Intermediate Review

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Intermediate Review
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Intermediate Review

• References
• Basic inheritance
• Time classes
• Date Classes
• File input/output
• Packages

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Memory

• One of the good things about java is that it
  abstracts memory away
• You dont have to worry about how things are
  being put into memory
• But you still need to be aware of how things are
  represented

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Basic Example

• int sum 100
• int arr

100
SUM
X
arr
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Next

• int sum 100
• int arr
• arr new int10

100
SUM
arr
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Question

• What happens on the last line ?
• int arr
• arr new int10
• arr0 12
• arr new int5

100
SUM
arr
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answer

• A new set of ints are allocated and WE LOSE all
  data in the old one
• You need to copy over data (and use a temp array)

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References

• So a primitive variable is mapped to a location
  in memory
• int x x 234
• Class Objects are a little more complicated since
  they have member variables and methods
• Memory references will point to a location which
  has been setup with the object you create
• miniVan mycar
• mycar new miniVan(.)

x
234
HondaOdyssey2000Red
mycar
ref
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References

• Create new primitive variable y, will result in
  another memory location and value copy
• int y x
• Create another miniVan instance in the following
  will simply make it point to same place (unless
  new is used)
• miniVan oCar mycar

x
234
HondaOdyssey2000Red
y
234
mycar
ref
oCar
ref
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Who cares ?

• So what is the difference ??

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Difference !

• Messing with x, wont affect y
• Messing with class reference will change both
  objects

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Difference II

• oCar.year 2005
• Surprise!

x
234
HondaOdyssey2005Red
y
234
mycar
ref
oCar
ref
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Why?

• If its such a bad idea, why have it at all?
• Any ideas why we would want to create object
  using references?

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Advantage

• If the class is huge
• Dont want to keep copying all the member
  variables if I plan on only reading it

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Back to Arrays

• Can have array of length 0
• not the same as null
• int numbers
• numbers new int0
• numbers null
• What is the difference here ?

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Two dimensional arrays

• You can create an array of any object, including
  arrays
• Person bunch new Person10
• int table new int1020
• int t tableij
• An array of an array is a two dimensional array

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Before coding

• Before we start to code one last thing
• How to get user input ??
• Most languages give you access to something
  called STANDARD out/in

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Standard IN/OUT

• Assume there is some way to talk to user
• And get user input
• Very low level
• Want something a little higher so dont have to
  worry for example how they enter the information
• In theory could be using hieroglyphics ?

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Reading Input through scanners

• Construct Scanner
• from input stream (e.g. System.in)
• Scanner in new Scanner(System.in)
• nextInt, nextDouble reads next int or double
• int n in.nextInt()
• hasNextInt, hasNextDouble test whether next token
  is a number
• next reads next string (delimited by whitespace)
• nextLine reads next line

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Eclipse

• Start Eclipse
• Start a new project (right click in project
  explorer)
• Double click, right click on default package and
  start a new class
• Call it InputTester
• Check off that you want comments
• Check off you want a main

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Code then run this

• public class InputTester
• public static void main(String args)
• Scanner in new Scanner(System.in)
• System.out.print("How old are you?")
• int age in.nextInt()
• age
• System.out.println("Next year, you'll be "
  age)

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coding

• Let start to code
• Tic tac toe game
• A program to allow two used to play a game
• Where to start ??
• What objects can you think of ?

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Basic parts

• Board tic tac toe board
• What kind of method would you need here ?
• Although its a 3 by 3 board, lets keep it
  general
• Move
• The part which can ask for a move and check if
  legal and place into board
• Front end
• We would put main in here
• It would start a game
• When done ask if you want to play again

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Board class

• Would need a constructor
• Reset method to set everything to blank
• Place move to put a move into the board
• Print out to see the board

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Note please

• The users in the game are represented by Xs and
  Os
• No reason we cant use 1,2
• If have a member , what does it mean ?
• public static final int X 1
• public static final int O 2

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• Board.X
• Board.O
• Dont need to instantiate to use it
• Makes it easier to speak a common language when
  using the class

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• public class TicTacToepublic static final int
  EMPTY 0public static final int X 1public
  static final int O 2private final int SIZE
  3 //for 3x3
• private int board
• //ok lets add a constructor

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coding

• Add constructor
• Add reset method
• Add toString method
• Use for loop
• Need to translate from 1,2 to X,O
• Dont hard code values, use your final statics

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Next

• Lets code the move class
• Very basic
• Need method to get the users next move
• Assume move is a move from each user
• Need to ask board if game is done
• Add another method to the board game
• If user enters bad movewhat do you want to do ?

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Finally

• Now lets code the front end with main
• Create a class MainGameTTT
• Have a main in it
• What do we need to do next ?

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Game logic

• Start a game
• Loop
• At the end ask user if they want to play another
  game ?
• This is a little tricky to loop.any ideas ?

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Yay!

• Ok you have a working game
• Test it out on all your friends ?

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Multiple dimensions

• No reason cant create 4,5,6 dimension arrays
• Gets hard to manage
• Better idea
• Think about another way of representing the data
• Often creating an object is a better approach

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Arrays further

• Need to explicitly copy contents of arrays when
  resizing arrays with temp one
• Better solution
• ArrayList
• Vector
• Full object versions of arrays
• Capacity can grow over time
• Useful methods bundles with them

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code

• Create a new class with a main called VectorTest
• Create a vector
• Put in some stuff
• Print them out
• Replace something in the middle
• Print it out
• Clear the vector
• Print it out

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Default values

• Should be aware if you forget to set values
• Might mess up your logic
• Think of multiplying a bunch of numbers and not
  setting one of them
• Compiler/IDE will let you know if you forgot to
  set values (warning)

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• Hope you had fun learning this!