T1 /Java OOP Yet again, with Kasper B. Graversen

T1 /Java OOPT1 /Java OOPYet again,Yet again, with Kasper B. Graversen with Kasper B. Graversen

Today’s planToday’s plan

• Number representation

• Logic operators and % operator

• Operator precedence

• Type casting

• JavaDoc

• Flow control

• Objects representing amounts

Nerdy, weirdeven boring!But necessary.

Useful and exciting - Stay awake!

On the right trackOn the right track

•The main properties of an object (instantiation, methods, attributes)

•The creation of objects with pointers pointing to them

•The difference between simple types and objects

DictionaryDictionary

• StaticStaticAccess to methods without object creation, only allowed in main() at this stage of the course!

•Constructor“method” automatically called at object creation

•“.” - Dot operatorAccess to attributes and methods in objects

Representation

100101 < 38== 26Binary Decimal Hexadecimal

37

Representation

1234

ones

tens

hundreds

thousands

100101one’s

two’sfour’s

eight’ssixteen’s

thirty-two’s

Formally:4*10 + 3*10 + 2*10 + 1*101 20 3

Equally:1*2 + 0*2 + 1*2 + 0*2 +...0 1 2 3

Representation

•Numbers are represented by bits.

•37 = 100101 , here rep. by 6 bits

•as int in Java (32 bit) =00000000 00000000 00000000 00100101

• Ranges of int:2 = 4.294.967.296 = ~-2.1 to ~2.1 billion

32

RepresentationRepresentation

•So, is a 64 bit number twice the size of a 32 bit number?

•No, the size grows exponentially, the range doubles for each new bit, ie. 1, 2, 4, 8, 16

•64 bit ~ 1,8*10

19

Calculations w. binary Calculations w. binary numbersnumbers

11001 + 0101 =11110

25

+ 5= 30

Representation of special Representation of special charschars

•How do we represent a ‘return’?– \b backspace

– \t tab

– \n linefeed

– \" double quote, "

– \' single quote, '

– \\ backslash, \System.out.println(”\”hello world\””);

•In programming ambiguity is forbidden, so we assign priorities to all the operators•With “(“ “)” we can change the order of evaluation to:

Operator precedenceOperator precedence

• What is 2+3*4 evaluated to? 14 or 20

+

2 3

4

*

+

2

3 4

*

Operator precedenceOperator precedence

• This is possible since () has the highest evaluation priority.

• Removing unnecessary () can clear up an expression

• To beginners () are clearer.

Operator precedenceOperator precedence1. ( )

2. *, /, % Multiplicative operators

3. +, - Additive operators

4. <, >, >=, <= Relational operators

5. ==, != Then do any comparisons for equality and inequality

6. & Bitwise and

7. | Bitwise or

8. && Logical and

9. || Logical or

10. ? : Conditional operator

11. = Assignment operator

Logic operatorsLogic operators

• Programs usually check for more than one condition.

• A bank account systemint withdraw(int dollars){ if(dollars > 0) { if(balance > dollars) { balance -= dollars; return balance; } }}

Tiresome!

if(dollars > 0 && balance > dollars){

balance -= dollars;return balance;

}


• Don’t use |, & or ^ they are bitwise operators

• && AND || OR ! NOT

• “Not both Pinocchio and Santa”

!(Pinocchio && Santa)

•Any milk: non fat, low fat, regular

nonfat || lowfat || regular


•The operators are very expressive.

•If v1,v2,v3 are “votes” when is the expression true?

(v1 && (v2||v3)) || (v2 && v3)

•The expression is true if at least two of the three votes are true

Alternative to logical op.Alternative to logical op.

•Another approach would be to count the votes manuallyint votes = 0;

if(v1) votes++;

if(v2) votes++;

if(v3) votes++;

if(votes >= 2)

{

}

if(v1 && (v2||v3) || v2 && v3) {

}

The % operatorThe % operator•The % (called modulus) returns

the remainder of a division.

•15/3 == 5 15%3 == 0

17/3 == 5 17%3 == 2

•xxx%3 returns numbers ranging 0-2

•How do we find out if number X is odd or even?

if(x%2 == 0) …if(x%2 == 1) …

Types and castingTypes and casting

•Explicit type casting

–simple types

–objects

•implicit type declaration (by syntax)

Explicit type castingExplicit type casting

•Formally: (Type) Expression– (int) 2.0

– (Horse) foo

•Inserted everywhere

– In parameters to method calls

– In front of method calls to alter return type


• Its needed when loss of data may occur, ie double put into an integer– int i = 9.0/4.0 (compiler error)

– int i = (int) 9.0/4.0

•Problems arise especially when types are intermixed– 1/2*3.5 = 0.0– 1.0/2*3.5 = 1.75


•Its needed when methods require other type of data than you have– double amount = 9.95;

withdraw(amount);

Must be changed to

withdraw((int)amount);

Implicit type declarationImplicit type declaration• 1 int

• 1L long

• 1.0 double

• 1.0ffloat

• ‘c’ char

• “c” String (object!)

• 01 Octal numbers!!!!

Implicit type declarationImplicit type declaration

•Two rare examplesWhat number does the following program print and why?

int a = 1000;int b = 0010;System.out.print(a-b);

Answer: 992

byte b1 = 2, b2 = 3, b3;b3 = b1 + b2;possible loss of precisionfound : intrequired byte b3 = b1 + b2; ^1 error

b3 = (byte)((int)b1 + (int)b2);

Control Of FlowControl Of Flow

•In java there are many possibilities of controlling the flow of the program. We will look at –if-else–for()

Control Of Flow: if-elseControl Of Flow: if-else

•We can branch the codecondition

thenstatements(code)

“meet at work”

if(floor.isDirty()){ floor.clean();}

“do the baking”

“meet at work”

“do the baking”

truefalse

Control Of Flow: if-elseControl Of Flow: if-else

•Sunday baker…if(day == sunday){ baker.makeDow(); Loaf l = baker.makeLoaf(); oven.insert(l); …}else{ baker.go(home);}…

condition

thenstatements(code)

true false

elsestatements(code)

Control Of Flow: if-elseControl Of Flow: if-else•What is the difference?

if(condition1){ statements1

}else if(condition2) { statements2

}else if(condition3) { statements3

}else { statements4

}

if(condition1){ statements1

}if(condition2) { statements2

}if(condition3) { statements3

}else { statements4

}

Dating for engineersDating for engineersint age;

System.out.print("How old are you: ");

age = Integer.parseInt(stdin.readLine());

if (age < 18)

System.out.print("Do you have an older sister/brother?");

else

if (age < 25)

System.out.print("Doing anything tonight?");

else

if (age < 35)

System.out.print("Do you have an younger sister/brother?");

else

if (age < 65)

System.out.print("Do you have a daughter/son?");

else

System.out.print("Do you have a granddaughter/grandson?");

-Andrew Taylor

How does the program run when ages 16 and 27 are met?

Control Of Flow: forControl Of Flow: for

•Loops are used to solve jobs which consist of one or more sub-jobs repeated.

–Reading a file a line at a time

–Interrogate all the suspects one by one

Control Of Flow: forControl Of Flow: for

•The syntaxinitial

statements

next

conditionfalse

truefor(initial ; condition ; next){ statements… }

Counting to 13int i;for(i = 0; i < 13; i++){ System.out.println(i);}

Why not int i = 0 instead of i = 0

““Quick Tips”Quick Tips”

•Creating the “quick tips” machine–There are 13 football matches a week

–Each match can either be won by either team or a draw, which is denoted 1, 2 or X


•A computer definition

–Do something 13 times

–At each time randomly find either 1, 2 or x

How do we do this?

Using random numbersUsing random numbers

•The javadoc explains• public static double random()

Returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0.

•The javadoc tophttp://java.sun.com/j2se/1.3/docs/api/index.html

http://java.sun.com/j2se/1.3/docs/api/java/lang/Math.html


•So (int) Math.random()*100

yields numbers from 0-100• (int)(Math.random()*100%3)

yields numbers from 0-2


The final programfor(int i = 0; i < 13; i++){ System.out.print(“Row “ +(i+1)+ “:”); int val = (Math.random()*100)%3; if(val == 0) System.out.println(“x”); else System.out.println(val);}

Why the green parenthesis?

Typed methodsTyped methods

•Are like methods, now they just return a value of a certain type

–Allows functionality which do more than print on the screen

–Allows splitting up a complex method into several sub-methods.


• FormallyreturnType methodName(Type input,…)

{

…

return someThingOfTheType;

}

• int celsiusToFarenheit(int celsius)

{

double fh = celsius * 9.0 / 0.5 + 32;

return Math.round((float) fh);

}

Typed methodsTyped methods•For our horse example from last

time:• class HorseEvaluator

{ /** return 0-100 where 100 is a super horse */

int evaluate(Horse h)

{

int grade; … ; return grade;

}

}

• class HorseDealer

{

HorseEvaluator friend;

void buyHorses()

{

for(int i = 0; i < noHorses; i++)

if(friend.evaluate(hx) > 90)

buyThatHorse();

} }


•If the horse evaluator only printed strings on the screen, the horse buyer could not ask for his advice!

Objects representing Objects representing amountsamounts

•We are to create a web store

•The store sells promotion pens, shirts and mugs


•Last time we looked at objects representing one item, ie. a horse.

•We looked at how to represent the horse and especially which attributes should be used in the program


•Each item in the store can be modeled as

–A name

–A description

–A price


•We can design the shop either as

Shop

Accountingboolean sell(int no)

shirts pens mugs

Shop

Accountingboolean sell(int no)

numberLeftdescriptionprice

or


•We must always balance between a too simplistic and a too complex model of the world.

•This should be evaluated against the current use of the program - as well as future use

String representation of String representation of objectsobjects

Horse h = new Horse(“Silas”,2);System.out.print(h);

A@73d6a5

• To make a better representation of an object, create a special toString() method

String representation of String representation of objectsobjects

class Horse{ … public String toString() { String s = “name: ”+name; s = s + “ energy: “ +energy; return s; }}

System.out.print(h) Will now give us more meaningful information

OOP in JavaOOP in Java

The end

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T1 /Java OOP Yet again, with Kasper B. Graversen