Slide on Methods.ppt

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Chapter 6 Methods

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Opening Problem

Find the sum of integers from 1 to 10, from 20 to 30, and

from 35 to 45, respeti!el"#

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Problem

int sum = 0;

for (int i = 1; i

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Problem

int sum = 0;

for (int i = 1; i

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$olution

public static int sum%int i1, int i2& '

  int sum ( 0)  for (int i ( i1) i *( i2) i++&

  sum +( i)

  return sum)

public static void main%$tring-. args& '

  $"stem#out#println%/$um from 1 to 10 is / + sum%1, 10&&)

  $"stem#out#println%/$um from 20 to 30 is / + sum%20, 30&&)

  $"stem#out#println%/$um from 35 to 45 is / + sum%35, 45&&)

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Obeti!es o define methods ith formal parameters %6#2

o in!oe methods ith atual parameters %i#e#, arguments& %6#2 o define methods ith a return !alue %6#3

o define methods ithout a return !alue %6#4

o pass arguments b" !alue %6#5

o de!elop reusable ode that is modular, eas" to read, eas" to debug, andeas" to maintain %6#6

o rite a method that on!erts headeimals to deimals %6#

o use method o!erloading and understand ambiguous o!erloading

%6#7

o determine the sope of !ariables %6#8

o appl" the onept of method abstration in softare de!elopment

%6#10

o design and implement methods using stepise refinement %6#10

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9efining Methods

: method is a olletion of statements that aregrouped together to perform an operation#

 public sttic int max(int num1, int num2) {

int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

9efine a method ;n!oe a method

int z = max(x, y);

atual parameters

%arguments&

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9efining Methods

: method is a olletion of statements that aregrouped together to perform an operation#

 public sttic int max(int num1, int num2) {

int result;

if (num1 > num2)

result = num1;

else

result = num2;

return result;

} 

modifier

return !alue

t"pemethod

nameformal

 parameters

return !alue

method

 bod"

method

header

 parameter list

9efine a method ;n!o4e a method

int z = max(x, y);

atual parameters

%arguments&

method

signature

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Method $ignature

 Method signature is the ombination of the method name and the

 parameter list#

 public sttic int max(int num1, int num2) {

int result;

if (num1 > num2)

result = num1;

else

result = num2;

return result;

} 

modifier

return !alue

t"pemethod

nameformal

 parameters

return !alue

method

 bod"

method

header

 parameter list

9efine a method ;n!o4e a method

int z = max(x, y);

atual parameters

%arguments&

method

signature

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Formal Parameters

he !ariables defined in the method header are non as formal parameters#

 public sttic int max(int num1, int num2) {

int result;

if (num1 > num2)

result = num1;

else

result = num2;

return result;

} 

modifier

return !alue

t"pemethod

nameformal

 parameters

return !alue

method

 bod"

method

header

 parameter list

9efine a method ;n!o4e a method

int z = max(x, y);

atual parameters

%arguments&

method

signature

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:tual Parameters

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=eturn >alue "pe: method ma" return a !alue# he return>alue"pe is the data t"pe

of the !alue the method returns# ;f the method does not return a!alue, the return>alue"pe is the e"ord !oid# For eample, the

return>alue"pe in the main method is !oid#

 public sttic int max(int num1, int num2) {

int result;

if (num1 > num2)

result = num1;else

result = num2;

return result;

} 

modifier

return !alue

t"pe methodname formal parameters

return !alue

method

 bod"

method

header

 parameter list

9efine a method ;n!o4e a method

int z = max(x, y);

atual parameters

%arguments&

method

signature

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Calling Methods

esting the max method

his program demonstrates alling a method ma

to return the largest of the int !alues

 TestMax TestMax   Run

Animation

http://var/www/apps/conversion/tmp/scratch_6/html%2FTestMax.bathttp://www.cs.armstrong.edu/liang/intro10e/html/TestMax.htmlhttp://var/www/apps/conversion/tmp/scratch_6/html%2FTestMax.bat

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Calling Methods, ont#

public static void main(String[] args) {

int i = 5;int j = 2;int k = max(i, j);

System.out.println("The maximum between " + i +" and " + j + " is " + k);

} 

public static int max(int num1, int num2) {

int result;

if (num1 > num2)result = num1;

elseresult = num2;

return result;} 

 pass the !alue of i pass the !alue of

animation

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rae Method ;n!oation

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println(

"The maximum between " + i +" and " + j + " is " + k);} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

i is no 5

animation

i i

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rae Method ;n!oation

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println(

"The maximum between " + i +" and " + j + " is " + k);} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

  is no 2

animation

i ti

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rae Method ;n!oation

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println(

"The maximum between " + i +" and " + j + " is " + k);} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

in!oe ma%i, &

animation

i ti

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rae Method ;n!oation

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println(

"The maximum between " + i +" and " + j + " is " + k);} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

in!oe ma%i, &

Pass the !alue of i to num1

Pass the !alue of to num2

animation

i ti

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rae Method ;n!oation

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println(

"The maximum between " + i +" and " + j + " is " + k);} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

delare !ariable result

animation

i ti

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rae Method ;n!oation

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println(

"The maximum between " + i +" and " + j + " is " + k);} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

%num1 ? num2& is true sine num1 is 5

and num2 is 2

animation

animation

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rae Method ;n!oation

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println(

"The maximum between " + i +" and " + j + " is " + k);} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

result is no 5

animation

animation

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rae Method ;n!oation

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println(

"The maximum between " + i +" and " + j + " is " + k);} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

return result, hih is 5

animation

animation

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rae Method ;n!oation

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println(

"The maximum between " + i +" and " + j + " is " + k);} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

return ma%i, & and assign the

return !alue to  

animation

animation

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rae Method ;n!oation

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println(

"The maximum between " + i +" and " + j + " is " + k);} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

else

result = num2;

return result;} 

@eute the print statement

animation

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C:A;OB: return statement is reuired for a !alueDreturning method# he

method shon belo in %a& is logiall" orret, but it has aompilation error beause the Ea!a ompiler thins it possible thatthis method does not return an" !alue#

o fi this problem, delete if (n < 0) in %a&, so that the ompiler illsee a return statement to be reahed regardless of ho the ifstatement is e!aluated#

  public sttic int sign(int n) {if (n > 0)

return 1;else if (n == 0)

return 0;

else if (n < 0)

return –1;

}

%a&

$hould be

%b&

 public sttic int sign(int n) {

if (n > 0)

return 1;else if (n == 0)

return 0;

else

return –1;

}

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=euse Methods from Other Classes

 BO@ One of the benefits of methods is for reuse# he ma

method an be in!oed from an" lass besides estMa# ;f

"ou reate a ne lass est, "ou an in!oe the ma method

using ClassBame#methodBame %e#g#, estMa#ma

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Call $tas

animation

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rae Call $ta 

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println("The maximum between " + i +" and " + j + " is " + k);

} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

elseresult = num2;

return result;} 

i is delared and initialiGed

he main method

is in!o4ed#

iF 5

animation

animation

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rae Call $ta 

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println("The maximum between " + i +" and " + j + " is " + k);

} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

elseresult = num2;

return result;} 

  is delared and initialiGed

he main method

is in!o4ed#

 0F 2

iF 5

animation

animation

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rae Call $ta 

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println("The maximum between " + i +" and " + j + " is " + k);

} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

elseresult = num2;

return result;} 

9elare  

he main method

is in!o4ed#

$pae reCuired for the

main method

4F

 0F 2

iF 5

animation

animation

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rae Call $ta 

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println("The maximum between " + i +" and " + j + " is " + k);

} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

elseresult = num2;

return result;} 

;n!oe ma%i, &

he main method

is in!o4ed#

$pae reCuired for the

main method

4F

 0F 2

iF 5

animation

animation

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rae Call $ta 

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println("The maximum between " + i +" and " + j + " is " + k);

} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

elseresult = num2;

return result;} 

 pass the !alues of i and to num1and num2

he ma method is

in!o4ed#

num2F 2

num1F 5

$pae reCuired for themain method

4F

 0F 2iF 5

animation

animation

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rae Call $ta 

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println("The maximum between " + i +" and " + j + " is " + k);

} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

elseresult = num2;

return result;} 

9elare result

he ma method is

in!o4ed#

resultF

num2F 2

num1F 5

$pae reCuired for themain method

4F

 0F 2iF 5

animation

animation

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rae Call $ta 

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println("The maximum between " + i +" and " + j + " is " + k);

} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

elseresult = num2;

return result;} 

%num1 ? num2& is true

he ma method is

in!o4ed#

resultF

num2F 2

num1F 5

$pae reCuired for themain method

4F

 0F 2iF 5

animation

animation

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rae Call $ta 

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println("The maximum between " + i +" and " + j + " is " + k);

} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

elseresult = num2;

return result;} 

:ssign num1 to result

he ma method is

in!o4ed#

$pae reCuired for thema method

resultF 5

num2F 2

num1F 5

$pae reCuired for themain method

4F

 0F 2iF 5

animation

animation

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rae Call $ta 

 public static void main(String[] args) {int i = 5;int j = 2;int k = max(i, j);

System.out.println("The maximum between " + i +" and " + j + " is " + k);

} 

public static int max(int num1, int num2) {int result;

if (num1 > num2)result = num1;

elseresult = num2;

return result;} 

=eturn result and assign it to  

he ma method is

in!o4ed#

$pae reCuired for thema method

resultF 5

num2F 2

num1F 5

$pae reCuired for themain method

4F5

 0F 2iF 5

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!oid Method @ample

his t"pe of method does not return a !alue# he method performs some ations#

 TestVoidMethod TestVoidMethod   Run

 TestReturnGradeMethod TestReturnGradeMethod   Run

http://var/www/apps/conversion/tmp/scratch_6/html%2FTestVoidMethod.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FTestReturnGradeMethod.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FTestReturnGradeMethod.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FTestVoidMethod.bat

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Passing Parameters public sttic !oi n#rintln(Strin$ mess$e% int n) &

for (int i = 0; i < n; i++)  System.out.println(mess$e);

'

$uppose "ou in!oe the method using

nPrintln%H

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Pass b" >alue

his program demonstrates passing !alues

to the methods#

IncrementIncrement   Run

http://var/www/apps/conversion/tmp/scratch_6/html%2FIncrement.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FIncrement.bat

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Pass b" >alue

esting Pass b" !alue

his program demonstrates passing !alues

to the methods#

 TestPassByValue TestPassByValue   Run

http://var/www/apps/conversion/tmp/scratch_6/html%2FTestPassByValue.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FTestPassByValue.bat

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Pass b" >alue, ont#

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ModulariGing Code

Methods an be used to redue redundant oding andenable ode reuse# Methods an also be used tomodulariGe ode and impro!e the ualit" of the

 program#

GreatestCommonDivisorMethodGreatestCommonDivisorMethod

Run

PrimeNumberMethodPrimeNumberMethod

Run

Case $tud" Con!erting Keadeimals

http://var/www/apps/conversion/tmp/scratch_6/html%2FGreatestCommonDivisorMethod.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FPrimeNumberMethod.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FPrimeNumberMethod.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FGreatestCommonDivisorMethod.bat

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Case $tud" Con!erting Keadeimals

to 9eimals

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O!erloading Methods

O!erloading the max Method

public sttic ouble m(ouble num1% oublenum2) &

if (num1 num2)  return num1;  else  return num2;

'

 TestMethod!verloadin" TestMethod!verloadin"   Run

http://var/www/apps/conversion/tmp/scratch_6/html%2FTestMethodOverloading.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FTestMethodOverloading.bat

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:mbiguous ;n!oation

$ometimes there ma" be to or more possible

mathes for an in!oation of a method, but the

ompiler annot determine the most speifimath# his is referred to as ambiguous

invocation# :mbiguous in!oation is a

ompile error#

: bi ; ti

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:mbiguous ;n!oation public clss *mbi$uous!erloin$ &

  public sttic !oi min(Strin$,- r$s) &

  System.out.println(m(1% 2));

'

 public sttic ouble m(int num1% ouble num2) &

if (num1 num2)

  return num1;

  else

  return num2;

  '

 public sttic ouble m(ouble num1% int num2) &

  if (num1 num2)  return num1;

  else

  return num2;

'

'

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$ope of oal >ariables

: loal !ariable a !ariable defined inside amethod#

$ope the part of the program here the

!ariable an be referened#he sope of a loal !ariable starts from its

delaration and ontinues to the end of the

 blo that ontains the !ariable# : loal!ariable must be delared before it an beused#

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$ope of oal >ariables, ont#

ou an delare a loal !ariable ith thesame name multiple times in different nonD

nesting blos in a method, but "ou annot

delare a loal !ariable tie in nested blos#

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$ope of oal >ariables, ont#

public static void method1() {int x = 1;

int y = 1;

for (int i = 1; i < 10; i++) {x += i;

}

for (int i = 1; i < 10; i++) {y += i;

}}

It is fine to declare i in twonon-nesting blocks

public static void method2() {

int i = 1;int sum = 0;

for (int i = 1; i < 10; i++)sum += i;

}

}

It is wrong to declare i intwo nesting blocks

$ope of oal >ariables ont

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$ope of oal >ariables, ont# /ine it no errors

 public sttic !oi correcteto() &  int = 1;

  int y = 1;

  i is eclre

for (int i = 1; i < 10; i++) &  += i;

  '

  i is eclre $in

  for (int i = 1; i < 10; i++) &

  y += i;

  '

'

$ope of oal >ariables ont

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$ope of oal >ariables, ont#

it errors

 public sttic !oi incorrecteto() &

  int = 1;

  int y = 1;

  for (int i = 1; i < 10; i++) &  int = 0;

  += i;

  '

'

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54

Method :bstration

ou an thin of the method bod" as a bla bothat ontains the detailed implementation for the

method#

Method Keader

Method bod"Lla Lo

Optional argumentsfor ;nput

Optional return!alue

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Lenefits of Methods

Q

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Case $tud" Renerating =andom

Charaters

Computer programs proess numerial data and haraters#ou ha!e seen man" eamples that in!ol!e numerial data#;t is also important to understand haraters and ho to

 proess them#

:s introdued in $etion 2#8, eah harater has a uniueAniode beteen 0 and FFFF in headeimal %65535 indeimal o generate a random harater is to generate arandom integer beteen 0 and 65535 using the folloing

epression %note that sine 0 *( Math#random%& * 1#0, "ouha!e to add 1 to 65535#&

%int&%Math#random%& %65535 + 1&&

Case $tud" Renerating =andom

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5

Case $tud" Renerating =andom

Charaters, ont#

 Bo let us onsider ho to generate a random

loerase letter# he Aniode for loerase letters

are onseuti!e integers starting from the Aniode

for SaS, then for SbS, SS, ###, and SGS# he Aniode for SaSis

%int&SaS

$o, a random integer beteen %int&SaS and %int&SGS is%int&%%int&SaS + Math#random%& %%int&SGS D %int&SaS + 1&

Case $tud" Renerating =andom

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Case $tud" Renerating =andom

Charaters, ont#

 Bo let us onsider ho to generate a random

loerase letter# he Aniode for loerase letters

are onseuti!e integers starting from the Aniode

for SaS, then for SbS, SS, ###, and SGS# he Aniode for SaSis

%int&SaS

$o, a random integer beteen %int&SaS and %int&SGS is%int&%%int&SaS + Math#random%& %%int&SGS D %int&SaS + 1&

Case $tud" Renerating =andom

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58

Case $tud" Renerating =andom

Charaters, ont#

:s disussed in Chapter 2#, all numeri operators

an be applied to the har operands# he har

operand is ast into a number if the other operand

is a number or a harater# $o, the preedingepression an be simplified as follos

SaS + Math#random%& %SGS D SaS + 1&

 $o a random loerase letter is

%har&%SaS + Math#random%& %SGS D SaS + 1&&

Case $tud" Renerating =andom

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60

Case $tud" Renerating =andom

Charaters, ont#

o generaliGe the foregoing disussion, a random harater

 beteen an" to haraters h1 and h2 ith h1 * h2

an be generated as follos

%har&%h1 + Math#random%& %h2 T h1 + 1&&

he =andomCharater Class

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he =andomCharater Class nomrcter.6!7 8enerte rnom crcters

 public clss nomrcter &

  99 8enerte rnom crcter beteen c1 n c2 9

  public sttic cr $etnomrcter(cr c1% cr c2) &  return (cr)(c1 + t.rnom() 9 (c2 : c1 + 1));

  '

99 8enerte rnom loercse letter 9

  public sttic cr $etnomoerseetter() &

  return $etnomrcter(% );

  '

 99 8enerte rnom uppercse letter 9

  public sttic cr $etnom>pperseetter() &

  return $etnomrcter(*% ?);

  '

99 8enerte rnom i$it crcter 9

  public sttic cr $etnom@i$itrcter() &

  return $etnomrcter(0% A);  '

99 8enerte rnom crcter 9

  public sttic cr $etnomrcter() &

  return $etnomrcter(Bu0000% Bu////);

  '

'

 TestRandomCharacter TestRandomCharacter

Run

RandomCharacterRandomCharacter

$ i = fi %O ti l&

http://var/www/apps/conversion/tmp/scratch_6/html%2FTestRandomCharacter.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FTestRandomCharacter.bat

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$tepise =efinement %Optional&

he onept of method abstration an be appliedto the proess of de!eloping programs#

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PrintCalender Case $tud" 

et us use the PrintCalendar eample to demonstrate thestepise refinement approah#

PrintCalendarPrintCalendar   Run

9esign 9iagram

http://var/www/apps/conversion/tmp/scratch_6/html%2FPrintCalendar.bathttp://var/www/apps/conversion/tmp/scratch_6/html%2FPrintCalendar.bat

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 printCalendar

%main&

read;nput  printMonth

get$tart9a"

 printMonthitle printMonthLod"

getotalBumOf9a"s

getBumOf9a"s;nMonth

getMonthBame

iseapear

64

9esign 9iagram

9esign 9iagram

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 printCalendar

%main&

read;nput  printMonth

get$tart9a"

 printMonthitle printMonthLod"

getotalBumOf9a"s

getBumOf9a"s;nMonth

getMonthBame

iseapear

65

9esign 9iagram

9esign 9iagram

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 printCalendar

%main&

read;nput  printMonth

get$tart9a"

 printMonthitle printMonthLod"

getotalBumOf9a"s

getBumOf9a"s;nMonth

getMonthBame

iseapear

66

9esign 9iagram

9esign 9iagram

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 printCalendar

%main&

read;nput  printMonth

get$tart9a"

 printMonthitle printMonthLod"

getotalBumOf9a"s

getBumOf9a"s;nMonth

getMonthBame

iseapear

6

9esign 9iagram

9esign 9iagram

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 printCalendar

%main&

read;nput  printMonth

get$tart9a"

 printMonthitle printMonthLod"

getotalBumOf9a"s

getBumOf9a"s;nMonth

getMonthBame

iseapear

67

9esign 9iagram

9esign 9iagram

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 printCalendar

%main&

read;nput  printMonth

get$tart9a"

 printMonthitle printMonthLod"

getotalBumOf9a"s

getBumOf9a"s;nMonth

getMonthBame

iseapear

68

9esign 9iagram

9esign 9iagram

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0

9esign 9iagram 

 printCalendar

%main&

read;nput  printMonth

get$tart9a"

 printMonthitle printMonthLod"

getotalBumOf9a"s

getBumOf9a"s;nMonth

getMonthBame

iseapear

;mplementation opD9on

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;mplementation opD9on

A $%eleton &or 'rintCalendarA $%eleton &or 'rintCalendar

opDdon approah is to implement one method in the

struture hart at a time from the top to the bottom# $tubsan be used for the methods aiting to be implemented# :

stub is a simple but inomplete !ersion of a method# he

use of stubs enables "ou to test in!oing the method from

a aller# ;mplement the main method first and then use a

stub for the printMonth method# For eample, let

 printMonth displa" the "ear and the month in the stub#

hus, "our program ma" begin lie this

;mplementation LottomDAp

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;mplementation LottomDAp

LottomDup approah is to implement one method in the

struture hart at a time from the bottom to the top# Foreah method implemented, rite a test program to test it#

Loth topDdon and bottomDup methods are fine# Loth

approahes implement the methods inrementall" and

help to isolate programming errors and maes debugging

eas"# $ometimes, the" an be used together#

Lenefits of $tepise =efinement

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Lenefits of $tepise =efinement

$impler Program

=eusing Methods

@asier 9e!eloping, 9ebugging, and esting

Letter Failitating eamor