OOP in Java Repetition Constants Selection Exception

1
OOP in JavaRepetition Constants
SelectionException
Auction

• by Kasper B. Graversen

2
Repetition

• 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

3
Repetition

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

4
Repetition
false

• Formallywhile(condition) code
• Unlike for we have to do the next code in the
  statements

true
statements
5
Repetition

• 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

statements
false
true
6
Repetition

• 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.

7
Repetition

• We can stop loops, ie. when looking for a person
  in a file

Student stud null do String name
in.readString() if(name.equals(hans))
stud new Student(name) break
while(stud null)
8
Repetition

• Given the formulacn c0 (1i/100)n

int interest(int i, int c0, int n) int
accumulated 0 for(int j 0 j accumulated 1i/100 int cn c0
accumulated return cn
9
OOP in JavaRepetition Constants
SelectionException

• by Kasper B. Graversen

10
Constants

• 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 name

class Deck public static final int DECSIZE
52
11
Constants

• Heavily used variable input which at compile time
  can be checked

class Student Student(String name,String
haircolor) if(haircolor.equals()) else
if() else Wrong haircolor

12
Constants
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)

• Students are then created with

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
13
Constants

• 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 -)

14
Constants

• Heavily used for arguments which can be checked
  at compile time

class Student Student(String name,String
haircolor) if(haircolor.equals()) else
if() else Wrong haircolor

15
Static 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

16
Static

• 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)

17
OOP in JavaRepetitionConstants
SelectionException

• by Kasper B. Graversen

18
Selection

• switch-case is an efficient and easy way to do
  selections

if(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
19
Selection
Limitations

• 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.

if(i 2 year 2001) if(age
25) if(surname.equals(Hans)
lastname.equals(Nielsen))
20
OOP in JavaRepetitionConstants
SelectionException

• by Kasper B. Graversen

21
Exceptions
lack of...

• V-shaped or chunked style
• of programming

if( ! -1) D if( ! -1) E
if( ! -1) F else
(ErrorA) else (ErrorB) else (Erro
rC)
if( -1) (ErrorA) return D if(
-1) (ErrorB) return E if( -1)
(ErrorC) return F
22
Exceptions

• 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.

23
Exceptions
class CardIndex try
setupWindow() loadPersons()
manipulate() savePersons()
catch(SetupException e)
catch(LoadException e) catch(SaveException
e) catch(Exception e)
24
Exceptions
try setupWindow() try
loadPersons() catch(IOException e)
manipulate() try savePersons()
catch(IOException e) catch(SetupExcepti
on e) catch(Exception e)
Sharing exceptions
25
Exceptions
Throwing exceptions

• Throw them explicitly
• Use classes which throws

private void setupWindow() throws
SetupException throw new
SetupException()
private void loadPersons() throws IOException
Stream in new Stream(file.txt) in.read()
26
Exceptions
In constructors

• Throwing an exception in a constructor terminates
  the construction of the object
• Student s new Student()
• if(s null)
• System.out.println(uhohh!)

27
Exceptions
Defining new ones

• Make a new class extending class Exception

class SetupException extends Exception public
SetupException(String s) super(s)
28
Case 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

29
Case study
whats relevant

• 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

30
Case 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

31
Case study

• A screen shot of the current program

32
The code
import 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())
33
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()
34
The code
private void setupWindow() window new
Display("Auction v1.0") window.println("Welco
me to auction v1.0") window.println("Today
we are selling " article)
window.println("Owned by " seller)
window.prompt("newBidmaker", " ")
window.prompt("newBid", bid100)
35
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()

36
The End