Admin. CS 112 Introduction to Programming. Recap: The Critter Class. Recap: Event-Driven Programming. State Machine Driven Cougar.

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1 Admin CS 112 Introduction to Programming q PS8 and class project questions? q Special session on Google App Engine Monday 8 pm at DL 120 Critters/Event-Driven Programming; Interface Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone: yry@cs.yale.edu 2 Recap: The Critter Class Recap: Event-Driven Programming // 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 q Key concepts: The simulator is typically implemented using polymorphism 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? % Recap: Cougar State Machine Driven Cougar q Write a critter class Cougar (among the dumbest of all animals): Method constructor public Cougar() eat fight getcolor getmove tostring Always eats. Always roars. Behavior Blue if the Cougar has never fought; red if he has. Walks west until he finds food; then walks east until he finds food; then goes west and repeats. "C" q getmove q Color Move West! Has fought eat() eat() Move East fight() Has fought 6 1

2 Recap: Snake Method Behavior 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" 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 ++ Non-EDP-> EDP: Guarding Condition Snake solution A non-event driven version steps < cyclelength import java.awt.*; // for Color public class Snake extends Critter { private int cyclelength; // # steps in curr. Horiz. cycle steps < cyclelength private int steps; // # of cycle's steps already taken cyclelength = 1; while (true) { steps = 0; while (steps < cyclelength) if cyclelength % 2 == 1 go East else go West steps ++; // invariant? go South cyclelength ++ if (cyclelength % 2 == 1) go East else go West steps++; steps == cyclelength go South cyclelength ++ steps=0; 9 public Snake() { cyclelength = 1; steps = 0; public Direction getmove() { if (steps < cyclelength) { steps++; if (cyclelength % 2 == 1) { else { else { steps = 0; return Direction.EAST; return Direction.WEST; cyclelength ++; return Direction.SOUTH; public String tostring() { return "S"; if (cyclelength % 2 == 1) go East else go West steps++; steps == cyclelength Go South cyclelength ++ steps=0; Comment: States Testing critters q Counting is helpful: How many total moves has this animal made? How many times has it eaten? Fought? q Remembering recent actions in fields may be helpful: Which direction did the animal move last? How many times has it moved that way? Did the animal eat the last time it was asked? How many steps has the animal taken since last eating? How many fights has the animal been in since last eating? q Focus on one specific critter of one specific type Only spawn 1 of each animal, for debugging q Make sure your fields update properly Use println statements to see field values q Look at the behavior one step at a time Use "Tick" rather than "Go" 2

3 Designing Bulldog Coordination q Be open minded q Think about strategies, e.g., How much state do your bulldogs keep and probe state? When does your Bulldog eat/mate? Is there an optimal fight strategy for a specific type of opponent? Do your bulldogs play disguise? Does your strategy change with time? Do your bulldogs coordinate their behaviors to form some kind of patterns? 13 Coordination 14 Critter exercise: Hipster q A group of hipster critters want to hangout together q Each hipster can suddenly become inspired and choose a random board location called an edgy bar q A hipster go north until reaches edgy bar s horizontal, then east until reaching the bar 15 Solution 1 (See Hipster.java) 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(); public Direction getmove(string[][] grid) { t ++; if (t == nextt) { edgybarx = rand.nextint( getwidth() ); edgybary = rand.nextint( getheight() ); 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(); public Direction getmove(string[][] grid) { q Problem: Each t ++; if (t == nextt) { hipster goes edgybarx = rand.nextint( getwidth() ); edgybary = rand.nextint( getheight() ); to a different bar. if (gety()!= edgybary) { return Direction.NORTH; else if (getx()!= edgybarx) { return Direction.EAST; else { return Direction.CENTER; public String tostring() { return "H(" + edgybarx + "," +edgybary + ") ; if (gety()!= edgybary) { return Direction.NORTH; else if (getx()!= edgybarx) { return Direction.EAST; else { return Direction.CENTER; We want all hipsters to share the same bar location. public String tostring() { return "H(" + edgybarx + "," +edgybary + ") ; 3

4 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(); public Direction getmove(string[][] grid) { t ++; if (t == nextt) { edgybarx = rand.nextint( getwidth() ); edgybary = rand.nextint( getheight() ); if (gety()!= edgybary) { return Direction.NORTH; else if (getx()!= edgybarx) { return Direction.EAST; else { return Direction.CENTER; public String tostring() { return "H(" + edgybarx + "," +edgybary + ") ; Recall: Static members q static: Part of a class, rather than part of an object. Object classes can have static methods and fields. Not copied into each object; shared by all objects of that class. object #1 state: int field1 double field2 behavior: public void method3() public int method4() public void method5() class state: private static int staticfielda private static String staticfieldb behavior: public static void somestaticmethodc() public static void somestaticmethodd() 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() Accessing static fields q From inside the class where the field was declared: fieldname // get the value fieldname = value; // set the value q From another class (if the field is public): ClassName.fieldName // get the value ClassName.fieldName = value; // set the value Example q We want to assign a unique, increasing employee ID for each Employee object (e.g., Secretary, Marketer, Lawyer) that we create generally static fields are not public unless they are final 22 Employee with Static public class Employee { private int empid; private static int nextempid = 1000; 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"); Example: The Employee Objects 1000 Employee.nextEmpID public class Employee { private int empid; private static int nextempid = 1000; emplid = nextempid; nextempid++; 24 4

5 Example: The Employee Objects Example: The Employee Objects larry: Lawyer name = Larry empid = 1000; Employee.nextEmpID larry: Lawyer name = Larry empid = 1000; Employee.nextEmpID public class Employee { private int empid; private static int nextempid = 1000; emplid = nextempid; nextempid++; mike: Marketer name = Mike empid = 1001; public class Employee { private int empid; private static int nextempid = 1000; emplid = nextempid; nextempid++; Lawyer larry = new Lawyer("Larry"); Marketer mike = new Marketer("Mike"); Back to Hipster Exercise 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); q 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) if (gety()!= edgybary) { return Direction.NORTH; else if (getx()!= edgybarx) { return Direction.EAST; else { return Direction.CENTER; public String tostring() { return "H(" + edgybarx + "," +edgybary + ") ; Comment: PS8 Development Strategy Summary: Polymorphism q Do one species at a time in ABC order from easier to harder debug printlns q polymorphism: Ability for the same code to be used with different types of objects and behave differently with each. q Simulator helps you debug smaller width/height fewer animals "Tick" instead of "Go" "Debug" checkbox drag/drop to move animals 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. 5

6 Polymorphic Array: Generic Programming Critter[] critters = { new Ant(), new Cougar(), new Snake(), new Bulldog() index 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 Single vs. Multiple Inheritance q 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 q The price: collisions, such as the same variable name, same method name in two parents, have to be resolved q Java design decision: single inheritance, meaning that a derived class can have only one parent class q But many concepts in real-life do have multiple perspectives 32 Motivating Example: Sorting Motivating Example: Sorting Employees public static void insertionsort(int[] elems) { int key = elems[index]; public static void insertionsort(int[] elems) { int key = elems[index]; && elems[insertpos - 1] > key) { // end of for // end of insertionsort 33 && elems[insertpos - 1] > 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 34 Motivating Example: Sorting Employees Motivating Example: Sorting Persons public static void insertionsort(employee[] elems) { Employee key = elems[index]; && elems[insertpos - 1].compareTo(key) >0 ) { // end of for // end of insertionsort public class Employee { int compareto(employee other) { return name.compareto(other.name); 35 public static void insertionsort(person[] elems) { Person key = elems[index]; && elems[insertpos - 1].compareTo(key) >0 ) { // end of for // end of insertionsort public class Person { private int age; int compareto(person other) { return age other.age; 36 6

7 Applying Sorting to General Class X public static void insertionsort(x[] elems) { X key = elems[index]; && elems[insertpos - 1].compareTo(key) >0 ) { // end of for // end of insertionsort Applies to any class X that has implemented the compareto method Summary q The sort method can be applied to any class X that implements the compareto method that we used when defining the sort method. q 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 Problem: An Employee/Person conceptually really is not an X. X is just an abstract property Interface Outline q An interface provides an abstraction to write reusable, general programs q 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 An interface is an abstraction for the common behaviors of these behaviors q Often interface represents abstract concepts q Admin and recap q Interface Motivation Syntax Interfaces Inheritance vs Interface q interface: A list of methods that classes can promise to implement. 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." q Inheritance gives you an is-a relationship and code-sharing. A Lawyer object can be treated as an Employee, and Lawyer inherits Employee's code. q Interface give you an is-a relationship without code sharing

8 Interface Syntax Interface: Example q 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 43 interface is a reserved word public interface Movable { public double getspeed(); public void setspeed(double speed); public void setdirection(int direction); public int getdirection(); This interface describes the behaviors common to all movable things. (Every Movable thing should have these methods.) A semicolon follows each method header immediately No method in an interface has a definition (body) 44 Implementing an interface Interface Implementation q general syntax: public class <name> implements <interface names> {... Example: public class Bicycle implements Movable {... (What must be true about the Bicycle class for it to compile?) q 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. Example: public class Bicycle implements Movable { The compiler error message: Bicycle.java:1: Bicycle is not abstract and does not override abstract method getspeed() in Movable Example: Shape interface q An interface for shapes: public interface Shape { public double area(); This interface describes the common features that all shapes should have in your design. (Every shape has an area.) 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;

9 Example: Rectangle class Outline // 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 * Math.power(weight, 0.425) * Math.power(height, 0.725); q Admin and recap q Interface Motivation Syntax Polymorphism through interface // other methods Polymorphic Reference through Interface q 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. When you invoke a method through the interface variable, the behavior is that of the object type. Interface Polymorphism: Example public static void printshapeinfo(shape s) { System.out.println("area : " + s.area()); System.out.println(); 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); 52 Interface Polymorphism: Example q 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 Highly Reusable Sorting public interface Comparable { public int compareto(comparable comp); public class Sort { public static void insertionsort(comparable[] elems) { Comparable key = elems[index]; && elems[insertpos - 1].compareTo(key) > 0) { // end of for // end of insertionsort

10 Defining a Comparable Interface Defining a Comparable Interface public interface Comparable { public int compareto(comparable comp); public class Employee extends Object implements Comparable { public int compareto(comparable emp) { String oname = ((Employee)emp).name(); return name.compareto( oname ); 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() ); Exercise q What if you want the ability to sort according to different attribute, e.g., name, pay Highly Reusable Sorting public interface Comparable2 { public int compareto(comparable2 comp, String attr); public class Sort { public static void insertionsort(comparable2[] elems, String attr) { Comparable2 key = elems[index]; && elems[insertpos - 1].compareTo(key, attr) > 0) { 57 // end of for // end of insertionsort 58 Defining a Comparable Interface Summary: Using Interface for General Programming public interface Comparable2 { public int compareto(comparable comp, String attr); public class Employee extends Object implements Comparable2 { 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; 59 q When defining a class or method (e.g., sorting), think about the essence (most general) properties/behaviors of the objects you require q Define those properties in an interface q Implement the class/method for the interface only so that your design is the most general! 60 10