oop in java repetition constants selection exception by kasper b. graversen

OOP in JavaOOP in JavaRepetitionRepetition Constants Constants SelectionSelectionExceptionException

by Kasper B. Graversen

(c) Kasper B. Graversen 2

RepetitionRepetition• 3 different kinds

– for– while– do-while

• Each can be transformed to any other

• Enables stop-condition to be placed before or after a repetition


RepetitionRepetition

• The for() construction we saw 2 weeks ago.

initial

statements

next

conditionfalse

true


RepetitionRepetition

• Formallywhile(condition){ code… }

• Unlike for we have to do the “next” code in the statements

condition

statements

false

true


RepetitionRepetition• Formallydo { code… } while(condition);

• Unlike for we have to do the “next” code in the code

• Unlike for/while we do condition after the first loop

condition

statements

falsetrue


RepetitionRepetition• When to use what?

I prefer– for when a fixed number of iterations are

to be performed i.e. do something X times– while when doing something an unknown

number of times, I.e. reading information from a file

– do-while when I need code to be performed before a condition can be checked.


RepetitionRepetition• We can stop loops, ie. when

looking for a person in a fileStudent stud = null;do{ String name = in.readString(); if(name.equals(“hans”)) { stud = new Student(name);

break; }}while(stud == null);


RepetitionRepetition• Given the formula

cn = c0 * (1+i/100)n

do 1+i/100 n times

int interest(int i, int c0, int n)

{

int accumulated = 0;

for(int j = 0; j < n; j++)

accumulated *= 1+i/100;

int cn = c0 * accumulated;

return cn;

}

OOP in JavaOOP in JavaRepetitionRepetition Constants Constants SelectionSelectionExceptionException



ConstantsConstants• Constants are values which never

change value– pi = 3.1415926535– cards = 52

• Use the final keyword to ensures the values stay unmodified. Use only CAPITALS in the nameclass Deck{ public static final int DECSIZE = 52;}


ConstantsConstants• Heavily used variable input which at

compile time can be checkedclass Student{ Student(String name,String haircolor) { if(haircolor.equals(…)) else if(…) … else …Wrong haircolor… }}


ConstantsConstants

Students are then created with

class Student{ public static final int REDHAIR = 0; public static final int BLONDHAIR = 1; public static final int BLUEHAIR = 2; Student(String name, int haircolor) { if(haircolor == REDHAIR) … }}

Student s = new Student(“Leo”,Student.REDHAIR);Spelling errors such as REDHAR are checked (since there are no variables of such name)Student s = new Student(“Leo”,223);is not checked at compile time


ConstantsConstants• Its also an easy way to tell the user of

the object which hair colors are available (particularly in Javadoc)

• Using constants, however, do not prevent you from typing the wrong haircolor (you wanted a redhead but typed blond :-)


ConstantsConstants• Heavily used for arguments which can

be checked at compile timeclass Student{ Student(String name,String haircolor) { if(haircolor.equals(…)) else if(…) … else …Wrong haircolor… }}


Static & constantsStatic & constants• static means “exist outside of

objects”. No matter how many objects there will be only one constant (ie. 2000 Deck objects but only one DECKSIZE with value 52)

• static allows use of constants before any objects of the class has been instantiated.

• All constants should be static


StaticStatic • Use static on methods if you find

the course easy• Use static on methods if they do

not require an object to work in (ie. Math.max(int,int))

• static is useful but not used often (check the javadocs)

OOP in JavaOOP in JavaRepetitionRepetitionConstantsConstants SelectionSelectionExceptionException



SelectionSelection• switch-case is an efficient and

easy way to do selectionsif(i == 0) …(a)… else if(i == 1) …(b)…else if(i == 2 || i == 3) …(c)…else …(d)…

switch(i){ case 0: …(a)… break; case 1: …(b)… break;

case 2: case 3: …(c)… break; default: …(d)… break;}


SelectionSelection• switch Applies only to simple type

integer values int,long,char• Not efficient with intervals• Can not handle && used in if• The examples below show cases where

a “translated” to switch-case is impossible.

LimitationsLimitations

if(i == 2 && year == 2001)if(age > 25)if(surname.equals(“Hans”) || lastname.equals(“Nielsen”));

OOP in JavaOOP in JavaRepetitionRepetitionConstantsConstants SelectionSelectionExceptionException



ExceptionsExceptions“V-shaped” or “chunked” styleof programming

lack of...lack of...

if(… != -1){ …D… if(… != -1) { …E… if(… != -1) {…F… } else {(ErrorA);} } else {(ErrorB);}}else{(ErrorC);}

if(… == -1){ (ErrorA) return;}…D… if(… == -1){ (ErrorB) return;}…E… if(… == -1){ (ErrorC) return;}…F…


ExceptionsExceptions• Using exceptions separates logic and

error handling• Result: two clean code blocks.• Methods have two choices

– Handle the exception ourselves– Throw it back to the one who called the

method

• Choice depends on the situation. Can the error be handled locally then do it.


ExceptionsExceptionsclass CardIndex{ … try { setupWindow(); loadPersons(); manipulate(); savePersons(); } catch(SetupException e) { … } catch(LoadException e){ … } catch(SaveException e){ … } catch(Exception e){ … }}


ExceptionsExceptionstry{ setupWindow(); try { loadPersons(); } catch(IOException e){ … } manipulate(); try { savePersons(); } catch(IOException e){ … }}catch(SetupException e) { … }catch(Exception e){ … }

Sharing exceptionsSharing exceptions


ExceptionsExceptions

private void setupWindow() throws SetupException{ … throw new SetupException();}

Throwing exceptionsThrowing exceptions

•Throw them explicitly

•Use classes which throws

private void loadPersons() throws IOException{ Stream in = new Stream(“file.txt”); in.read…()}



• Throwing an exception in a constructor terminates the construction of the object

Student s = new Student()

if(s == null)

System.out.println(“uhohh!”);

In constructorsIn constructors



• Make a new class extending class Exception

Defining new onesDefining new ones

class SetupException extends Exception{ public SetupException(String s) { super(s);}}


Case studyCase study• A program is needed for having an

electronic auction on articles sold by people using the public transportation.

• At central station a computer is setup.• People should be able to see the article,

the current bid and be able to bid themselves

• Since central station closes at night, the program must be able to handle this


Case studyCase study• A program is needed for having an

electronic auction on articles sold by people using the public transportation.

• At central station a computer is setup.• People should be able to see the article,

the current bid and be able to bid themselves

• Since central station closes at night, the program must be able to handle this

what’s what’s relevantrelevant


Case studyCase study• Implicit requirements

– New bids must be higher than the existing one

– A bidder must give information about him (in our simple case, a name)

– The program must be quick and easy to use


Case studyCase studyA screen shot of the current program


The codeThe codeimport java.io.*;import javagently.*;class Auction{ private String filename; // name of dat file private String article; // name of article to be sold private String seller; // name of seller private int bid; // highest bid so far private String bidmaker; // current buyer private Display window; public static void main(String[] args) { try { Auction auc = new Auction("auction.dat"); while(true) { auc.round(); } } catch(IOException e) { System.out.print("An error has occoured+ " +e.getMessage()); } }


public Auction(String filename) throws IOException { this.filename = filename; readDatFile(); setupWindow(); }

private void readDatFile() throws IOException { Stream in = new Stream(filename, Stream.READ); String header = in.readString(); if(!header.equals("auction1.0")) { in.close(); throw new IOException("Wrong dat file, header was \""+header+"\""); } article = in.readString(); seller = in.readString(); bid = in.readInt(); bidmaker = in.readString(); in.close(); }


The codeThe code private void setupWindow() { window = new Display("Auction v1.0"); window.println("Welcome to auction v1.0"); window.println("Today we are selling " + article); window.println("Owned by " + seller); window.prompt("newBidmaker", " "); window.prompt("newBid", bid+100); }


public void round() throws IOException { window.println("Highest bid so far: " + bid); window.println("Made by " + bidmaker); window.ready("Press ready when you have entered the fields"); String name = window.getString("newBidmaker"); int newbid = window.getInt("newBid"); if(name.length() > 2 && newbid > bid) { bid = newbid; bidmaker = name; // save to disc Stream out = new Stream(filename, Stream.WRITE); out.println("auction1.0"); // print the header out.println(article); out.println(seller); out.println(bid); out.println(bidmaker); out.close(); } }


The EndThe End

oop in java repetition constants selection exception by kasper b. graversen

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