Lecture 4 Class

8/6/2019 Lecture 4 Class

1/39

SOFTENG251 Lecture 4: Making Classes palette 1 / 31

SOFTENG 251:

Object-Oriented Software Construction

Lecture 4: Making Classes

Ewan TemperoDepartment of Computer Science


2/39

Potential Assessment Question

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields

Initialising fields (1)

Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell

Card: A first (static) go

Using static Card

Card: A second (static) go

Using static Card

Review


Which of the following statements is most correct with respect to the

following Java method?

p r i v a t e void method ( ) {char a , b , c ;

a = a ;

b = c ;

c = b ;

System . o u t . p r i n t l n ( + b ) ;

}

(a) The method will compile and when executed print the character b.

(b) The method will compile but will fail with an error when executed.

(c) The method will not compile due not putting void as the parameter.

(d) The method will not compile due to some other reason.

Justify your answer.
http://void/http://void/


3/39

Agenda

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review


Admin

Quiz 1

What is a class

Classes in Java Fields

Reading:

Lesson: Language Basics (finish)

Lesson: Classes and Objects (first 3 sections) Text: Chapter 5


4/39

Previously in SOFTENG 251

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review


public static void main(String[] args) {System.out.println("FreeCell started.");

....

placeCardsOnBoard(board, deck);....

displayBoard(board, foundations, cells);

}

/**

* Produce a string that provides an ASCII representation

* of the specified card. The card is an integer in the

* ...*/

private static String getCardASCIIDisplay(int card) {....

}


5/39

What is a class?

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review


A group of methods (and fields) identified by a name

A unit of encapsulation

Can be used to provide abstraction

Can provide objects

Can be used to for checking for errors


6/39

Where do Classes come from? (1)

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review


A good class consists of methods (and fields) that belong together or

do related things

A good class provides a single coherent concept (an abstraction)

A good class is generally easier to understand and easier to change

(because everything in the class is related to a single concept) One attribute of goodness of class is cohesion

find related things and put them in a class


7/39

Which of these things belong together?

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review



int[] deck = createDeck();shuffleDeck(deck);

// Board consists of 8 cascades of maximum length 52

int[][] board = new int[52][8];

placeCardsOnBoard(board, deck);

int[] foundations = new int[4]; // Stores the top card

int[] cells = new int[4];

// Print the board, including foundations and cellsdisplayBoard(board, foundations, cells);

}


8/39

The Board Class

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review


package se251.freecell.v4;

public class ASCIIBoard {

}


9/39

The Board Class

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review




public static void placeCardsOnBoard(int[][] board,int[] deck) {

.....

}public static void displayBoard(int[][] board,

int[] foundations,int[] cells) {

.....

}

}


10/39

The Board Class

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review





.....



.....

}

}


11/39

The Board Class

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review





.....



.....

System.out.print(FreeCell.getCardASCIIDisplay(card));

.....

}}


12/39

Alternate Design

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review


public class ASCIIBoard {public static void placeCardsOnBoard(int[][] board,

int[] deck) {...}

public static void displayBoard(int[][] board,int[] foundations,

int[] cells) {...}

private static String getCardASCIIDisplay(int card){...}

}


13/39

Alternate Design

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Using static Card

Review



int[] deck) {...}public static void displayBoard(int[][] board,

int[] foundations,

int[] cells) {...}


private static int getRank(int card) { ... }private static int getSuit(int card) { ... }

}


14/39

Alternate Design

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card

Card: A second (static) go Using static Card

Review



int[] deck) {...}public static void displayBoard(int[][] board,

int[] foundations,

int[] cells) {...}


private static int getRank(int card) { ... }private static int getSuit(int card) { ... }

}


15/39

Status check

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review



public class FreeCell {.....


int[] deck = createDeck();

shuffleDeck(deck);

// Board consists of 8 cascades of maximum length 52

int[][] board = new int[52][8];ASCIIBoard.placeCardsOnBoard(board, deck);

int[] foundations = new int[4]; // Stores the top card

int[] cells = new int[4];

// Print the board, including foundations and cells

ASCIIBoard.displayBoard(board, foundations, cells);

System.out.println("FreeCell stopped.");

}}


16/39

Fields

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review


Fields are variables that are accessible from code anywhere in a class

Fields are used to store state information that is available in multiple

places (e.g., different methods) and across time (e.g., different method

invocations)

Like methods, fields can be used with respect to a class (static) orwith respect to an object (instance variables)

Fields may be public or private (and other visibilities)


17/39

Fields

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review


Fields are variables that are accessible from code anywhere in a class

Fields are used to store state information that is available in multiple

places (e.g., different methods) and across time (e.g., different method

invocations)

Like methods, fields can be used with respect to a class (static) orwith respect to an object (instance variables)

Fields may be public or private (and other visibilities)

public fields are almost always a Bad Idea


18/39

Adding fields to ASCIIBoard

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review




int[] deck) { ... }public static void displayBoard(int[][] board,

int[] foundations,

int[] cells) { ... }}


public class ASCIIBoard {// Board consists of 8 cascades of maximum length 52

private static int[][] board;

private static int[] foundations; // Stores the top card

private static int[] cells;

public static void initialiseBoard() { ... }public static void placeCardsOnBoard(int[] deck) { ... }public static void displayBoard() { ... }

}


19/39

More Java conventions

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review


Put all field declarations at the top of the class declaration

Name (private) fields with leading underscore, E.g., board

This is not a Sun standard

http://java.sun.com/docs/codeconv
http://java.sun.com/docs/codeconvhttp://java.sun.com/docs/codeconv


20/39

Using fields

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review


public static void placeCardsOnBoard(int[] deck) {// Place cards on board

int row = 0;

int cascade = 0;

for (int card = 0; card < deck.length; card++) {if (cascade == board[row].length) {

cascade = 0;

row++;

}

// Dont need to test row because board is big enoughboard[row][cascade] = deck[card];

cascade++;

}}

Fields are declared outside methods, but are accessible from inside all

methods.

Fields retain their values between method calls


21/39


PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review


public static void initialiseBoard() {board = new int[52][8];

foundations = new int[4];

cells = new int[4];

}

It doesnt make sense to initialise fields in each method, so typicallythere will be a separate method to do this


22/39

Status Check

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review



int[] deck = createDeck();

shuffleDeck(deck);

ASCIIBoard.initialiseBoard();

ASCIIBoard.placeCardsOnBoard(deck);

ASCIIBoard.displayBoard();

System.out.println("FreeCell stopped.");

}


23/39

No initialisation

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review


se251 prompt> java -cp target/classes se251.freecell.v5.FreeCellFreeCell started.

Exception in thread "main" java.lang.NullPointerException

at se251.freecell.v5.ASCIIBoard.\placeCardsOnBoard(ASCIIBoard.java:20)

at se251.freecell.v5.FreeCell.main(FreeCell.java:118)


24/39

No initialisation

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review







25/39

No initialisation

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review







26/39

No initialisation

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review




at se251.freecell.v5.ASCIIBoard.\

placeCardsOnBoard(ASCIIBoard.java:20)


15 public static void placeCardsOnBoard(int[] deck) {16 // Place cards on board

17 int row = 0;18 int cascade = 0;

19 for (int card = 0; card < deck.length; card++) {20 if (cascade == board[row].length) {21 cascade = 0;

22 row++;

23 }24 // Dont need to test row because board is big enough

25 board[row][cascade] = deck[card];

26 cascade++;

27 }

28 }


27/39

Interpreting .

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting .

Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review


A decimal place 3.14159 Separating a contained package from a containing package

se251.freecell

Separating the package part of a class name from the class name

se251.freecell.v1.FreeCell Accessing a member of a class:

deck.length length is a field of the array named deck

ASCIIBoard.initialiseBoard() initialiseBoard() is

a method on the class (se251.freecell.v5.)ASCIIBoard System.out.println() out is a field of the class

(java.lang.)System and println() is a method of out


28/39

Another Class

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation

Interpreting . Deck class

Refactoring

Making Classes

FreeCell


Using static Card


Review



public class Deck {private static int[] deck;

public static void create() {// Create sorted card deck

deck = new int[52];

....

}

public static void shuffle() {...

}

public static int[] getCardArray() {return deck;

}}


29/39

Using Deck

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review


class ASCIIBoard {...

public static void placeCardsOnBoard() {int[] cards = Deck.getCardArray();

// Place cards on board

int row = 0;int cascade = 0;

for (int card = 0; card < cards.length; card++) {if (cascade == board[row].length) {

cascade = 0;

row++;}// Dont need to test row because board is big enough

board[row][cascade] = cards[card];

cascade++;

}}


30/39

Refactoring

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review


Changing the design without changing the functionality with the intent of

reducing future costs

One example of changing the design is to move code into methods.

Other examples of changing the design include

creating/modifying/deleting classes, moving methods or/and fieldsbetween classes

When functionality changes (e.g. changing presentation of cards) then

that is not considered refactoring, but better designs makes it easier to

also improve/add functionality so often happens during refactoring work.


31/39

Where do Classes come from? (2)

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review


A good class represents a concept Concepts are often described using nouns

One way to find candidates for classes is to look at the nouns in the

description of the system

A way to find candidates for methods of a class is to look at the verbsassociated with the nouns

A way to find candidates for fields of a class is to look at the properties

or attributesassociated with the nouns


32/39

FreeCell

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review


The FreeCell board starts with the 52 cards of a standard

card deck shuffled and laid out face up in 8 vertical cascades,

the first 4 cascades with 7 cards and the remaining 4 with 6

cards. There are also 4 empty cells and 4 empty foundation

piles, one for each suit. The object of FreeCell is to play all

cards onto the foundation piles in ascending order of rank.

Any card at the bottom end of a cascade (called the

uncovered card) may be moved into an empty cell. An

uncovered card or card in a cell may be moved onto afoundation pile if it is the next highest card in the suit (starting

with Ace if the foundation pile is empty). An uncovered card or

a card in a cell may be moved to the end of any cascade,

provided the colour of the suits of the two cards are different,

and the rank of the moved card is one less than the rank of

the card being covered.


33/39

FreeCell

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review

















34/39

FreeCell

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review

















35/39


PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review



public class Card {private static int code;

public static void setCode(int code) {code = code;}public static String getASCIIDisplay() { ... }private static int getRank() { ... }

private static int getSuit() { ... }}


36/39

Using static Card

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review


public static void displayBoard() {...

while (not finished) {....

Card.setCode( board[row][cascade]);

System.out.print(Card.getASCIIDisplay());

...

}


37/39


PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review



public class Card {public static String getASCIIDisplay(int card) {

...}private static int getRank() { ... }private static int getSuit() { ... }

}


38/39

Using static Card

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review


public static void displayBoard() {...

while (not finished) {...

int card = board[row][cascade];

System.out.print(Card.getASCIIDisplay(card));

...

}


39/39

Review

PAQ

Agenda

Review

Classes

Classes

ASCIIBoard

Status check

Fields

ASCIIBoard fields

Java Conventions

Using fields


Status Check

No initialisation


Refactoring

Making Classes

FreeCell


Using static Card


Review

Learnt something about:

Classes

what properties good classes should have

how to find them what they look like in Java

Fields

what fields are

what they look like in Java

A multiple-class program

More Java code conventions

Documents

Lecture 4 Class