CS 112 Introduction to Programming

Transcription

1 CS 112 Introduction to Programming User-Defined Data Types: Examples Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone: Admin q PS6 (Sukoku) questions q Friday s class 2 1

2 Recap: Design and Implementation Methodology: Procedural Based q Design (goal oriented) top-down stepwise goal-driven method decomposition methods designed are those needed for the current goal verb driven q Program implementation and testing bottom-up 3 Recap: Design and Implementation Methodology: Object-Oriented q Design Identify objects that are part of the problem domain or solution Each object has state (variables) Each object has behaviors (methods) Often do not consider one specific goal, but rather a context noun driven 4 2

3 Side-by-Side Comparison Func%on- oriented public class DrawRocket{ public static void main(string args[]){ for (int size = 3; size < 10; size++){ drawrocket(size); public static void drawrocket(int scale){ printcap(scale); public static void printcap(int scale){... Object- oriented public class RocketDrawing{ public static void main(string args[]){ for (int size = 3; size < 10; size++){ Rocket currocket = new Rocket(size); currocket.draw(); public class Rocket{ public int rocketsize; public Rocket(int rocketsize){ this.rocketsize = rocketsize; public void draw(){ printcap(); public void printcap(){... 5 Recap: Class Definition Components Variables fields (instance variables per object) static variables (shared by all objects) Methods static methods (method usable with or without object) Can access only static variables instance methods (can be used only on objects; can access both static and instance variables) Constructors Accessors (do not change object state) Mutators (modify object state) 6 3

4 Point class: Variables public class Point { // fields (instance variables, attributes) int x; int y; // static variables shared by all objects static int numpoints = 0; Point class: Constructors public class Point { // constructors public Point(int x, int y) { this.x = x; this.y = y; numpoints ++; // a shared variable to // keep track of all // Points created. public Point() { this(0, 0); 4

5 Point class: Static Methods public class Point { // static methods: // cannot access any instance variables // because one may call Point.readpoint(input) // which has no implicit parameter public static Point readpoint(scanner scan) { Point p = new Point(); p.x = scan.nextint(); p.y = scan.nextint(); return p; public static int totalpoints() { return numpoints; Point class: Instance Methods // mutators public void translate(int dx, int dy) { x = x + dx; y = y + dy; public void setlocation(int x, int y) { this.x = x; this.y = y; // accessors public double distancefromorigin() { return Math.sqrt(x * x + y * y); public String tostring() { return "(" + x + ", " + y + ") ; public void draw() { StdDraw.filledCircle(x, y, 3); StdDraw.textLeft(x, y, tostring() ); 5

6 Point class as blueprint Point class state: int x, y behavior: translate(int dx, int dy) draw() Point object #1 state: x = 5, y = -2 behavior: translate(int dx, int dy) draw() Point object #2 state: x = -245, y = 1897 behavior: translate(int dx, int dy) draw() Point object #3 state: x = 18, y = 42 behavior: translate(int dx, int dy) draw() The class (blueprint) will describe how to create objects. Each object will contain its own data and methods. Outline q Admin and recap q Defining classes o Motivation and basic syntax Ø Examples 12 6

7 Design and Implementation Methodology: Object-Oriented Design Identify objects (nouns) and their behaviors Often do not consider one specific goal, but rather a context Often lead to more reusable, extensible software 13 Example 1: A Coin Class We define a Coin class to model a coin in heads-tails games Design questions: State: what field(s) do we need to represent the state of a coin? face, an integer (or boolean) that represents the current face Operations: what are some common behaviors/operations of a coin in a game context? a Coin constructor, to set up the object a isheads method, to return if face is HEADS a tostring method, to return a string description of the current state a flip method, to flip the coin Q: introduce a setface() method? See Coin.java, CountFlips.java, FlipRace.java 14 7

8 Instance Data: The Two Coins in FlipRace class Coin coin1: Coin int face; face = 0 coin2: Coin face = 1 15 Example 2: The BankAccount Class We define an BankAccount class to model a bank account Design questions: State: what field(s) do we need to represent the state of a bank acct? acctnumber,an integer acctname, a string balance, an integer Behaviors/operations: what are some common behaviors of a bank account in a simple banking context? A constructor, to set up the object a getbalance method, to return balance a tostring method, to return a string description of the current state a withdraw method, to withdraw from the account a deposit method, to deposit into the account a addinterest method, to add interest See BankAccount.java, Transactions.java 16 8

9 Example 2: Account and Transactions public class BankAccount { final double RATE = 0.035; long acctnumber; String acctname; double balance; public BankAccount (String owner, long account, double initial) { acctname = owner; acctnumber = account; balance = initial; public static void main (String[] args) { BankAccount aliceacct = new BankAccount ( Alice", 11111, ); BankAccount bobacct = new BankAccount ( Bob", 22222, ); BankAccount charlesacct = new BankAccount ( Charles", 33333, ); bobacct.deposit (30.00); public double deposit (double amount) { balance = balance + amount; return balance; 17 Example 2: The Three BankAccount Objects in Transactions aliceacct: BankAccount acctnumber = acctname = Alice balance = aliceacct: BankAccount acctnumber = acctname = Alice balance = bobacct: BankAccount acctnumber = acctname = Bob balance = bobacct: BankAccount acctnumber = acctname = Bob balance = charlesacct: BankAccount acctnumber = acctname = Charles balance = charlesacct: BankAccount acctnumber = acctname = Charles balance = After bobacct.deposit (30.00); 18 9

10 Example 3: The Ball Class We define a Ball class to model self-bouncing balls 19 Example 3: The Ball Class Design questions: State: what field(s) do we need to represent the state of a selfbouncing ball? rx, ry, current position vx, vy, speed color, current color left, right, bottom, top:cage (boundaries) Behaviors/operations: what are some common behaviors of a selfbouncing ball? A default constructor, to set up a random ball in unit square A constructor, to set up a ball with given parameters A move method, to update position A draw method, to display See Ball.java, BouncingBalls.java 20 10

11 Example 3: Bouncing Ball in Unit Square public class Ball { double rx, ry; double vx, vy; double radius; instance variables public Ball() { rx = ry = 0.5; vx = Math.random() * 0.03; vy = Math.random() * 0.03; radius = Math.random() * 0.01; Ball.java constructor public void move() { if ((rx + vx > 1.0) (rx + vx < 0.0)) vx = -vx; if ((ry + vy > 1.0) (ry + vy < 0.0)) vy = -vy; rx = rx + vx; ry = ry + vy; bounce public void draw() { StdDraw.filledCircle(rx, ry, radius); methods Object References q Allow client to manipulate an object as a single entity. q Essentially a machine address (pointer). Ball b1 = new Ball(); Ball b2 = new Ball(); b2 = b1; addr value main memory (64- bit machine) 11

12 Object References q Allow client to manipulate an object as a single entity. q Essentially a machine address (pointer). Ball b1 = new Ball(); Ball b2 = new Ball(); b2 = b1; b1 100 addr value registers main memory (64- bit machine) Object References q Allow client to manipulate an object as a single entity. q Essentially a machine address (pointer) Ball b1 = new Ball(); Ball b2 = new Ball(); b2 = b1; b1 100 addr value 0.55 registers main memory (64- bit machine) 12

13 Object References q Allow client to manipulate an object as a single entity. q Essentially a machine address (pointer). Ball b1 = new Ball(); Ball b2 = new Ball(); b2 = b1; b1 100 addr value registers main memory (64- bit machine) Object References q Allow client to manipulate an object as a single entity. q Essentially a machine address (pointer). Ball b1 = new Ball(); Ball b2 = new Ball(); b2 = b1; b1 100 b2 addr value registers main memory (64- bit machine) 13

14 Object References q Allow client to manipulate an object as a single entity. q Essentially a machine address (pointer). Ball b1 = new Ball(); Ball b2 = new Ball(); b2 = b1; b1 100 b2 107 addr value registers main memory (64- bit machine) Object References q Allow client to manipulate an object as a single entity. q Essentially a machine address (pointer). Ball b1 = new Ball(); Ball b2 = new Ball(); b2 = b1; Data stored in for bit recycler (garbage collection). b1 100 b2 100 addr value registers main memory (64- bit machine) 14

15 Object References q Allow client to manipulate an object as a single entity. q Essentially a machine address (pointer). Ball b1 = new Ball(); Ball b2 = new Ball(); b2 = b1; Moving b2 also moves b1 since they are aliases that reference the same object. b1 100 b2 100 addr 100 value registers main memory (64- bit machine) Creating Many Objects q Each object is a data type value. Use new to invoke constructor and create each one. public class BouncingBalls { public static void main(string[] args) { int N = Integer.parseInt(args[0]); Ball balls[] = new Ball[N]; for (int i = 0; i < N; i++) balls[i] = new Ball(); create and ini=alize N objects while(true) { StdDraw.clear(); for (int i = 0; i < N; i++) { balls[i].move(); balls[i].draw(); StdDraw.show(20); anima=on loop 15

16 Example 4: Continuous Line Graph What is a base class to allow drawing of continuous line graphs? 31 Example: Turtle We define a Tutle class to keep track of drawing turtle (cursor) Design questions: State: what field(s) do we need to represent the state of a drawing turtle? x,y, current position angle: the current drawing direction Operations: what are some common behaviors/operations on a Turle to allow flexible drawing? A Turtle constructor, to set up the object A goforward( stepsize) method a turnleft( angle ) method 32 16

17 Turtle Graphics public class Turtle { double x, y; // turtle is at (x, y) double angle; // facing this direction public Turtle(double x0, double y0, double a0) { x = x0; y = y0; angle = a0; public void turnleft(double delta) { angle += delta; public void goforward(double d) { double oldx = x; double oldy = y; x += d * Math.cos(Math.toRadians(angle)); y += d * Math.sin(Math.toRadians(angle)); StdDraw.line(oldx, oldy, x, y); 33 N-gon What is the angle of each turn? angle = 360/ N What is the initial degree? angle /

18 N-gon public class Ngon { public static void main(string[] args) { int N = Integer.parseInt(args[0]); double angle = / N; Turtle turtle = new Turtle(0.5, 0, angle/2.0); double step = Math.sin(Math.toRadians(angle/2.0)); for (int i = 0; i < N; i++) { turtle.goforward(step); turtle.turnleft(angle); Spira Mirabilis public class Spiral { public static void main(string[] args) { int N = Integer.parseInt(args[0]); double decay = Double.parseDouble(args[1]); double angle = / N; double step = Math.sin(Math.toRadians(angle/2.0)); Turtle turtle = new Turtle(0.5, 0, angle/2.0); for (int i = 0; i < 10 * N; i++) { step /= decay; turtle.goforward(step); turtle.turnleft(angle); 36 18

19 Spira Mirabilis public class Spiral { public static void main(string[] args) { int N = Integer.parseInt(args[0]); double decay = Double.parseDouble(args[1]); double angle = / N; double step = Math.sin(Math.toRadians(angle/2.0)); Turtle turtle = new Turtle(0.5, 0, angle/2.0); for (int i = 0; i < 10 * N; i++) { step /= decay; turtle.goforward(step); turtle.turnleft(angle); Spira Mirabilis in Nature 38 19

20 Caution René Magritte. "This is not a pipe." Java. This is not a Turtle. Turle myturtle = new Turtle(0.5, 0, 45); myturle.turnleft( 90 ); OOP. Natural vehicle for studying abstract models of the real world. Example 5: A Complex Class A complex number (a + bi) is a quintessential mathematical abstraction (a + bi) + (c + di) = a + c + (b + d)i (a + bi) x (c + di) = ac - bd + (ad + bc)i a = 3 + 4i, b = i a + b = 1 + 7i a b = i a = 5 Many applications of complex numbers Fractals. Impedance in RLC circuits. Signal processing and Fourier analysis. Control theory and Laplace transforms. Quantum mechanics and Hilbert spaces

21 Argos Antennas 41 A Complex Class Design questions: State: what field(s) do we need to represent the state of a complex number? re, a number representing the real part im, a number representing the imaginary part Behaviors: what are some common behaviors of a complex number? a Complex constructor, to set up the object A abs method, to return the magnitude a tostring method, to return a string description of a complex number Mathematical operations such as +, -, * a plus method to add current complex number with another complex number a times method to multiply current complex number with another complex number 42 21

22 The Complex Class: Design Question public class Complex { double re; double im; public Complex(double real, double imag) { re = real; im = imag; public?? plus(complex b) { - What is the return type of plus? - Should plus change the state of the number 43 The Consistency (Familiarity) Design Principle Suppose a, b, and c are standard numbers (Complex numbers are numbers after all) Does a + b (think a.+(b) ) change a? no What is the return of a + b (think a.+(b)? The value of a + b so that we can write a + b + c State (no)change and return type of plus public Complex plus(complex b) { double real = re + b.re; double imag = im + b.im; return new Complex(real, imag); 44 22

23 Complex.java public class Complex { double re; double im; instance variables public Complex(double real, double imag) { re = real; im = imag; constructor public String tostring() { return re + " + " + im + "i"; public double abs() { return Math.sqrt(re*re + im*im); public Complex plus(complex b) { double real = re + b.re; double imag = im + b.im; return new Complex(real, imag); refers to b's instance variable creates a Complex object, and returns a reference to it public Complex times(complex b) { double real = re * b.re im * b.im; double imag = re * b.im + im * b.re; return new Complex(real, imag); methods 45 Immutability: Advantages and Disadvantages Immutable data type. Object's state does not change once constructed. Complex is an example of Immutable objects. String defined by Java is another example. Advantages. Avoid aliasing bugs. Makes program easier to debug. Limits scope of code that can change values. Pass objects around without worrying about modification. Disadvantage. New object must be created for every value. 23

24 A Simple Client public static void main(string[] args) { Complex a = new Complex( 3.0, 4.0); Complex b = new Complex(-2.0, 3.0); Complex c = a.times(b); System.out.println("a = " + a.tostring() ); System.out.println("b = " + b.tostring() ); System.out.println("c = " + c.tostring() ); % java TestClient a = i b = i c = i 47 24