oop in java by kasper b. graversen1 oop in java /ebuss kasper b. graversen lecture 2

OOP in Java by Kasper B. Graversen 1

OOP in Java /EBUSSOOP in Java /EBUSS

Kasper B. GraversenLecture 2


Course changeCourse change• Due to lack of computers

– Labs from 9-12 in 3.15 + 3.16 – Lectures 13-16 in 3.14– 7 out of 9 homework assignments must be

passed– First one will be given next Wednesday

• Today stops at 12• Course homepage

http://www.it-c.dk/~kbilsted


Questions/commentsQuestions/comments

• Did the book make any sense?• Do you have a question for today’s

chapter?


On the right trackOn the right track• Code-Compile-Execute

cycle• Installed and played with

the compiler

•Smallest Java programclass A{ public static void main(String[] args) { System.out.println(“java is fun”); }}


On the right track 2On the right track 2• Stay on the right side of the

tracks. • Use the compiler and play with

the examples the book presents

• Flunking-% on this type of course at IT-C is ~40%


Course dictionaryCourse dictionary• Attribute,variable:Attribute,variable:

defines a property of an object• Method,function:Method,function:

defines functionality of an object• Class:Class:

Defines an object• Syntax:Syntax:

What we write• Semantics:Semantics:

The meaning of what we write


Today’s programToday’s program

• Documentation 101• Simple types• Objects• Coding style


Comments 101Comments 101• // till end of the line• /* spanning multiple lines */

• /** special comments spanning multiple lines */

• Comments always above or to the right of what is being commented on!

• Nesting is not allowed!


The Java languageThe Java language

“In Java everything is an object!”

*

*Except for simple types

i.e. the car/shop


Simple types in JavaSimple types in Java

• Simple types are– boolean: boolean (logiske værdier)– integer: int long (heltal)– float: float double (kommatal)– character: char (a-Z-0-9)

• No ambiguity!– The current speed of a car is not just “a value”

its representation must be clearly expressed.


Simple types Simple types In earlier languages

• Simple types was all one had• It’s type was determined by its name

– i,j,k,l were integers a,b,c,d were chars

• In “BASIC” a variable could change type runtime

• In “C” these could be combined in structures - like a class with no functionality


Simple typesSimple types

• Why simple types?– Speed, storage space

• Why is a string not a simple type– Because it has functionality, ie.

String s = “hamburger”;System.out.print(s.substring(4,8));

gives “urge” not “burg”


How we count when How we count when programmingprogramming

• We always count from 0!– Seen in particularly old literature

• Thus the first position is in a string is position 0

• However, the length of a string is equal to the number of characters in the string, ie “foobar” == 6


How we represent How we represent characters in a computercharacters in a computer

• Each character is nothing but a integer value in the computer.

• These values are called ASCII values• In the computer world only the first 127

values are standardized. • Danish characters lies outside 127, thus it

can be problematic to transfer raw text from ie Windows to Mac

• Try holding down the alt key and type a number between 0-255 on the numeric pad and release all keys. (65 will produce an “A”)


How we represent simple How we represent simple types in a computertypes in a computer

• Is a predefined small chunk of the computers memory.

• These chunks are measured in “bits” and “bytes” - more on this in the next lecture.


How to use simple typesHow to use simple types

• Nothing new • We now know

how to specify these attributes and how we can represent them.

Wheel Wheel

WheelWheelMotor

Steering mechanism

Bodyspeed

colorprice cleanness

• Or maybe we are just confused on a higher level?


How would you represent How would you represent the attributes?the attributes?

class Motor{ int speed; // meter/hour}

What happens if I choose meter/second?

10 m/s = 36 km/h11 m/s = 39,6 km/hinsecurity of speed = 3,6 km/hfirst choice insecurity = 1 m/h

The motor

Motor

speed


How would you represent How would you represent the attributes?the attributes?

class Body{ int price; // in $ String color; // only “blue”, “black”, “green” ??? cleanness;}

How do we ensure only legal values are used? Is “bluegreen” a valid color and can the system handle it? Compare it? Etc.Sometimes the solution is to not use simple types.

The bodyBody

colorprice cleanness


Comments and attributesComments and attributes

• Still using comments to specify the units are important. A unit confusion between pounds and newtons led to NASA loosing $94 million on a Mars Climate Orbiter!


The Java languageThe Java language

• Simple types• Classes• “Special classes”

– Integrated in the syntax• String• Array

– Always exists• System


Roles of objectsRoles of objects

• Data container– A king (name, birthyear, deathyear, sons etc.)

• Functionality provider– A dictionary, converters

• Mix– Bank account

• data: balance, name,...• functionality: deposit, withdraw, interest rate,...

– Human


Object theoryObject theory• An object is an encapsulation of

attributes and functionality.• Notably, it allows concealing of

internal representation and supplying easy to use methods.– speedup(int km) in a car. – deposit(int cent) in a bank account – match(Colour othercar)

• Allows creating a new kinds of types


Object TheoryObject Theory

Object

Internal representation

Objectinterfaces(aka methods)


Object TheoryObject Theory• Plato (428-348 BC) perceived the

world as flawed copies of ideas. All horses derive from the abstract idea “horse”

• This explained the different appearances of horses



• In Java a class is “the idea” • An object is “the copy”

– formally called an instantiation of the class.

• Designing classes is a matter of defining static structure.


Object TheoryObject Theory• How to model a horse?

class Horse{ String name; int energy; // limits 0-2

void walk() { if(energy > 0) energy--; } void appearance() { if(energy == 0) System.out.println(“dead”); if(energy == 1) System.out.println(“drowsy”); if(energy == 2) System.out.println(“fresh”); } void eat() { energy = 2; }}



• To create a horse, we must create an objectHorse h1 = new Horse();

• and set appropriate valuesh1.name = “lucas”;

h1.energy = 2;

• The dot-operator (.) gives access to the objects internals.



• FormallyHorse h1 = new Horse();

• isType nameOfPointer = new Type();

• h1 is of the type Horse• h1 has the ability to point to horse objects

• h1 is pointing at a freshly created object


Object TheoryObject TheoryHorse h1 = new Horse();h1.name=“silas”; h1.energy=2;

Horse h2 = new Horse();h2.name=“superhorse”; h2.energy=99;

h1

h2

Horse

Silas2

Horse

Superhorse99


Object TheoryObject Theoryint h3 = new Horse();h3.name=“jürgen”; h3.energy=2;

h1 < h2

h1

h2

Horse

Silas2

Horse

Superhorse99

h1.energy < h2.energy

h1 = h2

h1 == h2



Horse h1 = new Horse();Horse h2 = new Horse();

h1.name=“silas”; h2.name=“superhorse”;

h1.energy=2; h2.energy=99;

h1.appearance(); h2.appearance();

h1.walk(); h2.walk();






Poor Silas DIED due to a simple cheat!

...racing the horses...racing the horses



• The program was “mechanically” correct

• Unexpected input made it perform faulty

• Programming is more than working out the “mechanics” also the input/data needs be secured within expected limits.

……programs & inputprograms & input



• Setting up constraints in the comments clearly doesn’t suffice!

• Additionally its easy to forget to initialize all attributes

• Solution: add a constructor

……constructing valid objectsconstructing valid objects

Horse(String name, int e){ this.name = name; energy = e;}

Easy to type,hard to read!

No return type



• But constructors can also check input against the constraints of the class

……constructing valid objectsconstructing valid objects

Horse(String name, int energy){ this.name = name; if(energy < 3) this.energy = energy; else this.energy = 2;}


Object TheoryObject Theory• Constructors defines an interface

to creating objects• When the constructor is defined

Horse foo = new Horse(); is now illegal

• The new operator must obey the constructorHorse foo = new Horse(“max”, 2);

A string An integer


Starting programsStarting programs• How do we get

started?– Functionality resides

inside objects– And can only be

executed when objects are created

– It takes functionality to create objects

Horse h1 = new Horse(“silas”,2); Horse h2 = new Horse(“super”,99);h1.appearance();h2.appearance();h1.walk(); h2.walk();h1.appearance();h2.appearance();h1.walk(); h2.walk();h1.appearance();h2.appearance();h1.walk(); h2.walk();h1.appearance();h2.appearance();

Our horse race


Starting programsStarting programs

• Java has a bootstrapping mechanism called main() which is automatically called when the virtual machine is started with a class.

• The static allows Java to access the main method before object creation.


Starting programsStarting programs

• javac *.java• java HorseRace

class HorseRace{ public static void main(String[] args) {

Horse h1 = new Horse(“silas”, 2);Horse h2 = new Horse(“superhorse”, 99);

h1.appearance(); h2.appearance();h1.walk(); h2.walk();h1.appearance(); h2.appearance();h1.walk(); h2.walk();h1.appearance(); h2.appearance();h1.walk(); h2.walk();h1.appearance(); h2.appearance();

}}

How do we get on from here?


Order of executionOrder of execution

• One line at a time• In order to use a variable/object it

must first be created

main(…){

beers = beers+1;int beers;

}


staticstatic is forbidden! is forbidden!

During compilation the compiler reports

class SomethingAbstract{ public static void main(String[] args) { f(); } void f() {…}}

A.java:5: non-static method f() cannot be referenced from a static context f(); ^

Must be read as: you forgot a new somewhere

static


Code conventionsCode conventions

• Class names are always spelled with a starting capital letter

• The opposite for methods and attributes

• Attributes can not contain spaces so multiple words are concatenated– int theDogsMothersName


Code conventionsCode conventionsThe codes are identical to the compiler,are they to you too?

class A{public static void main(String[] args){f();}void f(){System.out.print("foo");}}

class A{ public static void main(String[] args) { f(); } void f() { System.out.print("foo");} }


Code conventionsCode conventionsclass A{ int a; String b;

A(String b, int a){…} void b(){…} void c(){…} void d(){…}}

class Horse{ String name; int energy; Horse(String name, int energy){…} void walk(){…} void appearance(){…} void eat(){…}}

Which do you prefer?

Spend time on proper naming.


Special classesSpecial classesAs String is a class not a simple type, writing

String s = “foo”;

means

String s = new String(“foo”);

Horse

Silas2

Should therefore be depicted as

Horse

name2

String

Silas


The endThe end

oop in java by kasper b. graversen1 oop in java /ebuss kasper b. graversen lecture 2

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