1/27/2014. OO design approach with examples. UML and project clarifications (2) Project clarifications 1. UML

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1 OO design approach with examples Sharma Chakravarthy Information Technology Laboratory (IT Lab) Computer Science and Engineering Department The University of Texas at Arlington, Arlington, TX URL: 1. UML UML and project clarifications Each class should be represented using appropriate notation for public, private, protected, and static attributes and methods. Constructors should be identified as such. Make sure parameters, return types are properly indicated Relationships between classes should be represented using labels, cardinality information (where appropriate). Later, Is a, has a, and uses a need to be represented Later, abstract classes and interfaces need to be represented See (but make sure you use modifiers and constructors, parameters and return types) UML and project clarifications (2) 3. Course and textbook There is a prescribed textbook for the course which means that you need to read it Many questions I am getting about the project (e.g., UML diagrams) are also answered in the textbook My lecture notes are not sufficient for doing well on the quizzes/exams or projects Chapters covered are mentioned on my web along with homework problems for each chapter Please post all class related (project or otherwise) questions on the blackboard Project clarifications 1. Many students are asking me as to what chapters one needs to read to do the projects Since the chapters (as organized in the book) do not give all the information needed to finish a project Attending classes Going over my slides Heeding pointers and clarifications I provide are going to be useful for doing projects. 1. Also, my quizzes and exams cover not only the chapters indicated, but also the material covered in the class 1

2 Design of a class (general guidelines) Choosing attributes/methods as private makes them visible only inside a class Choosing attributes/methods as public makes them visible everywhere (with in a class, within subclasses, within same packages, and in other packages) (C-style code in Java!) Hence, you have to choose access modifiers judiciously! How do you do that? 5 Design of a class (general guidelines) One approach is to make everything private and relax as needed to get maximum encapsulation! Relax from private one visibility step (package-private, protected) at a time to accomplish the above In other words, do not make any attribute/method public unless absolutely necessary Use the principle of need to know 6 Design of a class Want to design an alarm clock Need to identify visible attributes, interfaces and separate them from implementation If it is an alarm clock combined with a radio, you will have to think of these objects that together form the system (recall has-a association) Do not combine separate concepts into a single class Example of an object Alarm clock as an object What are its private attributes/fields? Current second (a value in the range 0 59) Current minute (a value in the range 0 59) Current hour (a value in the range 1 12) The time the alarm is set for (a valid hour and minute) Whether the alarm is on or off Am, pm indicator The fields merely describe values that define the current state of the alarm clock The above cannot be changed unless knobs are provided for that purpose (methods/visible interface in our case) 7 8 2

3 Example of an object (2) Alarm clock as an object What are its public attributes? There may not be any public attributes! What are its public methods (visible front end)? Set time (to a valid time) Set alarm time (to a valid time) Turn alarm on/off (one or two methods) Read time Read current alarm time Each method manipulates one or more fields The above are public methods 9 Example of an object Alarm clock as an object What are its private methods? Increment the current second Increment the current minute Increment the current hour Sound alarm Note that these methods also change the attributes of the clock class (or state of that object). In addition, they have to manipulate the fields correctly When the second reaches 59, minute is incremented by 1 and second is set to zero, wrap around of hours etc. Similarly for alarm time 10 Class Design Hints 1. Always keep as much data private as possible This is important. Otherwise, it violates encapsulation. You may have to write accessor/observer and/or mutator methods, but you are still better off keeping the instance fields private. Provides separation from implementation Experience shows that data representation or implementation is likely to change more often than usage. Plus, bugs are easier to detect 2. Always initialize class attributes Although Java initializes instance and class attributes of objects, defaults may not make sense always. Explicitly initialize them (e.g., default date) Class Design Hints (2) 3. Don t use too many basic types in a class The idea is to replace multiple related uses of basic types with other classes. This keeps your classes small, easier to understand, and manage. For example, replace the following fields in a class private String street; private String city; private int zip; private STATE state; // using enum This is how you build new classes using existing ones With a new class called Address. This way you can cope with changes to addresses and deal with international addresses. Will not disturb other classes You will be using these ideas in your projects 3

4 Class Design Hints (3) 4. Not all fields need individual accessors and mutators You may need to get and set an Employee s name. Hiring date, for example, does not change once the object is constructed. Quite often objects have fields that you don t want others to get or set (this is part of information hiding) Employee s salary may not be visible to everyone. Hence not public. It may be protected or packageprivate so that only required modules can access it. Class Design Hints (3) 5. Use a standard layout for class definition (in this order) Attributes Methods Within each of the above, list static members public members Package private members, and private members Some put the attributes at the bottom of the class. Class Design Hints (4) 6. Breakup classes that have too many (independent) responsibilities This may be a bit vague and subjective. If you can decompose one intermingled class into two simpler classes, do it. However, classes with only one/very few attributes/methods may be an overkill public class CardDeck{ private int[] value; //private int[] value = new int[52]; private String[] suit; public CardDeck() { public void shuffle() { //contrast with Date public int gettopvalue() { public String gettopsuit() { Public?? draw () { Class Design Hints (5) In the previous example Two concepts are intermingled: card and deck. shuffle applies to the Deck gettopvalue() and gettopsuit() refer to cards. draw applies to the deck but returns a card (i.e., 2 values) You cannot return multiple values in a programming language except as an object! Therefore, it is better to separate this into two concepts/classes with their own responsibilities 4

5 CardDeck classes public class Card{ //good design private int value; //default does not make sense! private String suit; //can be enum type public Card(int avalue, String asuit) { public int getvalue() { public String getsuit() { public class CardDeck{ //better design private Card[] deck; //private card[] deck = new Card[52]; public CardDeck() { public void shuffle() { private Card gettopcard() { Public Card draw() { CardDeck classes with enum public class Card{ //good design private enum Suits {DIAMOND, HEART, SPADE, CLUB private int value; //default does not make sense! private Suits suit; public Card(int avalue, String asuit) { public int getvalue() { public String getsuit() { public class CardDeck{ //better design private Card[] deck; //private card[] deck = new Card[52]; public CardDeck() { public void shuffle() { private Card gettopcard() { Public Card draw() { Card acard = new Card(1, DIAMOND ); System.out.println(aCard.value +, + acard.suit); // will print 1,DIAMOND Card instance/class attributes class Card implements CardDeckConstants{ private static enum Suits {SPADE, CLUB, DIAMOND, HEART; private static String[] cardvalues = { "A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K" ; private String value; // default does not make sense! private Suits suit; Card class constructor public Card(int avalue, String asuit) { if (!isvalid(avalue, asuit.touppercase())){ System.out.println("1: Invalid card: (" + avalue + ", " + asuit + ")"); System.exit(ABNORMAL_EXIT); value = avalue; suit = Suits.valueOf(aSuit.toUpperCase()); // constructor public class CardDeck implements CardDeckConstants{ private Card[] deck = new Card[CARDS_IN_A_DECK]; cse 1325: chakravarthy 19 cse 1325: chakravarthy 20 5

6 CardDeck constructor public CardDeck() { System.out.println("Create a valid Deck without jokers:\n"); int card = 0; for (String s : cardsuits) for (int i=1; i <= CARDS_IN_A_SUIT; i++) deck[card++] = new Card(i, s); public void shuffle(int randomseed) { System.out.println("Shuffling the deck using " + randomseed + " as seed"); Random randgen = new Random(randomSeed); int times = TIMES_TO_SHUFFLE + randgen.nextint(times_to_shuffle); for (int i = 0; i < times; i++) { int pos_1 = randgen.nextint(cards_in_a_deck); // 0 to 51 int pos_2 = randgen.nextint(cards_in_a_deck); Card tmpcard = deck[pos_1]; deck[pos_1] = deck[pos_2]; deck[pos_2] = tmpcard; cse 1325: chakravarthy 21 Class Design Hints (7) 7. Make names of your classes and methods reflect their responsibilities Analogous to attributes and local variables having meaningful names that reflect what they represent A good convention is to use nouns for class names (e.g., Order) or a noun preceded by an adjective (e.g., RushOrder) or a gerund (e.g., BillingAddresss) Begin with a lowercase get for accessor methods Begin with a lowercase set for mutator methods Summary Identify classes each should have an independent existence and have its own properties; distinguishability of objects is provided by Java Identify attributes/fields decide its characteristics (public vs. private (or protected or package-private) static vs. non static, final (constants) Identify constructors one or more depending on need Include proper comments for javadoc Summary Identify methods: public depending on how objects are manipulated from outside and private how objects are manipulated from within the class Identify static and non static methods; also what each method returns including void Use principle of minimality for all of the above Do the UML diagram for classes (go over associations) Test each class individually (unit testing) before integrating it into the larger project (with a main in each class) 6

7 UML for the CardDeck <<Class>> CardDeck <<Class>> +Card What are its attributes/fields? What are its methods? We want to capture a date consisting of month, day, and year and manipulations of date For example, does date1 precede date2? If I add 40 days to date1, what will be the resulting date? How many full months are there between date1 and date2? Attributes/fields month to represent the month (mm) date to represent a day (dd) year to represent a year (yyyy) What are their types and visibility? Who makes sure that day is in the range 1 to 31 depending on the month (in the range 1 to 12)? Who handles the correctness of a date? or public class Date{ private final int month;// month (between 1 and 12) private final int day;// day (between 1 and 31) private final int year; // year Why final? Makes the object immutable! It is not difficult to check months for range However, it is not so easy to do it for days as the number of days vary from month to month How can we cope with that? Do not want to construct a date that is not valid! 7

8 public class Date implements Proj2Constants{ private static final int[] DAYS = { 0,31,29,31,30,31,30,31,31,30,31,30,31 ; private final int month;// month (between 1 and 12) private final int day;// between 1 and DAYS[month]) private final int year; // year Let us look at constructors Methods A default constructor initialized to default values does not seem appropriate for this class as it results in an invalid date (0 0 0)! /** * Constructor: Does bounds-checking to ensure object represents a valid date m represents the month between 1 and 12 d represents the date between 1 and the corresponding number from array DAYS y represents the year RuntimeException if the date is invalid */ public Date(int m, int d, int y) { if (!isvalid(m, d, y)) throw new RuntimeException( 1: Invalid date"); month = m; day = d; year = y; public Date(String sdate){ int m, d, y; String [] chopdate = sdate.split( -"); m = Integer.parseInt(chopDate[ZEROI]); d = Integer.parseInt(chopDate[ONEI]); y = Integer.parseInt(chopDate[TWOI]); if (!isvalid(m, d, y)) throw new RuntimeException( 2: Invalid date"); month = m; day = d; year = y; /** * Is the given date valid? month, day and year false if month exceeds 12 or is less than 1 false if day exceeds the corresponding days for a month from array false if leap year is not correct */ Why is this method static? private static boolean isvalid(int m, int d, int y){ if (m < 1 m > 12) return false; if (d < 1 d > DAYS[m]) return false; if (m == 2 && d == 29 &&!isleapyear(y)) return false; return true; // note short circuit operators 8

9 /** * is y a leap year? y represents the year specified true if year divisible by 400 false if year divisible by 100 and not by 400 */ private static boolean isleapyear(int y) { if (y % 400 == 0) return true; if (y % 100 == 0) return false; return (y % 4 == 0); The order of check is very important! Date and Time Class Summary You can take a look at the rest of the methods Can add more methods as needed User of the does not have to know the implementation details Can change implementation as long the public interface is kept the same Follow this example for your design and implementation DateTime Class Suppose you are asked to create a DateTime class that can manage both date and time (Oracle has something like this) Would you make a separate class? Why? Or would you combine all attributes into one class? Or would you add Date to Time class or Time to Date class? Why? What are the implications of the above? What are the methods (public and private) You may want to do this as homework to understand the design, and concept separation discussed earlier! DateTime Class (2) Time has been implemented to wrap around That is, if you add 2 hours to 23:00:00, you get 01:00:00 This is not appropriate for DateTime class where if you add 2 hours to , 23:00:00, you should get , 01:00:00 (not , 01:00:00) To accomplish the above, you need to know when the wraparound happens! See how you can do the above problem! 9

Project 2 Is about designing a few classes based on the given specification. Implement constructors and methods in each class Do the UML for all the classes Thank You!

10 Project 2 Is about designing a few classes based on the given specification. Implement constructors and methods in each class Do the UML for all the classes Thank You! Identify attributes of each class Identify visibility of attributes of each class Identify visibility of methods of each class We will also give you code for reading input Please study the code given to you Follow good programming practices and javadoc annotations as shown in the code given to you January 13, 2010 cse 1325: chakravarthy 38 10

OOP, Java classes and Objects (Chapter 3 and 8)

OOP, Java classes and Objects (Chapter 3 and 8) Sharma Chakravarthy Information Technology Laboratory (IT Lab) Computer Science and Engineering Department The University of Texas at Arlington, Arlington,