Learning java in 180 minutes

1
Learning java in 180 minutes
.
.

• With Kasper B. Graversen

2
Todays program

• Understanding classesattributes
• methods
• constructors
• access modifiers
• final, static
• Understanding scopes and references
• Classes using other classes
• Stockmarket
• Linked lists

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The linked list

• The linked list works as Vector
• It differs in that it is never and the
  underlaying structure is a linked list rather
  than a big array
• The methods are still the same
• get(),set(),size(),remove()
• LinkedList

4
Chapter 8

• Don't panic over chapter 8
• It shortly tells about advanced features in java
  such as classes inside classes.
• However, you should relate and understand the
  clone() and iterator()
• The coffee example is a bit long but worth
  reading when you
• However, mind that linked list can get fairly
  advanced should you think of functionality not
  covered in the book. So read it and try best
  possible to understand it

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Chapter 8

• Iterators may to you seem like a strange
  construction when you have for-loops and
  while-loops.
• At this stage just understand it as an
  alternative to the looping mechanisms in java.
• In later programming courses you will see that
  the idea "iterator" is very usefull for different
  traversals of a set of data.

6
Drawing UML

• Two kinds of UML
• Dynamic
• Runtime - hence dynamic
• Showing a situation at a certain point of the
  program execution (typically after basic
  initialization)
• Static
• Compiletime - hence static
• Shows the overall relations between classes which
  can be read directly from the source code

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Drawing UML

1

• Static

Stock
Dynamic
Stock
LinkedList
User 400
Stock
Sam
Stock
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Class building

• Define the appropiate attributes
• Proper names, not (a,b,c)
• Choose the proper types
• Simple types int,long,float,double, boolean,char
• Pointers to objects
• May consider differentiating between attributes
  for the object and for accounting
• for a customer object the name and the total
  amount of money spent
• Attributes define the states in which object can
  be in

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Class building

• Decide the valid data needed to create objects.
• Constructors must be build utilizing this data

class User String name double balance
User(String name, double balance)
this.balancebalance this.name name
User u new User("Kasper", 10000.0)
same order
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Class building

• If you have more than one constructor we say the
  constructor is overloaded.
• Each constructor must have a different signature
  from the others
• A signature is the number of arguments and their
  types

User(String name) User(String name, double
balance) User(String foo, String bar)
User u1 new User("Bent", 3.4) User u2 new
User("Bent","Sam") User u3 new User("Bent")
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Class building

• At this stage we now need to define the
  functionality of the object.
• However, not all objects have functionality - ie.
  the Stock object
• For each functionality we create a method.
• If methods can be overloaded aswell as
  constructors - as long as the overloaded methods
  has different signatures (you can't overload the
  return value)

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Class building

• After building a class we apply access modifiers
  to protect the object.
• The more restricted the object is the easier it
  is to use/re-use
• And the more secure we are that nothing
  completely weird happens
• We apply public in front of all attributes,
  constructors and methods which may be accessed by
  others
• And we apply private in front of those which are
  only for internal purposes.

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Study Case Stock trading

• A stock trading central is a place where stocks
  can be bought and sold by users.
• Users can hold several different stocks including
  stocks from the same compagny but with different
  values.
• User interact with the trading central through a
  GUI.

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Study Case Stock trading

• The screen seen from the user

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Object Oriented Design

• How do we create this program in Java?
• We identify building blocks (objects) which
  together makes the system
• We identify the needed functionality and
  attributes needed in these building blocks

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Object Oriented Design

• We try to isolate functionality so that only one
  object is responsible for a certain task
• This object may use other objects to further
  split the task, however, it should be the only
  one accessing these "helper objects"

account -ing
main program
internet connect
load/ save
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Study Case Stock trading

• A stock trading central is a place where stocks
  can be bought and sold by users.
• Users can hold several different stocks including
  stocks from the same compagny but with different
  values.
• User interact with the trading central through a
  GUI.

18
Study Case Stock trading

• A stock trading central is a place where stocks
  can be bought and sold by users.
• Users can hold several different stocks including
  stocks from the same compagny but with different
  values.
• User interact with the trading central through a
  GUI.

classes green
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Study Case Stock trading

• A stock trading central is a place where stocks
  can be bought and sold by users.
• Users can hold several different stocks including
  stocks from the same compagny but with different
  values.
• User interact with the trading central through a
  GUI.

classes green functionality purple
20
Study Case Stock trading

• A stock trading central is a place where stocks
  can be bought and sold by users.
• Users can hold several different stocks including
  stocks from the same compagny but with different
  values.
• User interact with the trading central through a
  GUI.

classes green functionality purple
attributes yellow
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Study Case Stock trading

• Static UML

1

Stock
User


1
1
StockCentral
1
1
GUI
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Study Case Stock trading
User String name -double balance -LinkedList
stocks buyStocks(Stock stock) sellStocks(int
StockNo) printInfo()
StockCentral -Display display -LinkedList
users -LinkedList stocks buyStocks(usr,stock,amou
n) sellStocks(usr,stock,amount) User
userInfo() addUser(String,int)
turn() -stockChange() -findUser(String
name) -findStock(String name)
denotes public - denotes private Due to lack of
space types are not shown
GUI
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Stock trading StockGUI
import javagently.Display class StockGUI
StockCentral central Display display new
Display("Stock central")
StockGUI(StockCentral central)
this.central central display.prompt("nam
e", " ") display.prompt("what",
"buy/sell") display.prompt("no",
"number") display.println("Welcome to the
stock central") display.ready()
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Stock trading StockGUI
((continued)) public void fetchValues()
"fetch the values" if(display.getString("
what").equals("buy")) central.buyStocks()
if(display.getString("what").equals("sell"))
central.sellStocks() public void
println(String s) display.println(s)

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Study Case Stock trading
User String name -double balance -LinkedList
stocks buyStocks(Stock stock) sellStocks(int
StockNo) printInfo()
StockCentral -Display display -LinkedList
users -LinkedList stocks buyStocks(usr,stock,amou
n) sellStocks(usr,stock,amount) User
userInfo() addUser(String,int)
turn() -stockChange() -findUser(String
name) -findStock(String name)
denotes public - denotes private Due to lack of
space types are not shown
GUI
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Stock trading Stock

• The Stock is a simple "container class" as there
  is no functionality.
• What does it represent? an amount of stocks

class Stock public String name public
double val public int amount public
Stock(String name, double val, int amount)
this.name name this.val val
this.amount amount
"this" refers to the variables in the class scope
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Stock trading User

• Users is roughly a collection of stocks
  identified by a name
• Notice that the StockCentral is the central class
  in the system (like an octapussy holding on to
  all essential parts). It has full control. The
  user has no way of cheating.

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Study Case Stock trading
User String name -double balance -LinkedList
stocks buyStocks(Stock stock) sellStocks(int
StockNo) printInfo()
StockCentral -Display display -LinkedList
users -LinkedList stocks buyStocks(usr,stock,amou
n) sellStocks(usr,stock,amount) User
userInfo() addUser(String,int)
turn() -stockChange() -findUser(String
name) -findStock(String name)
denotes public - denotes private Due to lack of
space types are not shown
GUI
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Stock trading User
class User public String name private
double balance private LinkedList stocks
public User(String name, double balance)
this.name name this.balance balance
stocks new LinkedList() public
buyStocks(Stock stock) stocks.add(stock)
balance - stock.amount val
public sellStocks(int stockNo, int amount)
Stock stock (Stock) stocks.get(stockNo)
if(amount lt stock.amount) balance
amoung stock.val stock.amount -
amount if(stock.amount 0)
stocks.remove(stockNo)
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Stock trading User
(((class User continued))) public void
printInfo(Display display)
display.println(name " you have the following
stocks") for(int i 0 i lt stocks.size()
i) Stock stock (Stock)
stocks.get(i) display.println("" i "
" stock.name " " stock.amount "
at " stock.val) display.println("An
d a balance of " balance)
How to expand it so it will print the total
wealth the user has?
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Stock trading User
At each iteration we accumulate the value for the
set of stocks
public void printInfo(Display display) double
totalwealth 0 display.println(name " you
have the following stocks") for(int i 0 i lt
stocks.size() i) Stock stock
(Stock) stocks.get(i) display.println(""
i " " stock.name " "
stock.amount " at " stock.val)
totalwealth stock.amount stock.val
display.println("And a balance of " balance)
display.println("total balance " totalwealth)
If stocks change then this number only shows the
money spent!
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Repetition Variables

• What does the statement b mean? ie. in a b c
• It means "read the value of the variable b" (if
  the variable is a simeple type)
• Or tell which object b is pointing at(if the
  variable is not a simeple type)
• The order of evaluation
• Evaluate the rigth side and set the value to the
  left side, ie. a b a(read the value of b and
  a and add them, and assign the variable a to the
  result

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Study Case Stock trading
User String name -double balance -LinkedList
stocks buyStocks(Stock stock) sellStocks(int
StockNo) printInfo()
StockCentral -Display display -LinkedList
users -LinkedList stocks buyStocks(usr,stock,amou
n) sellStocks(usr,stock,amount) User
userInfo() addUser(String,int)
turn() -stockChange() -findUser(String
name) -findStock(String name)
denotes public - denotes private Due to lack of
space types are not shown
GUI
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Stock trading StockCentral
import javagently. class StockCentral
StockGUI display LinkedList users
LinkedList stocks StockCentral()
display new StockGUI(this) users new
LinkedList() stocks new LinkedList()
users.add(new User("Hansi", 1000.0))
users.add(new User("Liza", 2000.0))
stocks.add(new Stock("IBM", 1000, 2000))
stocks.add(new Stock("RedHat", 1000, 888))
stocks.add(new Stock("Jedit", 10, 40))
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Stock trading StockCentral
(((continued))) public void buyStocks(String
usrnam, String stocknam, int amt) try User
user findUser(usrnam) //get at reference to
the user Stock stock findStock(stocknam,amt)
// get new stock object user.buyStocks(stock)
catch(Exception e) display.println(e.getMess
age()) private User findUser(String name)
User user null for(int i 0 i lt
users.size() i)//search the list of users
user (User) users.get(i) // get the
i'th user if(user.name.equals(name))
return user // if it's the
user return throw new Exception("User not
found")
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Stock trading StockCentral
(((continued))) private Stock findStock(String
stockname, int amount) Stock stock null
for(int i 0 i lt stocks.size() i)//search
the stocks stock stocks.get(i)
// get the i'th stock if(stock.name.equal
s(stockname)) if(stock.amount gt amount)
stock.amount - amount
stock new Stock(stock.name, amount)
return stock throw new
Exception("Not enough stocks") throw
new Exception("The stock was not found")
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Stock trading StockCentral

• Why do we create a new Stock and not a new User?
• The amount the User has and the amoun which are
  at the StockCentral are different
• The User and the StockCentral should not share
  the same stocks, as the Stock's in StockCentral
  changes value all the time

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Stock trading StockCentral
(((continued))) public void turn()
stockChange() display.fetchValues() //
change the value of all stocks private void
stockChange() Stock stock for(int i
0 i lt stocks.size() i) stock
stocks.get(i) stock.val
(Math.random()-0.5)stock.val
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Stock trading StockCentral
(((continued))) public static void
main(Stringargs) StockCentral central
new StockCentral() for(int i 0 i lt
365 i) central.round()
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Scopes Test

• What does the following program output?

class Foo int x, y void bar(int x, int y)
this.x x this.y y x
calc() y calc() int
calc()return -100 public static void
main(String args) Foo bar new
Foo() bar.bar(1,2)
System.out.println(bar.x " " bar.y)
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Linked Lists

• Each object has a pointer to the next object

SOS
BBQ
TA
BT
class Stock String name double val
Stock next Stock(String n,double v,
Stock ne) namenvalv next ne

Stock s new Stock("SOS",55, null) s new
Stock("BBQ", 222, s) s new Stock("TA", 8,
s) Stock head new Stock("BT",123, s)
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The end...

• Happy holidays
• The rest of the course will be lectured by Uffe
  Kofoed
• bye bye merry X-mas -)