monthmonth == 2) && == 2) && isLeapYear (year)isLeapYear (year)) {) { answeranswer = = answeranswer + 1; + 1; } } return return answeranswer;; } } }}

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

e

R2R2




R1R1S

een

befo

re

See

n be

fore

See

n be

fore

See

n be

fore


/**/** * @param year ( * @param year (assumeassume: : 1 ≤ year1 ≤ year) ) * * @return Whether the year is a leap year@return Whether the year is a leap year */ */ private b private boolean isLeapYear (oolean isLeapYear (int yearint year) {) { returnreturn ((divides divides (4, year) && !(4, year) && !divides divides (100, year))(100, year)) || || divides divides (400, year)(400, year);; } }

/**/** * @param year ( * @param year (assumeassume: : 1 ≤ year1 ≤ year) ) * * @return Whether the year is a leap year@return Whether the year is a leap year */ */ private b private boolean isLeapYear (oolean isLeapYear (int yearint year) {) { returnreturn ((divides divides (4, year) && !(4, year) && !divides divides (100, year))(100, year)) || || divides divides (400, year)(400, year);; } }

Com

plet

e

Com

plet

e

Com

plet

e

Com

plet

e

R2R2



private int day; private int day; // 1 ≤ day ≤ daysInMonth// 1 ≤ day ≤ daysInMonth private int month; private int month; // 1 ≤ month ≤ 12// 1 ≤ month ≤ 12 private int year; private int year; // 1 ≤ year// 1 ≤ year ... setDate, setToNextDay, toString methods... setDate, setToNextDay, toString methods

private int daysInMonth ()private int daysInMonth () { ... } { ... }

private boolean private boolean isLeapYear isLeapYear ()() { ... } { ... }

private boolean private boolean legalDate legalDate ()() { ... } { ... } }}


private int day; private int day; // 1 ≤ day ≤ daysInMonth// 1 ≤ day ≤ daysInMonth private int month; private int month; // 1 ≤ month ≤ 12// 1 ≤ month ≤ 12 private int year; private int year; // 1 ≤ year// 1 ≤ year ... setDate, setToNextDay, toString methods... setDate, setToNextDay, toString methods

private int daysInMonth ()private int daysInMonth () { ... } { ... }

private boolean private boolean isLeapYear isLeapYear ()() { ... } { ... }

private boolean private boolean legalDate legalDate ()() { ... } { ... } }}

R1R1




... setDate, setToNextDay, toString methods... setDate, setToNextDay, toString methods

private int daysInMonth (int month, int year)private int daysInMonth (int month, int year) { ... } { ... }

private boolean private boolean isLeapYear isLeapYear (int year)(int year) { ... } { ... }

private boolean private boolean legalDatelegalDate (int day, int month, int year)(int day, int month, int year) { ... } { ... } }}



... setDate, setToNextDay, toString methods... setDate, setToNextDay, toString methods

private int daysInMonth (int month, int year)private int daysInMonth (int month, int year) { ... } { ... }

private boolean private boolean isLeapYear isLeapYear (int year)(int year) { ... } { ... }

private boolean private boolean legalDatelegalDate (int day, int month, int year)(int day, int month, int year) { ... } { ... } }}

R2R2


OBSERVATION:OBSERVATION:

R2R2

We could have used the We could have used the fully parametrisedfully parametrised versions: versions:

int daysInMonth (int month, int year)int daysInMonth (int month, int year) boolean boolean isLeapYear isLeapYear (int year)(int year) boolean boolean legalDate legalDate (int day, int month, int year)(int day, int month, int year)

in the version of in the version of DateDate, as well as in version ., as well as in version .R1R1

They do not depend on any chosen They do not depend on any chosen fieldfield representation. representation.

They may, therefore, be worth They may, therefore, be worth factoring outfactoring out into a separate into a separate class, class, DateStuffDateStuff, , which either representation forwhich either representation for DateDate could usecould use..


class DateStuff {class DateStuff {


public int daysInMonth (int month, int year)public int daysInMonth (int month, int year) { ... } { ... }

public boolean public boolean isLeapYear isLeapYear (int year)(int year) { ... } { ... }

public boolean public boolean legalDatelegalDate (int day, int month, int year)(int day, int month, int year) { ... } { ... }

private private boolean divides (int d, int n)boolean divides (int d, int n) { { ... } ... }

}}







}}


But, now that they are But, now that they are publicpublic methods, methods, daysInMonth daysInMonth and and isLeapYearisLeapYear must be protected against misuse. must be protected against misuse.

Note: Note: legalDatelegalDate is already secure – dealing with bad data is, is already secure – dealing with bad data is, after all, its purpose!after all, its purpose!







DateStuffDateStuff


/**/** * @param month ( * @param month (1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year ( * @param year (1 ≤ year1 ≤ year) ) * * @return The number of days in the month@return The number of days in the month */ */public int daysInMonth (int month, int year)public int daysInMonth (int month, int year){{ if ((month < 1) || (month > 12) || (year < 1)) { if ((month < 1) || (month > 12) || (year < 1)) { System.out.printlnSystem.out.println ("*** daysInMonth: bad argument(s)" + ("*** daysInMonth: bad argument(s)" + " – returning zero"); " – returning zero"); return 0; return 0; }} int int answeranswer = = monthDays[month – 1]monthDays[month – 1];; if (( if ((monthmonth == 2) && == 2) && isLeapYear (year)isLeapYear (year)) {) { answeranswer = = answeranswer + 1; + 1; } } return return answeranswer;;}}

/**/** * @param month ( * @param month (1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year ( * @param year (1 ≤ year1 ≤ year) ) * * @return The number of days in the month@return The number of days in the month */ */public int daysInMonth (int month, int year)public int daysInMonth (int month, int year){{ if ((month < 1) || (month > 12) || (year < 1)) { if ((month < 1) || (month > 12) || (year < 1)) { System.out.printlnSystem.out.println ("*** daysInMonth: bad argument(s)" + ("*** daysInMonth: bad argument(s)" + " – returning zero"); " – returning zero"); return 0; return 0; }} int int answeranswer = = monthDays[month – 1]monthDays[month – 1];; if (( if ((monthmonth == 2) && == 2) && isLeapYear (year)isLeapYear (year)) {) { answeranswer = = answeranswer + 1; + 1; } } return return answeranswer;;}}

DateStuffDateStuff


/**/** * @param year ( * @param year (1 ≤ year1 ≤ year) ) * * @return Whether year is a leap year @return Whether year is a leap year */ */public boolean isLeapYear (int year)public boolean isLeapYear (int year){{ if (year < 1){ if (year < 1){ System.out.printlnSystem.out.println ("*** isLeapYear: bad argument " + ("*** isLeapYear: bad argument " + "– returning false"); "– returning false"); return false; return false; }} returnreturn ((divides divides (4, year) && !(4, year) && !divides divides (100, year))(100, year)) || || divides divides (400, year)(400, year);; } }}}

/**/** * @param year ( * @param year (1 ≤ year1 ≤ year) ) * * @return Whether year is a leap year @return Whether year is a leap year */ */public boolean isLeapYear (int year)public boolean isLeapYear (int year){{ if (year < 1){ if (year < 1){ System.out.printlnSystem.out.println ("*** isLeapYear: bad argument " + ("*** isLeapYear: bad argument " + "– returning false"); "– returning false"); return false; return false; }} returnreturn ((divides divides (4, year) && !(4, year) && !divides divides (100, year))(100, year)) || || divides divides (400, year)(400, year);; } }}}

DateStuffDateStuff


/**/** * @param day ( * @param day (legallegal: : 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month ( * @param month (legallegal: : 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year ( * @param year (legallegal: 1 ≤ year): 1 ≤ year) * * * * @return Whether the parameters are legal@return Whether the parameters are legal * * */ */ public public boolean legalDateboolean legalDate (int day, int month, int year)(int day, int month, int year) { { return return (1 <= year) &&(1 <= year) && ((1 <= month) && ((1 <= month) && (month <= 12)) && (month <= 12)) && ((1 <= day) && ((1 <= day) && (day <= (day <= daysInMonth (month, year)daysInMonth (month, year)))));;

} }

/**/** * @param day ( * @param day (legallegal: : 1 ≤ day ≤ daysInMonth1 ≤ day ≤ daysInMonth)) * @param month ( * @param month (legallegal: : 1 ≤ month ≤ 121 ≤ month ≤ 12)) * @param year ( * @param year (legallegal: 1 ≤ year): 1 ≤ year) * * * * @return Whether the parameters are legal@return Whether the parameters are legal * * */ */ public public boolean legalDateboolean legalDate (int day, int month, int year)(int day, int month, int year) { { return return (1 <= year) &&(1 <= year) && ((1 <= month) && ((1 <= month) && (month <= 12)) && (month <= 12)) && ((1 <= day) && ((1 <= day) && (day <= (day <= daysInMonth (month, year)daysInMonth (month, year)))));;

} }

DateStuffDateStuff

Uncha

nged

Uncha

nged

Uncha

nged

Uncha

nged**

except that nowexcept that now it’s it’s publicpublic.. except that nowexcept that now it’s it’s publicpublic.. **








}}







}}

Just

see

n

Just

see

n

Just

see

n

Just

see

n








}}







}}



public public staticstatic int daysInMonth ( ... ) int daysInMonth ( ... ) { ... } { ... }

public public staticstatic boolean boolean isLeapYear isLeapYear (int year)(int year) { ... } { ... }

public public staticstatic boolean boolean legalDatelegalDate (int day, int month, int year)(int day, int month, int year) { ... } { ... }

private private staticstatic boolean divides (int d, int n)boolean divides (int d, int n) { { ... } ... }

}}







}}

New

Jav

a

New

Jav

a co

ncep

t

conc

ept

New

Jav

a

New

Jav

a co

ncep

t

conc

ept


New Java Concept:New Java Concept:

MethodsMethods, as well as , as well as fieldsfields, may be declared , may be declared staticstatic..

staticstatic methodsmethods may work with may work with fieldsfields of its class, but only if of its class, but only if they are they are staticstatic..

staticstatic methodsmethods may invoke other may invoke other methodsmethods of its class, but of its class, but only if they are only if they are staticstatic..

In In DateStuffDateStuff, the methods , the methods isLeapYearisLeapYear and and dividesdivides work work only with their parameters – no only with their parameters – no fieldsfields..In In DateStuffDateStuff, the methods , the methods isLeapYearisLeapYear and and dividesdivides work work only with their parameters – no only with their parameters – no fieldsfields..

In In DateStuffDateStuff, the methods , the methods daysInMonthdaysInMonth and and legalDatelegalDate work with their parameters and (the work with their parameters and (the staticstatic fieldfield)) monthDaysmonthDays..In In DateStuffDateStuff, the methods , the methods daysInMonthdaysInMonth and and legalDatelegalDate work with their parameters and (the work with their parameters and (the staticstatic fieldfield)) monthDaysmonthDays..

In In DateStuffDateStuff, , daysInMonthdaysInMonth invokes invokes isLeapYearisLeapYear, which , which

invokes invokes dividesdivides. And . And legalDatelegalDate invokes invokes daysInMonthdaysInMonth..In In DateStuffDateStuff, , daysInMonthdaysInMonth invokes invokes isLeapYearisLeapYear, which , which

invokes invokes dividesdivides. And . And legalDatelegalDate invokes invokes daysInMonthdaysInMonth..








}}







}}

New

Jav

a

New

Jav

a co

ncep

t

conc

ept

New

Jav

a

New

Jav

a co

ncep

t

conc

ept






staticstatic fieldsfields and and methodsmethods belong to the belong to the classclass. They are . They are shared by all shared by all objectsobjects (if any) of the class. (if any) of the class.staticstatic fieldsfields and and methodsmethods belong to the belong to the classclass. They are . They are shared by all shared by all objectsobjects (if any) of the class. (if any) of the class.

To invoke a To invoke a staticstatic methodmethod of another class, we don’t need of another class, we don’t need an an objectobject of that other class. We invoke it prefixed with the of that other class. We invoke it prefixed with the name of its owning classname of its owning class (followed by the usual dot). (followed by the usual dot).

To invoke a To invoke a staticstatic methodmethod of another class, we don’t need of another class, we don’t need an an objectobject of that other class. We invoke it prefixed with the of that other class. We invoke it prefixed with the name of its owning classname of its owning class (followed by the usual dot). (followed by the usual dot).

Normally:Normally: we invoke a we invoke a methodmethod on an on an objectobject. We invoke it . We invoke it prefixed with the prefixed with the name of the variable referencing the objectname of the variable referencing the object (followed by the usual dot).(followed by the usual dot).

Normally:Normally: we invoke a we invoke a methodmethod on an on an objectobject. We invoke it . We invoke it prefixed with the prefixed with the name of the variable referencing the objectname of the variable referencing the object (followed by the usual dot).(followed by the usual dot).




See

n be

fore

See

n be

fore

See

n be

fore

See

n be

fore

R2R2




/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { if ( if (DateStuffDateStuff..legalDate (day, month, year)legalDate (day, month, year)) {) { setLegalDate (day, month, year)setLegalDate (day, month, year) } else { } else { date = 1; date = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 1970-01-01"); " 1970-01-01"); }} } }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { if ( if (DateStuffDateStuff..legalDate (day, month, year)legalDate (day, month, year)) {) { setLegalDate (day, month, year)setLegalDate (day, month, year) } else { } else { date = 1; date = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 1970-01-01"); " 1970-01-01"); }} } }

R2R2


/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { this.day = day;this.day = day; this.month = month; this.month = month; this.year = year; this.year = year; if (! if (!legalDate ()legalDate (){{ this.day = 1;this.day = 1; this.month = 1; this.month = 1; this.year = 1; this.year = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 0001-01-01"); " 0001-01-01"); }} } }


See

n be

fore

See

n be

fore

See

n be

fore

See

n be

fore

R1R1




/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { this.day = day;this.day = day; this.month = month; this.month = month; this.year = year; this.year = year; if (! if (!DateStuffDateStuff..legalDate (day, month, year) legalDate (day, month, year) {{ this.day = 1;this.day = 1; this.month = 1; this.month = 1; this.year = 1; this.year = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 0001-01-01"); " 0001-01-01"); }} } }

/** /** * Set the date to a particular day * Set the date to a particular day */ */ public void setDate public void setDate (int day, int month, int year) (int day, int month, int year) { { this.day = day;this.day = day; this.month = month; this.month = month; this.year = year; this.year = year; if (! if (!DateStuffDateStuff..legalDate (day, month, year) legalDate (day, month, year) {{ this.day = 1;this.day = 1; this.month = 1; this.month = 1; this.year = 1; this.year = 1; System.out.printlnSystem.out.println ("*** setDate: Bad parameters" + ("*** setDate: Bad parameters" + " ... setting date to" + " ... setting date to" + " 0001-01-01"); " 0001-01-01"); }} } }

R1R1






staticstatic fieldsfields and and methodsmethods belong to the belong to the classclass. They are . They are shared by all shared by all objectsobjects (if any) of the class. (if any) of the class.staticstatic fieldsfields and and methodsmethods belong to the belong to the classclass. They are . They are shared by all shared by all objectsobjects (if any) of the class. (if any) of the class.

To invoke a To invoke a staticstatic methodmethod of another class, we don’t need of another class, we don’t need an an objectobject of that other class. We invoke it prefixed with the of that other class. We invoke it prefixed with the namename of its owning class (followed by the usual dot). of its owning class (followed by the usual dot).

To invoke a To invoke a staticstatic methodmethod of another class, we don’t need of another class, we don’t need an an objectobject of that other class. We invoke it prefixed with the of that other class. We invoke it prefixed with the namename of its owning class (followed by the usual dot). of its owning class (followed by the usual dot).

Classes with Classes with onlyonly staticstatic fieldsfields and and methodsmethods need no objects need no objects constructed from them.constructed from them.Classes with Classes with onlyonly staticstatic fieldsfields and and methodsmethods need no objects need no objects constructed from them.constructed from them.Classes with Classes with onlyonly staticstatic fieldsfields and and methodsmethods need no objects need no objects constructed from them. constructed from them. So … hide the constructor!!So … hide the constructor!!Classes with Classes with onlyonly staticstatic fieldsfields and and methodsmethods need no objects need no objects constructed from them. constructed from them. So … hide the constructor!!So … hide the constructor!!



... ... monthDaysmonthDays (private static field)(private static field)

... daysInMonth... daysInMonth,, isLeapYearisLeapYear,, legalDate legalDate (public static methods)(public static methods)

... ... dividesdivides (private static method)(private static method) }}







private DateStuff () {private DateStuff () { // deliberately empty// deliberately empty }}





private DateStuff () {private DateStuff () { // deliberately empty// deliberately empty }}



constructorconstructor

This class doesn’t construct any instances of itself. Its This class doesn’t construct any instances of itself. Its constructorconstructor is is privateprivate, so nobody else can. There can be no , so nobody else can. There can be no DateStuffDateStuff objects. objects.


OBSERVATION:OBSERVATION:

R2R2

We could have used the We could have used the fully parametrisedfully parametrised versions: versions:

int daysInMonth (int month, int year)int daysInMonth (int month, int year) boolean boolean isLeapYear isLeapYear (int year)(int year) boolean boolean legalDate legalDate (int day, int month, int year)(int day, int month, int year)

in the version of in the version of DateDate, as well as in version ., as well as in version .R1R1

They do not depend on any chosen They do not depend on any chosen fieldfield representation. representation.


See

n be

fore

See

n be

fore

See

n be

fore

See

n be

fore



DateStuffDateStuff contains a set of utility contains a set of utility functionsfunctions that work just that work just with their with their parametersparameters. They need no persistent state to be . They need no persistent state to be held in fields. Such things are very held in fields. Such things are very reusablereusable..

DateStuffDateStuff can be used with either the can be used with either the R1R1 or or R2R2 choice choicefor representation of a for representation of a datedate in the in the DateDate class. class.

Factoring out these low-level functions into Factoring out these low-level functions into DateStuffDateStuff leaves the leaves the DateDate class ( class (R1R1 or or R2R2) ) shortershorter..

Collections of purely Collections of purely staticstatic methods are how we used to methods are how we used to do things do things – before Object-Orientation– before Object-Orientation. . Don’t make a habit Don’t make a habit of it!of it! Objects carrying their own state are powerful!!Objects carrying their own state are powerful!!





Review

Review

Review

Review






Nested Loop Rule:Nested Loop Rule:when we when we have a nestedhave a nested

loop, loop, move the inner loop move the inner loop into a separate methodinto a separate method..


Review

Review

Review

Review


Nested Loop Rule:Nested Loop Rule:when we when we have a nestedhave a nested

loop, loop, move the inner loop move the inner loop into a separate methodinto a separate method..



Code Duplication Rule:Code Duplication Rule:when we when we repeat the same repeat the same

code, code, move that code into a move that code into a separate method and repeat separate method and repeat

an invocation insteadan invocation instead.. Implement it later.Implement it later.

Review

Review

Review

Review

Review

Review

Review

Review

Mañana – Specific Forms

• Special Case rule

• Hard Problem rule

• Heavy Functionality rule

• Nested Loop rule

• Code Duplication rule

Review

Review

Review

Review

Utility Classes

publicpublic static static methodsmethods, as well as , as well as privateprivate static static fieldsfields..publicpublic static static methodsmethods, as well as , as well as privateprivate static static fieldsfields..

Utility classes (like Utility classes (like DateStuffDateStuff) have only the above kind of ) have only the above kind of methodsmethods and and fieldsfields..Utility classes (like Utility classes (like DateStuffDateStuff) have only the above kind of ) have only the above kind of methodsmethods and and fieldsfields..

publicpublic static static methodsmethods are invoked via the are invoked via the classclass name – no name – no objectobject is needed. is needed.publicpublic static static methodsmethods are invoked via the are invoked via the classclass name – no name – no objectobject is needed. is needed.

Utility classes (like Utility classes (like DateStuffDateStuff) have only ) have only privateprivate constructors constructors – no – no objectsobjects can be constructed. can be constructed.Utility classes (like Utility classes (like DateStuffDateStuff) have only ) have only privateprivate constructors constructors – no – no objectsobjects can be constructed. can be constructed.

STREAM STREAM A software process The Mañana Principle The Mañana Principle Don’t try to...

Documents

Transcript of STREAM STREAM A software process The Mañana Principle The Mañana Principle Don’t try to...