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Java Coding 5

Java Coding 5. To object or not…. David Davenport Computer Eng. Dept., Bilkent University Ankara - Turkey. email: david@bilkent.edu.tr. IMPORTANT…. Students…

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Java Coding 5

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  1. Java Coding 5 To object or not… David Davenport Computer Eng. Dept., Bilkent UniversityAnkara - Turkey. email: david@bilkent.edu.tr

  2. IMPORTANT… • Students… This presentation is designed to be used in class as part of a guided discovery sequence. It is not self-explanatory! Please use it only for revision purposes after having taken the class. Simply flicking through the slides will teach you nothing. You must be actively thinking, doing and questioning to learn! • Instructors… You are free to use this presentation in your classes and to make any modifications to it that you wish. All I ask is an email saying where and when it is/was used. I would also appreciate any suggestions you may have for improving it. thank you,David.

  3. From the beginning… • History of programming paradigms • GoTo Programming (spaghetti code!) • Structured Programming • Object-Oriented Programming • ??? Aspect-Oriented… • Paradigm changes response to • Need to build ever larger programs • Correctly • On time • On budget

  4. Key Attributes of OOP • Abstraction, Encapsulation, Inheritance, Polymorphism What? • Ease of reuse • Speeds implementation & facilitates maintenance. • Component-based approach • Easy to use existing code modules • Easy to modify code to fit circumstances! • A natural way to view/model world • Makes design quicker, easier & less error-prone.

  5. The world as we see it… • Look around & what do you see? • Things (books, chairs, tables, people!) • Actually, see individual things! • Ayse, David, my textbook, that chair, Mehmet’s pencil, etc. • The world is • a set of things • interacting with each other.

  6. Individual Category Describing the world (1) • Describe a particular person • Ayse has long blond hair, green eyes, is 1.63m tall, weighs 56Kg and studies computer engineering. Now lying down asleep. • Mehmet studies electronics, has short black hair and brown eyes. He is 180cm and 75 kilos. Now running to class! • Notice how all have specific values of • name, height, weight, eye colour, state, …

  7. Category Individual Describing the world (2) • Type/category determine an object’s properties & functionality • Person • has name, height, weight, can run, sleep, … • Category gives default properties • “Ayse is a person with green eyes.” We infer/assume she has two of them, as well as two legs, arms, nose, mouth, hair, can speak, run, sleep, etc! • Can concentrate on “relevant” properties

  8. Java OOP terminology • Class - Category • Properties/states • Functionality/Services(examines/alters state) data methods • object - Individual/unique thing(an instance of a class) • Particular value for each property/state • & functionality of all members of class.

  9. Ayse David Java OOP Software • Software System • Set of objects • Which interact with each other Created (instantiated) from class definitions One object will send a message to another object asking it to do a particular task. The first object does not need to know how the task is done (only how to request that it be done.) This corresponds to calling one of the second object’s methods! Person David: Say your name “David”

  10. Person name, age, salary, comments sayName, getNetSalary getComments setComments increaseAge … Name: “David” Age: 22 Salary: 2000 Comments:“Teaches CS101” Name: “Ayse” Age: 18 Salary: 500 Comments:“Good student” In more detail • Create & manipulate person objects

  11. String name; int age; double salary; String comments; public Person( String theName, int theAge ) { name = theName; age = theAge; comments = “”; } public void sayName() { System.out.println( name); } Coding Java Classes // header public class Person { // properties // constructors // methods }

  12. Coding Java Classes public String getName() { return name; } public String getComments() { return comments; } public void setComments( String someText) { comments = someText; } “get” & “set” methods for some properties(no setName!) public void increaseAge() { age = age + 1; } public double getNetSalary() { double netSalary; netSalary = salary - TAX; return netSalary; } Variables which are not parameters or properties must be defined locally.

  13. salary: name: age: “Ayse” 18 0.0 aStudent{Person} comments: “” Creating & Using Objects • Always • Declare variable to “hold” object • Create object using “new” statement • Call object’s methods Person aStudent; aStudent = new Person( “Ayse”, 18); aStudent.sayName(); Put this in method of another class, (e.g main method)

  14. salary: salary: name: age: name: age: “Ayse” 18 “David” 22 0.0 0.0 friend{Person} aStudent{Person} comments: comments: “” “” Creating & Using Objects Person aStudent; aStudent = new Person( “Ayse”, 18); Person friend; friend = new Person( “David”, 22); 23 “Good student” friend.increaseAge();aStudent.setComments( “Good student”);

  15. Examples: existing classes • Random class Random die; die = new Random(); int face = die.nextInt(6) + 1; System.out.println( face); • StringTokenizer class StringTokenizer tokens; tokens = new StringTokenizer( “to be or not to be”); while ( tokens.hasMoreTokens() ) { aWord = tokens.nextToken(); System.out.println( aWord); } System.out.println( “done”);

  16. Writing Your Own Classes • Coins • Dice • Traffic lights • TV • Video • Wallet • Robo • Music CD • Time/Date (in various formats!)

  17. david{Person} derya{Person} CS101 instructor{Person} Derya’s dad{Person} david2{Person} Object vs. Reference • In the “real” world So too in Java!

  18. david{Person} david2{Person} derya{Person} CS101 instructor{Person} Derya’s dad{Person} David’sclone Object vs. Reference • In the Java world David Derya

  19. myQCd{CD} Title B.R. Artist Queen Date 1976 Length 3:50 Title Best of Artist Genesis Date 1983 Length 2:40 myCd{CD} yourCd{CD} Title B.R. Artist Queen Date 1976 Length 3:50 yourQCd{CD} Same or Different? (1) • Comparing objects if ( myCd == yourCd) System.out.println( “Same”); else System.out.println( “Different”); if ( myCd == yourQCd) System.out.println( “Same”); else System.out.println( “Different”);

  20. myQCd{CD} Title B.R. Artist Queen Date 1976 Length 3:50 Title Best of Artist Genesis Date 1983 Length 2:40 myCd{CD} yourCd{CD} Title B.R. Artist Queen Date 1976 Length 3:50 yourQCd{CD} Same or Different? (2) • Define an “equals” method if ( myCd.equals( yourCd) ) System.out.println( “Same”); else System.out.println( “Different”); if ( myCd.equals( yourQCd) ) System.out.println( “Same”); else System.out.println( “Different”);

  21. Copying • in primitive vs. Object types… int i, j; i = 5; j = i; i++; Sys… ( i, j); Person me, x; me = new Person( …); x = me; me.setComments( “nice!”); Sys… ( me.getComments() + x.getComments(), ); Different Same!

  22. myCd{CD} yourQCd{CD} favouriteCd{CD} Copy vs. Clone Title B.R. Artist Queen Date 1976 Length 3:50 Title B.R. Artist Queen Date 1976 Length 3:50 favouriteCd = myCd; yourQCd = myCd.clone();

  23. aCd{CD} aCd = null; Lost objects • Java collects its garbage! Title B.R. Artist Queen Date 1976 Length 3:50 Title Best of Artist Genesis Date 1983 Length 2:40 yourCd{CD} myCd{CD} myCd = yourCd;

  24. main xyz a i b Parameter Passing (1) • Primitive types… public int xyz( int i) { i++; return i; } 5 5 6 6 main int a, b; a = 5; b = xyz( a); Sys… ( a, b);

  25. main xyz a x b Parameter Passing (2) • Object types… David221000“” public Person xyz( Person x) { x.setComments(“Nice); return x; } Nice main Person a, b; a = new Person( …); b = xyz( a); Sys… ( a.getComments() + b.getComments() );

  26. main xyz a x b Parameter Passing (3) • Object types… David221000“” Derya18500“” public Person xyz( Person x) { x = new Person( …); x.setComments(“Nice); return x; } Nice main Person a, b; a = new Person( …); b = xyz( a); Sys… ( a.getComments() + b.getComments() );

  27. Person static nameagesalarycomments instance David222000“Quiet” Derya18500“Nice” Gunes211500“Sunny” Ayse251000“Happy” a b c d Static vs. instance • Keep count of number of Person objects count 0 4 3 1 2

  28. All Objects… • automatically have • boolean equals( Object) • Object clone() • String toString() • BUT • they may not do what you would like/expect, so implement yourself!

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