1. CS 112 Introduction to Programming Critters/Event-Driven Programming; Interface Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone: 432-6400 Email: yry@cs.yale.edu

2. 2 Admin  PS8 and class project questions?  Special session on Google App Engine m Monday 8 pm at DL 120

3. Recap: The Critter Class // abstract class means not implement every method public abstract class Critter { public boolean eat() public Attack fight(String opponent) // ROAR, POUNCE, SCRATCH, FORFEIT public Color getColor() public Direction getMove(String[][] grid) // NORTH, SOUTH, EAST, WEST, CENTER public String toString() … // read the class for other methods available }

4. Recap: Event-Driven Programming  Key concepts: m The simulator is typically implemented using polymorphism m The simulator is in control, NOT an animal. An animal must keep state (as fields) so that it can make a single move, and know what moves to make later. We say that EDP focuses on writing the callback functions of objects % Next move?

5. Recap: Cougar  Write a critter class Cougar (among the dumbest of all animals): MethodBehavior constructor public Cougar() eat Always eats. fight Always roars. getColor Blue if the Cougar has never fought; red if he has. getMove Walks west until he finds food; then walks east until he finds food; then goes west and repeats. toString"C"

6. State Machine Driven Cougar 6 Move West Move East eat()  getMove  Color ! Has fought Has fought fight()

7. Recap: Snake MethodBehavior constructor public Snake() eat Never eats fight random pounce or roar getColor Color(20, 50, 128) getMove 1 E, 1 S; 2 W, 1 S; 3 E, 1 S; 4 W, 1 S; 5 E,... toString"S"

8. Non-EDP Version A non-event driven version cycleLength = 1; while (true) { steps = 0; while (steps < cycleLength) if cycleLength % 2 == 1 go East else go West steps ++; go South cycleLength ++ }

9. Non-EDP-> EDP: Guarding Condition 9 steps < cycleLength if (cycleLength % 2 == 1) go East else go West steps++; steps == cycleLength go South cycleLength ++ steps=0; A non-event driven version cycleLength = 1; while (true) { steps = 0; while (steps < cycleLength) if cycleLength % 2 == 1 go East else go West steps ++; // invariant? go South cycleLength ++ }

10. Snake solution import java.awt.*; // for Color public class Snake extends Critter { private int cycleLength; // # steps in curr. Horiz. cycle private int steps; // # of cycle's steps already taken public Snake() { cycleLength = 1; steps = 0; } public Direction getMove() { if (steps < cycleLength) { steps++; if (cycleLength % 2 == 1) { return Direction.EAST; } else { return Direction.WEST; } } else { steps = 0; cycleLength ++; return Direction.SOUTH; } public String toString() { return "S"; } steps < cycleLength if (cycleLength % 2 == 1) go East else go West steps++; steps == cycleLength Go South cycleLength ++ steps=0;

11. Comment: States  Counting is helpful: m How many total moves has this animal made? m How many times has it eaten? Fought?  Remembering recent actions in fields may be helpful: m Which direction did the animal move last? How many times has it moved that way? m Did the animal eat the last time it was asked? m How many steps has the animal taken since last eating? m How many fights has the animal been in since last eating?

12. Testing critters  Focus on one specific critter of one specific type m Only spawn 1 of each animal, for debugging  Make sure your fields update properly  Use println statements to see field values  Look at the behavior one step at a time m Use "Tick" rather than "Go"

13. Designing Bulldog  Be open minded  Think about strategies, e.g., m How much state do your bulldogs keep and probe state? m When does your Bulldog eat/mate? m Is there an “optimal” fight strategy for a specific type of opponent? m Do your bulldogs play disguise? m Does your strategy change with time? m Do your bulldogs coordinate their behaviors to form some kind of patterns? 13

14. Coordination 14 https://www.youtube.com/watch?v=_sUeGC-8dyk

15. Coordination 15

16. Critter exercise : Hipster  A group of hipster critters want to hangout together  Each hipster can suddenly become inspired and choose a random board location called an edgy bar  A hipster go north until reaches edgy bar’s horizontal, then east until reaching the bar

17. Solution 1 (See Hipster.java) public class Hipster extends Critter { private Random rand; private int edgyBarX, edgyBarY; private int nextT, t; private final int FLASH_INTERVAL = 200; public Hipster() { rand = new Random(); t = 0; nextT = rand.nextInt(FLASH_INTERVAL); } public Direction getMove(String[][] grid) { t ++; if (t == nextT) { edgyBarX = rand.nextInt( getWidth() ); edgyBarY = rand.nextInt( getHeight() ); t = 0; nextT = rand.nextInt(FLASH_INTERVAL); } if (getY() != edgyBarY) { return Direction.NORTH; } else if (getX() != edgyBarX) { return Direction.EAST; } else { return Direction.CENTER; } public String toString() { return "H(" + edgyBarX + "," +edgyBarY + ")”; }

18. Solution 1 (See Hipster.java) public class Hipster extends Critter { private Random rand; private int edgyBarX, edgyBarY; private int nextT, t; private final int FLASH_INTERVAL = 200; public Hipster() { rand = new Random(); t = 0; nextT = rand.nextInt(FLASH_INTERVAL); } public Direction getMove(String[][] grid) { t ++; if (t == nextT) { edgyBarX = rand.nextInt( getWidth() ); edgyBarY = rand.nextInt( getHeight() ); t = 0; nextT = rand.nextInt(FLASH_INTERVAL); } if (getY() != edgyBarY) { return Direction.NORTH; } else if (getX() != edgyBarX) { return Direction.EAST; } else { return Direction.CENTER; } public String toString() { return "H(" + edgyBarX + "," +edgyBarY + ")”; }  Problem: Each hipster goes to a different bar. We want all hipsters to share the same bar location.

19. Solution 1 (See Hipster.java) public class Hipster extends Critter { private Random rand; private static int edgyBarX, edgyBarY; private int nextT, t; private final int FLASH_INTERVAL = 200; public Hipster() { rand = new Random(); t = 0; nextT = rand.nextInt(FLASH_INTERVAL); } public Direction getMove(String[][] grid) { t ++; if (t == nextT) { edgyBarX = rand.nextInt( getWidth() ); edgyBarY = rand.nextInt( getHeight() ); t = 0; nextT = rand.nextInt(FLASH_INTERVAL); } if (getY() != edgyBarY) { return Direction.NORTH; } else if (getX() != edgyBarX) { return Direction.EAST; } else { return Direction.CENTER; } public String toString() { return "H(" + edgyBarX + "," +edgyBarY + ")”; }

20. Recall: Static members  static: Part of a class, rather than part of an object. m Object classes can have static methods and fields. m Not copied into each object; shared by all objects of that class. class state: private static int staticFieldA private static String staticFieldB behavior: public static void someStaticMethodC() public static void someStaticMethodD() object #1 state: int field1 double field2 behavior: public void method3() public int method4() public void method5() object #2 state: int field1 double field2 behavior: public void method3() public int method4() public void method5() object #3 state: int field1 double field2 behavior: public void method3() public int method4() public void method5()

21. Accessing static fields  From inside the class where the field was declared: fieldName // get the value fieldName = value ; // set the value  From another class (if the field is public ): ClassName. fieldName // get the value ClassName. fieldName = value ; // set the value  generally static fields are not public unless they are final

22. 22 Example  We want to assign a unique, increasing employee ID for each Employee object (e.g., Secretary, Marketer, Lawyer) that we create

23. Employee with Static public class Employee { private String name; private int empID; private static int nextEmpID = 1000; public Employee(String name) { this.name = name; empID = nextEmpID; nextEmpID++; } public static int nextEmpID() { return nextEmpID; } … } public class Firm { public static void main(String[] args) { Lawyer larry = new Lawyer(“Larry"); Marketer mike = new Marketer("Mike"); Lawyer lynn = new Lawyer(”Lynn"); }

24. 24 Example: The Employee Objects Employee.nextEmpID 1000 public class Employee { private String name; private int empID; private static int nextEmpID = 1000; public Employee(String name) { this.name = name; emplID = nextEmpID; nextEmpID++; } … }

25. 25 Example: The Employee Objects larry: Lawyer name = “Larry” empID = 1000; Lawyer larry = new Lawyer("Larry"); Employee.nextEmpID 1000 1001 public class Employee { private String name; private int empID; private static int nextEmpID = 1000; public Employee(String name) { this.name = name; emplID = nextEmpID; nextEmpID++; } … }

26. 26 Example: The Employee Objects larry: Lawyer name = “Larry” empID = 1000; mike: Marketer name = “Mike” empID = 1001; Marketer mike = new Marketer("Mike"); Employee.nextEmpID 1001 1002 public class Employee { private String name; private int empID; private static int nextEmpID = 1000; public Employee(String name) { this.name = name; emplID = nextEmpID; nextEmpID++; } … }

27. Back to Hipster public class Hipster extends Critter { Random rand; private static int edgyBarX, edgyBarY; private int nextT, t; public Hipster() { rand = new Random(); t = 0; nextT = rand.nextInt(200); } public Direction getMove(String[][] grid) { t ++; if (t == nextT) { edgyBarX = rand.nextInt(60); edgyBarY = rand.nextInt(50); t = 0; nextT = rand.nextInt(200); } if (getY() != edgyBarY) { return Direction.NORTH; } else if (getX() != edgyBarX) { return Direction.EAST; } else { return Direction.CENTER; } public String toString() { return "H(" + edgyBarX + "," +edgyBarY + ")”; }

28. Exercise  Design a type of critters that can conduct formation, e.g., move around a circle with equal distance (enough to earn you full 20 points of Part 2/PS8)

29. Comment: PS8 Development Strategy  Do one species at a time m in ABC order from easier to harder m debug printlns  Simulator helps you debug m smaller width/height m fewer animals m "Tick" instead of "Go" m "Debug" checkbox m drag/drop to move animals

30. Summary: Polymorphism  polymorphism: Ability for the same code to be used with different types of objects and behave differently with each.  System.out.println can print any type of object. Each one displays in its own way on the console.  CritterMain can interact with any type of critter. Each one moves, fights, etc. in its own way.

31. Polymorphic Array: Generic Programming index0123 Critter[] critters = { new Ant(), new Cougar(), new Snake(), new Bulldog() } while (true) { for (i=0; i<critters.length; i++) pos = critters[i].getMove(); disp = critters[i].toString(); draw disp at pos Not dependent on any specific critters but only the generic Critter concept

32. 32 Single vs. Multiple Inheritance  Some object-oriented languages allow multiple inheritance, which allows a class to be derived from two or more classes, inheriting the members of all parents  The price: collisions, such as the same variable name, same method name in two parents, have to be resolved  Java design decision: single inheritance, meaning that a derived class can have only one parent class  But many concepts in real-life do have multiple perspectives

33. Motivating Example: Sorting 33 public static void insertionSort(int[] elems) { for (int index = 1; index < elems.length; index++) { int key = elems[index]; int insertPos = index; while (insertPos > 0 && elems[insertPos - 1] > key) { // shift larger values to the right elems[insertPos] = elems[insertPos - 1]; insertPos--; } elems[insertPos] = key; } // end of for } // end of insertionSort

34. Motivating Example: Sorting Employees 34 public static void insertionSort(int[] elems) { for (int index = 1; index < elems.length; index++) { int key = elems[index]; int insertPos = index; while (insertPos > 0 && elems[insertPos - 1] > key) { // shift larger values to the right elems[insertPos] = elems[insertPos - 1]; insertPos--; } elems[insertPos] = key; } // end of for } // end of insertionSort Goal: design a sorting method to sort Employee objects by name. Q: Which line(s) of sorting depend on int instead of Employee

35. Motivating Example: Sorting Employees 35 public static void insertionSort(Employee[] elems) { for (int index = 1; index < elems.length; index++) { Employee key = elems[index]; int insertPos = index; while (insertPos > 0 && elems[insertPos - 1].compareTo(key) >0 ) { // shift larger values to the right elems[insertPos] = elems[insertPos - 1]; insertPos--; } elems[insertPos] = key; } // end of for } // end of insertionSort public class Employee { private String name; int compareTo(Employee other) { return name.compareTo(other.name); }

36. Motivating Example: Sorting Persons 36 public static void insertionSort(Person[] elems) { for (int index = 1; index < elems.length; index++) { Person key = elems[index]; int insertPos = index; while (insertPos > 0 && elems[insertPos - 1].compareTo(key) >0 ) { // shift larger values to the right elems[insertPos] = elems[insertPos - 1]; insertPos--; } elems[insertPos] = key; } // end of for } // end of insertionSort public class Person { private int age; int compareTo(Person other) { return age – other.age; }

37. Applying Sorting to General Class X 37 public static void insertionSort(X[] elems) { for (int index = 1; index < elems.length; index++) { X key = elems[index]; int insertPos = index; while (insertPos > 0 && elems[insertPos - 1].compareTo(key) >0 ) { // shift larger values to the right elems[insertPos] = elems[insertPos - 1]; insertPos--; } elems[insertPos] = key; } // end of for } // end of insertionSort Applies to any class X that has implemented the compareTo method

38. 38 Summary  The sort method can be applied to any class X that implements the compareTo method that we used when defining the sort method.  Q: Implement using already covered technique  Define a base class X public class X { public int compareTo(X other); }  Define that Employee, Person, … extends X and overrides compareTo m Problem: An Employee/Person… conceptually really is not an X. X is just an abstract property.

39. 39 Interface  An interface provides an abstraction to write reusable, general programs  Instead of writing a program for a single class of objects, we want to write a program to handle all classes with a given set of behaviors/properties m An interface is an abstraction for the common behaviors of these behaviors  Often interface represents abstract concepts

40. 40 Outline  Admin and recap  Interface  Motivation  Syntax

41. 41 Interfaces  interface: A list of methods that classes can promise to implement. m Analogous to non-programming idea of roles or certifications "I'm certified as a CPA accountant. The certification assures you that I know how to do taxes, perform audits, and do management consulting."

42. 42 Inheritance vs Interface  Inheritance gives you an is-a relationship and code-sharing. m A Lawyer object can be treated as an Employee, and Lawyer inherits Employee's code.  Interface give you an is-a relationship without code sharing.

43. 43 Interface Syntax  An interface is a collection of constants and abstract methods  abstract method: a method header without a method body; we declare an abstract method using the modifier abstract  since all methods in an interface are abstract, the abstract modifier is usually left off

44. 44 Interface: Example public interface Movable { public double getSpeed(); public void setSpeed(double speed); public void setDirection(int direction); public int getDirection(); } interface is a reserved word No method in an interface has a definition (body) A semicolon follows each method header immediately This interface describes the behaviors common to all movable things. (Every Movable thing should have these methods.)

45. 45 Implementing an interface  general syntax: public class implements {... } m Example: public class Bicycle implements Movable {... } (What must be true about the Bicycle class for it to compile?)

46. 46 Interface Implementation  If we write a class that claims to be an interface (e.g., Movable ), but doesn't implement all of the methods defined in the interface, it will not compile. m Example: public class Bicycle implements Movable { } m The compiler error message: Bicycle.java:1: Bicycle is not abstract and does not override abstract method getSpeed() in Movable

47. 47 Example: Shape interface  An interface for shapes: public interface Shape { public double area(); } m This interface describes the common features that all shapes should have in your design. (Every shape has an area.)

48. 48 Example: Circle class // Represents circles. public class Circle implements Shape { private double radius; // Constructs a new circle with the given radius. public Circle(double radius) { this.radius = radius; } // Returns the area of this circle. public double area() { return Math.PI * radius * radius; } }

49. 49 Example: Rectangle class // Represents person. public class Person implements Shape { private double weight; private double height; … public Person(double weight, double height) { this.weight = weight; this.height = height; } // Returns the area of a person using Du Bois formula public double area() { return 0.007184 * Math.power(weight, 0.425) * Math.power(height, 0.725); } // other methods }

50. 50 Outline  Admin and recap  Interface  Motivation  Syntax  Polymorphism through interface

51. Polymorphic Reference through Interface  A variable of interface type T can hold an object of any class implementing T. Movable mobj = new Bicyle();  You can call any methods defined in the Movable interface on mobj. m When you invoke a method through the interface variable, the behavior is that of the object type.

52. 52 Interface Polymorphism: Example public static void printShapeInfo(Shape s) { System.out.println("area : " + s.area()); System.out.println(); } m Any object that implements the interface may be passed as the parameter to the above method. Circle circ = new Circle(12.0); Person john = new Person(60, 175); printShapeInfo(circ); printShapeInfo(john);

53. 53 Interface Polymorphism: Example  We can create an array of an interface type, and store any object implementing that interface as an element. Circle circ = new Circle(12.0); Person john = new John(60, 175); YaleStudent nicole = new YaleStudent(); Shape[] shapes = {circ, john, nicole}; for (int i = 0; i < shapes.length; i++) { printShapeInfo(shapes[i]); }  Each element of the array executes the appropriate behavior for its object when it is passed to the printShapeInfo method

54. Highly Reusable Sorting 54 public interface Comparable { public int compareTo(Comparable comp); } public class Sort { public static void insertionSort(Comparable[] elems) { for (int index = 1; index < elems.length; index++) { Comparable key = elems[index]; int insertPos = index; while (insertPos > 0 && elems[insertPos - 1].compareTo(key) > 0) { // shift larger values to the right elems[insertPos] = elems[insertPos - 1]; insertPos--; } elems[insertPos] = key; } // end of for } // end of insertionSort }

55. Defining a Comparable Interface 55 public interface Comparable { public int compareTo(Comparable comp); } public class Employee extends Object implements Comparable { private String name; public Employee(String name) { this.name = name; } public int compareTo(Comparable emp) { String oName = ((Employee)emp).name(); return name.compareTo( oName ); } … }

56. Defining a Comparable Interface 56 public public class Staff { private Employee[] staffList; public Staff() { staffList = new Employee[4]; staffList[0] = new Lawyer("Lisa"); staffList[1] = new Secretary("Sally"); staffList[2] = new LegalSecretary("Lynne"); staffList[3] = new Hourly("Holly", 100); } public void payday() { Sort.insertionSort(staffList); for (int count = 0; count < staffList.length; count++) { System.out.printf("%-10s: ", staffList[count].name()); System.out.printf("$%.2f\n", staffList[count].pay() ); }

57. 57 Exercise  What if you want the ability to sort according to different attribute, e.g., name, pay

58. Highly Reusable Sorting 58 public interface Comparable2 { public int compareTo(Comparable2 comp, String attr); } public class Sort { public static void insertionSort(Comparable2[] elems, String attr) { for (int index = 1; index < elems.length; index++) { Comparable2 key = elems[index]; int insertPos = index; while (insertPos > 0 && elems[insertPos - 1].compareTo(key, attr) > 0) { // shift larger values to the right elems[insertPos] = elems[insertPos - 1]; insertPos--; } elems[insertPos] = key; } // end of for } // end of insertionSort }

59. Defining a Comparable Interface 59 public class Employee extends Object implements Comparable2 { private String name; public Employee(String name) { this.name = name; } public int compareTo(Comparable emp, String attr) { if (attr.equals(“name”)) { String oName = ((Employee)emp).name(); return name.compareTo( oName ); else if (attr.equals(“pay”)) { double diff = pay() – ((Employee)emp).pay(); if (diff > 0) return 1; else return -1; } else return 0; } … } public interface Comparable2 { public int compareTo(Comparable comp, String attr); }

60. Summary: Using Interface for General Programming  When defining a class or method (e.g., sorting), think about the essence (most general) properties/behaviors of the objects you require  Define those properties in an interface  Implement the class/method for the interface only so that your design is the most general ! 60