Lecture 5 Outline

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Lecture 5 Outline

• Project
• Arrays
• Full example class and objects

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Project

• BOSS submit - end date
• Ignore message not registered
• Automated marking so your output needs to be
  exactly as stated - else your program will fail
  the checks
• Must compile else score 0
• E.g
• Exercise 2 Write a program called
  MultiplesOfThree that will calculate the first n
  multiples of 3. You should ask the user to input
  how many multiples (i.e. n) they want to display
  on screen.
• An example of the expected output is
• How Many Numbers? 4
• Check your file name MultiplesOfThree case
  sensitive
• Submit only java files .java
• Check you give the correct command line arguments
  your program needs to be able to handle them
  correctly

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Command line arguments

• Public static void main(String args)
• Int a Integer.parseInt(args0)
• String - array starts0 java
• Int a Integer.parseInt(args0)

Use in your program
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Arrays
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Lecture Goals

• Revise arrays and two-dimensional arrays
• Work through a full example - designing classes
  and objects

Continued
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What is an Array

• one-dimensional arrays
• An array has a fixed number of elements and all
  elements are of the same type
• Each box has an index which in java and C
  starts with 0
• Here is an array which holds the ages of 10
  people

0 1 2 3 4
5 6 7 8 9
32 34 56 32 12 67
21 34 21 45
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Arrays

• Array Sequence of values of the same type
• Construct array
• Store in variable of type double

new double10
double data new double10
Continued
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Arrays

• When array is created, all values are initialized
  depending on array type
• Numbers 0
• Boolean false
• Object References null

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Arrays
Figure 1An Array Reference and an Array
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Arrays

• Use to access an element

data2 29.95
Figure 2Storing a Value in an Array
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Arrays

• Using the value stored
• Get array length as data.length.
• Index values range from 0 to length - 1

System.out.println("The value of this data item
is " data4)
Continued
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Arrays

• Accessing a nonexistent element results in a
  bounds error
• Limitation Arrays have fixed length

double data new double10data10 29.95
// ERROR
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Syntax Array Construction
 new typeNamelength Example  new
double10 Purpose To construct an array with a
given number of elements
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Syntax Array Element Access
arrayReferenceindex Example  data2 Purpose
To access an element in an array
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• Manipulation of arrays

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Lab

• Example SortTest.java
• Using arrays in sorting technique

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• public class SortTest
• /
• sort - sorts the values in the array x
  into ascending order
• /
• public static void sort(double x)
• int i,j
• double temp
• for (ix.length-1 igt0 i--)
• for (j0 jlti j)
• if (xjgtxj1) // swap
  round if out of order
• tempxj
• xjxj1

J j1 3 2 Temp 3
J j1 2 Temp 3
J j1 2 Temp 3
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/ main - tester for the method sort /
public static void main(String args)
int i,n double b
// Enter data used for testing MaInput
Manew MaInput() nMa.readInt("How many
numbers") // now when we know how many
numbers we want to sort // allocate the
memory bnew doublen // read the
data for (i0 iltn i)
biMa.readDouble("number "(i1)" ")

// Call the sort methods sort(b)
System.out.println("gt After sort - numbers
sorted lt") // Print the sorted array
for (i0 iltn i)
System.out.println("number "(i1)" "bi)
// End of the class
SortTest
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Two-Dimensional Arrays

• When constructing a two-dimensional array, you
  specify how many rows and columns you need
• You access elements with an index pair aij

final int ROWS 3final int COLUMNS
3String board new StringROWSCOLUMNS
boardij "x"
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A Tic-Tac-Toe Board
Figure 6A Tic-Tac-Toe Board
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Traversing Two-Dimensional Arrays

• It is common to use two nested loops when filling
  or searching

for (int i 0 i lt ROWS i) for (int j 0
j lt COLUMNS j) boardij " "
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File TicTacToe.java
01 / 02 A 3 x 3 tic-tac-toe board. 03
/ 04 public class TicTacToe 05 06
/ 07 Constructs an empty board. 08
/ 09 public TicTacToe() 10 11
board new StringROWSCOLUMNS 12 //
Fill with spaces 13 for (int i 0 i lt
ROWS i) 14 for (int j 0 j lt
COLUMNS j) 15 boardij "
" 16 17
Continued
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File TicTacToe.java
18 / 19 Sets a field in the board.
The field must be unoccupied. 20 _at_param i
the row index 21 _at_param j the column
index 22 _at_param player the player ("x" or
"o") 23 / 24 public void set(int i, int
j, String player) 25 26 if
(boardij.equals(" ")) 27
boardij player 28 29 30
/ 31 Creates a string representation of
the board, such as 32 x o 33
x 34 o 35 _at_return the string
representation 36 /
Continued
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Full example how to plan a program

• Noughts and Crosses
• Tick-tac-toe
• Look at building classes
• How to use Objects
• Difference between instance and class
  variables/methods
• 2-dimensional arrays

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The game Noughts and Crosses

• The Game problem definition
• Two players
• 3 by 3 board of squares
• One player crosses (X) , other noughts (O)
• Players take turns in placing a nought or cross
  in one of the nine squares
• A player wins if they have three of their pieces
• Placed horizontally, diagonally or vertically

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Planning a solution

• The problem definition broken down into parts
• Part 1 a 3-by-3 noughts and crosses board
• Part 2 two players
• Part 3 the game
• We can use Objects and Classes in Java to
  represent this problem/game

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But how?

• The board
• Represent board as 2- dimensional array
• Define the board class
• Define the methods associated with board class
• Instance methods for Board class, class methods
  for Board class
• Defining what is a winner

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The board

• The board contains
• A 3-by-3 set of squares
• Each square can contain a nought, a cross, or a
  blank
• Each square needs to be updateable (move)

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Representing the board using a 2-dimensional
array

• Assume the noughts and crosses are chars
• Char cross X
• Char nought O
• Char blank
• The board consists of an array of columns
• And is an array of rows
• Char board new charROWSCOLUMNS
• Char board new char33

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The board as 2-dimensional array

• This data is encapsulated in a board class
• Should the data belong to an instance or to the
  class itself?

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• Lets remind ourselves of instance variables and
  class variables

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Remember instance variables

• An object of a class is also referred to as an
  instance of that class
• When you create an object the object will contain
  all the variables that were included in the class
  definition.

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• However the variables in a class definition are
  not all the same there are two kinds
• One kind of variable is associated with each
  object uniquely each instance of the class will
  have its own copy of each of these variables ,
  with its own value assigned.
• These differentiate one object from another,
  giving an object its individuality name,
  address, telephone number Person class. These are
  referred to as instance variables

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Remember Static fields / class fields

• 2) The other kind of class variable is associated
  with the class and is shared by all objects of
  the class.
• There is only one copy of each of these kinds of
  variables no matter how many class objects are
  created
• and they exist even if no objects have been
  created

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• Called class variable because variable belongs to
  class. - or called static because used with
  static keyword
• They can be referenced by any object or class
• If the value of a class variable is changed then
  the new value is available in all the objects of
  that class.
• This is different from the instance variables
  where changing a value for one object does not
  affect the values in other objects

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Shared between all objects Sphere.PI
Class Sphere Definition --------------------------
------ Public class Sphere // class
variable Static double PI3.14 //instance
variables double xCenter double xCenter double
xCenter double xCenter
3.14
globe xCenter yCenter zCenter radius
Sphere Objects

• Ball
• xCenter
• yCenter
• aCenter
• radius

Each object gets its own copy
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Access modifiers in Java
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• Back to our program

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The board as 2-dimensional array

• This data is encapsulated in a board class
• Should the data belong to an instance or to the
  class itself?
• Static data - belongs to the class
• Non-static data - belongs each instance of
  the class
• Os and Xs ?
• Will be the same for each instance of the game,
  so they should be class based
• The board array?
• Is different for each instance of the game, so
  should be object (or instance) based

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The Board Class

• Class Board
• private static char cross X
  //class variables
• private static char nought O
• private static char blank
• private char board new char 33
  //instance variable
• Board()
• for (inti0 Iltboard.lenghtj) //for
  each row
• for (int j 0 jltboard0.lenghtJ)
  //for each column
• boardijblank

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Methods for the Board class

• Instance methods for the board class
• Printing the board (accessor/get method)
• public void printBoard() // INSTANCE METHOD
• System.outprintln()
• for int(i0 I lt board.length i) //
  for each row
• for (int j 0 jltboard0.length j)
• System.out.print( boardij
  )
• System.out.println()
• System.out.println()
• This is an instance method, it is non-static,
  because the data that we are getting is
  non-static

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Class methods for Board class

• Public static void setCross (char c)
• cross c
• Public static void setNought (char c)
• nought c
• Public static void setBlank (char c)
• blank c
• Using the methods
• instance methods boardObject.printBoard()
• Class methods Board.setBlank( )
• Static methods do not operate on an object but
  they are still defined inside the class

Why? because data is static
Prefix with name of the instance of the object
Prefix with name of class
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Detecting a winning move

• Add a data field which states whether a game has
  been won
• private boolean winner
• This is a non-static, as the win is associated
  with a particular instance of the board
• Set it to false in the constrcutor method
• Winner false
• Add a (non-static) method to access data field
• Public boolean isWinner()
• winner detectWinner() //to be defined..
• Return winner

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Defining a winner

• A winning move is one in which
• A row is completed with all noughts and crosses
• A column is completed with all noughts and
  crosses
• A diagonal is completed with all noughts and
  crosses
• private boolean detect()
• if (rowWinner() columnWinner()
  diagonalWinner())
• Return true
• Else return false

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• private boolean rowWinner()
• for (int 0 iltboard.length i) //for
  each row
• if ((boardi0 boardi1)(boardi1
  boardi2))
• if (boardi0!blank) return true
• return false
• private boolean columnWinner()
• for (int 0 iltboard.length i) //for each
  column
• if ((board0i board1i)(board1i
  board2i))
• if (board0i!blank) return true
• return false
• private boolean diagonalWinner()
• If((board00board11(board11board
  22))
• if (board00!blank) return true
• If((board02board11(board11board
  20))

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