1. Data Abstraction - Interfaces and Implementations Cmput 115 - Lecture 1 Department of Computing Science University of Alberta ©Duane Szafron 1999 Some code in this lecture is based on code from the book: Java Structures by Duane A. Bailey or the companion structure package Revised 12/16/99

2. ©Duane Szafron 1999 2 About This Lecture In this lecture we will learn how Java objects and classes can be used to build abstract data types.

3. ©Duane Szafron 1999 3Outline Object Concepts Interface Use Implementation Java interfaces

4. ©Duane Szafron 1999 4 What is an Object? some thing that has: - identity - state - set of defined behaviours * operations (methods) the object does * requests operations (methods) other objects do Example Object: student (061342, Mary Doe, 1st year, Computing Science, 12 Bellvue Towers, mdoe@cs.ualberta.ca, takes CMPUT 115, swims, gets a computer account)

5. ©Duane Szafron 1999 5 What is a Class? share a set of things that share (have in common): - an identification technique - state variables - set of defined behaviours * operations (methods) the object does * requests operations (methods) other objects do Example Class: student class (student id, name, year, program, address, email*, takes courses, activities*, special needs*)

6. ©Duane Szafron 1999 6Abstraction/Generalization/Specialization student’ class (student id, name, year, program, address, takes courses) cmput-student class is a subclass of student’(email, gets a computer account) phys-ed-student class is a subclass of student’(activities) student’ cmput-studentphys-ed-student isa

7. ©Duane Szafron 1999 7 Objects are everywhere!

8. ©Duane Szafron 1999 8 Types of Objects Objects in the “real world” User interface objects - e.g., windows, mouse drivers System objects - e.g., server, clients, proxies Persistent management objects - e.g., files, databases, versions, configurations User objects - e.g., user preferences

9. ©Duane Szafron 1999 9 Example of Data Structure Object Properties of queue - request is serviced from the front of the queue, new requests are placed at the rear of the queue. Implement a queue of players wishing to make their next move. Develop a computer game in which resources are either shared or competed for between players. How do you manage these resources? A common way is to use a queue. Queue player = new QueueList() // create a queue of players player.add(name) // add a player to the queue player-done = player.remove //remove a player from the queue

10. ©Duane Szafron 1999 10 Object Concepts An object has a public interface and a private state. The interface is the set of all resources the class provides. Objects that share the same interface are organized into a group called a class and each object in the class is called an instance of the class. (object = instance of a class) The state of an object differentiates it from other objects in the same class.

11. ©Duane Szafron 1999 11 class Object Class, Interface and State public interface private state public interface private state public interface private state external request public interface private state

12. ©Duane Szafron 1999 12 Interface, Implementation & Use There are three aspects for any class: –Interface: a description of the resources that the class provides. –Implementation: the code that describes the private state of instances and implements the computational resources of the class. –Use: code in a using class causes new instances of the used class to be created and invokes computations on these instances.

13. ©Duane Szafron 1999 13Interface In Java, the interface of a class includes: –The name of the superclass –Public variable declarations –Constructor declarations –Message declarations –Static method declarations Note that in Java, the term interface has a specific meaning that is slightly different than our generic use of the term interface. We will discuss Java interfaces later.

14. ©Duane Szafron 1999 14 Interface - Ratio Class -1 public class Ratio { /* an object for storing a fraction */ public Ratio(int top, int bottom) /* pre: bottom != 0 post: constructs a ratio equivalent to top/bottom */ public int getNumerator() /* post: return the numerator of the fraction */ public int getDenominator() /* post: return the denominator of the fraction */ Do not be concerned about the “pre:” and “post:” notation. It will be explained in the next lecture. code based on Bailey pg. 8

15. ©Duane Szafron 1999 15 Interface - Ratio Class -2 public double value() /* post: returns the real value equivalent to ratio */ public Ratio add(Ratio other) /* pre: other is non-null post: return new fraction - the sum of this and other */ code based on Bailey pg. 9

16. ©Duane Szafron 1999 16Use In Java, a class A can use another class B by: –Creating instances of class B. –Sending messages to instances of class B. –Invoking static methods from class B. –Using variables that are declared in class B. –Using message arguments that are declared to be of class B.

17. ©Duane Szafron 1999 17 Use - Ratio Class public static void main(String[] args) { Ratio r = new Ratio(1,1); // r == 1.0 r = new Ratio(1,2); // r == 0.5 r.add(new Ratio(1,3)); // r still 0.5 r = r.add(new Ratio(1,4)); // r == 0.75 System.out.println(r.value()); // 0.75 printed } code based on Bailey pg. 9

18. ©Duane Szafron 1999 18Implementation In Java, a class implementation includes: –Constructor code –Message code (instance methods) –Static (class) method code –Bindings for static variables

19. ©Duane Szafron 1999 19 Implementation - Ratio Class -1 import structure.*; public class Ratio { /* an object for storing a fraction */ protected int numerator; // numerator of ratio protected int denominator; // denominator of ratio public Ratio(int top, int bottom) { /* pre: bottom != 0 post: constructs a ratio equivalent to top/bottom */ Assert.pre(bottom != 0, "Denominator must not be 0"); this.numerator = top; this.denominator = bottom; } code based on Bailey pg. 8

20. ©Duane Szafron 1999 20 Implementation - Ratio Class -2 public int getNumerator() { /* post: return the numerator of the fraction */ return this.numerator; } public int getDenominator() { /* post: return the denominator of the fraction */ return this.denominator; } code based on Bailey pg. 8

21. ©Duane Szafron 1999 21 Implementation - Ratio Class -3 public double value() { /* post: returns the real value equivalent to ratio */ return (double)this.numerator /(double)this.denominator; } public Ratio add(Ratio other) { /* pre: other is non-null post: return new fraction - the sum of this and other */ Assert.pre(other != null, "Other must not be null"); return new Ratio(this.numerator*other.denominator+ this.denominator*other.numerator, this.denominator*other.denominator); } code based on Bailey pg. 9

22. ©Duane Szafron 1999 22 Java Interfaces For many classes, the interface and the implementation are in the same file. However, Java has a specific feature called an interface that can be used to separate the interface of a class from its implementation. This is not usually done! A Java interface contains no code, only message signatures.

23. ©Duane Szafron 1999 23 Java Interface - PlanarPoint -1 public interface PlanarPoint { public double getX(); /* post: returns the x coordinate. */ public double getY(); /* post: returns the y coordinate. */ public double getR(); /* post: returns the distance from the origin. */ public double getTheta(); /* post: returns the angle in radians from the x axis. */

24. ©Duane Szafron 1999 24 Graphical Interpretation X Y  R (x,y) (r,  )

25. ©Duane Szafron 1999 25 Java Interface - PlanarPoint -2 public void transform(double dr, double dTheta); /* pre: if dr >= 0 or |dr| < current r post: changes the r and theta coordinates by adding the given values to them. */ public void translate(double dx, double dy); /* post: changes the x and y coordinates by adding the given values to them. */ public PlanarPoint add(PlanarPoint aPoint); /* pre: aPoint is non-null post: return new PlanarPoint - the sum of this and other */ }

26. ©Duane Szafron 1999 26 Implementing Java Interfaces A Java class is used to implement a Java interface. The class must provide code for each message in the Java interface. The class name must be different from the Java interface name.

27. ©Duane Szafron 1999 27 Class - CartesianPoint -1 public class CartesianPoint implements PlanarPoint { protected double x; // x coordinate of point protected double y; // y coordinate of point public CartesianPoint(double x, double y) { /* post: constructs a Point with the given coordinates */ this.x = x; this.y = y; } PlanarPoint CartesianPoint PolarPoint

28. ©Duane Szafron 1999 28 Class - CartesianPoint -2 public double getX() { /* post: returns the x coordinate. */ return this.x; } public double getY() { /* post: returns the y coordinate. */ return this.y; }

29. ©Duane Szafron 1999 29 Class - CartesianPoint -3 public double getR() { /* post: returns the distance from the origin. */ return Math.sqrt((this.x*this.x) + (this.y*this.y)); }

30. ©Duane Szafron 1999 30 Class - CartesianPoint -4 public double getTheta() { /* post: returns the angle in radians from the x axis. */ double angle; angle = Math.atan(this.y/this.x); if (this.x < 0) angle = angle + Math.PI; return angle; }

31. ©Duane Szafron 1999 31 Graphical Interpretation X Y R (x,y) Assume x < 0 (r,  +  )   ++

32. ©Duane Szafron 1999 32 Class - CartesianPoint -5 public void transform(double dr, double dTheta) { /* pre: if dr < 0 then |dr| < current r post: changes the r and theta coordinates by adding the given values to them. */ double r; double theta; Assert.pre((dr>=0)|| (Math.abs(dr) <= this.getR()), ”require that dr >= 0 or abs(dr) <= r"); r = this.getR() + dr; theta = this.getTheta() + dTheta; this.x = r * Math.cos(theta); this.y = r * Math.sin(theta); }

33. ©Duane Szafron 1999 33 Graphical Interpretation X Y  (r 1,   )  (r 1 +r 2,   +   ) Transform Method

34. ©Duane Szafron 1999 34 Class - CartesianPoint -6 public void translate(double dx, double dy) { /* post: changes the x and y coordinates by adding the given values to them. */ this.x = this.x + dx; this.y = this.y + dy; }

35. ©Duane Szafron 1999 35 Class - CartesianPoint -7 public PlanarPoint add(PlanarPoint aPoint) { /* pre: other is non-null post: return new PlanarPoint - the sum of this and other */ Assert.pre(other != null, "Other must not be null"); return new CartesianPoint(this.x + aPoint.getX(), this.y + aPoint.getY()); } /* Note that we cannot use aPoint.x or aPoint.y since aPoint might not be a Cartesian Point. It may be an instance of some other class that implements the PlanarPoint interface. */ }

36. ©Duane Szafron 1999 36 Multiple Classes can Implement a Java Interface We can have an arbitrary number of classes that implement the same interface. For example, we could have another class that implements the PlanarPoint interface, named the PolarPoint class (see the web for code). We can declare variables to be the Interface type, but must create instances of the implementation classes to bind them to.

37. ©Duane Szafron 1999 37 Using PlanarPoint public void main(String args[]) { PlanarPoint cartPoint, polarPoint, sumPoint; cartPoint = new CartesianPoint(3.0, 4.0); polarPoint = new PolarPoint(5.0, Math.atan(4.0 / 3.0)); sumPoint = cartPoint.add(polarPoint); System.out.println(sumPoint); /*(6.0, 8.0) */ sumPoint = polarPoint.add(cartPoint); System.out.println(sumPoint); /* */ } X Y  (r 1,   )  (r 1 +r 2,   +   ) (3.0,4.0) (3+3, 4+4)

38. ©Duane Szafron 1999 38 Some Principles from the Textbook 1.The principled programmer understands a principle well enough to form an opinion about it. 2.Free the future: reuse code. 3.Design and abide by the interfaces as though you were the user. 4. Declare the data fields protected-- that way you cannot access them from outside the class and you are forced to access through the interface. principles from Bailey ch. 1