Admin. CS 112 Introduction to Programming. Recap: OOP. Math.random() Design. Static Math.random() method vs Random Objects

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1 Admin CS 112 Introduction to Programming User-Defined Data Types: Using static to hide OOP; Encapsulation Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone: q PS7 (GuitarHero) Walkthrough Tuesday at 8 pm q Project Please start to work on teaming as soon as you can If you need to find a partner, please send me a note so that we can arrange meetings at office hours 2 Recap: OOP Recap: Side-by-Side Comparison q Identify objects that are part of the problem domain or solution q An object includes both state and behaviors Focus on external behaviors (more shortly) 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 drawrocket(){ printcap(); public void printcap(){ 3 4 Math.random() Design Static Math.random() method vs Random Objects public static double random() Returns a random number between 0 and 1 Most static methods that we encountered are deterministic methods: given the same input parameter, gives the same output But the random() method cannot be, i.e., it is non-deterministic 5 6 1

2 A Little Peek into Random Number Generation The random numbers generated by Java are actually pseudo-random numbers Suppose you get a random number R n, the next time you call it to get R n+1, it returns: R n+1 = R n * it then converts to the right range to you! This method is proposed by D. H. Lehmer in mathematical jargon, Java uses a type of linear congruential pseudorandom number generator Implication: the previously returned random number must be remembered. 7 The Random class A Random object generates random numbers as preceding slide Class Random is found in the java.util package. import java.util.random; Method name Example: Description Random(long seed) Create a random number using a seed (R 0 ) Random() setseed(seed) nextint(<max>) Create a random number using a seed derived from time Initialize the random number generator Returns a random integer in the range [0, max) in other words, 0 to max-1 inclusive nextdouble() Returns a random real number in [0.0, 1.0) Random rand = new Random(); // Default, seed by time int randomnumber = rand.nextint(10); // 0-9 Q: What is the state of a Random object? Using Random Numbers Math.random() Example: to get a random number from 1 to 20 int n = rand.nextint(20) + 1; // 1-20 inclusive To get a number in arbitrary range [min, max] inclusive: <name>.nextint(<size of range>) + <min> Where <size of range> is (<max> - <min> + 1) Example: A random integer between 4 and 10 inclusive: int n = rand.nextint(7) + 4; public static double random() When this method is first called, it creates a single new pseudorandom-number generator, exactly as if by the expression new java.util.random() This new pseudorandom-number generator is used thereafter for all calls to this method. java/lang/math.html#random() 10 How May Math.random()be Implemented? Advantage and Issue of Using Math.random() A static variable, also called a singleton. Advantage Hide object-oriented programming, simplifying programming public class Math { static Random rand; public static double random() { if (rand == null) rand = new Random(); return rand.nextdouble();.. 11 Issue A single Random object: Every request for a random number is handled by one single random object, creating a bottleneck client 1 client 1 client 2 client 3 client 2 client

3 DrawingPanel Developed for the Back to Basics Textbook, using an OOP approach. Two key classes: StdDraw vs DrawingPanel DrawingPanel: A window on the screen. Not part of Java; provided by the textbook. Graphics: A "pen" to draw shapes and lines on a window. from Java standard class library 13 DrawingPanel Graphics A "Canvas" object that represents windows/drawing surfaces To create a window: DrawingPanel name = new DrawingPanel(width, height); Example: DrawingPanel panel = new DrawingPanel(300, 200); See SimplePanels.java DrawingPanel draws shapes / lines / characters /imagines using another object of type Graphics. Graphics: Your painting toolset: "Pen" or "paint brush" objects to draw lines and shapes; fonts for character, Access it by calling getgraphics on a DrawingPanel object. Graphics g = panel.getgraphics(); Draw shapes by calling methods on the Graphics object. g.fillrect(10, 30, 60, 35); g.filloval(80, 40, 50, 70); Graphics Coordinate System Some Graphics Methods Each (x, y) position is a pixel ("picture element"). Position (0, 0) is at the window's top-left corner. x increases rightward and the y increases downward. A rectangle from (0, 0) to (200, 100) looks like this: (0, 0) x+ (200, 100) y+ Method name g.setcolor(color); Description set Graphics to paint any following shapes in the given color g.drawline(x1, y1, x2, y2); line between points (x1, y1), (x2, y2) g.drawoval(x, y, width, height); outline largest oval that fits in a box of size width * height with top-left at (x, y) g.drawrect(x, y, width, height); outline of rectangle of size width * height with top-left at (x, y) g.drawstring(text, x, y); text with bottom-left at (x, y) g.filloval(x, y, width, height); fill largest oval that fits in a box of size width * height with top-left at (x, y) g.fillrect(x, y, width, height); fill rectangle of size width * height with top-left at (x, y) See SimplePanels.java 3

4 Benefit and Issue of StdDraw StdDraw Similar to Math.random() creating a single Random object, StdDraw creates a single drawing object Benefit Hides OOP programming complexity (No objects) Simplifies programming by converting the native Java coordinate system (up-side down) to a more intuitive coordinate system Adds additional methods such as square, point Potential issue Only 1 drawing surface in your program See SimplePanels.java Recap: 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) { - How to design the plus instance method? Recap: The Consistency (Familiarity) Design Principle Suppose a, b, and c are standard numbers (Complex numbers are numbers after all) What does a + b (think a.+(b) ) do? Returns the value of a + b so that we can write a + b + c; the value of a is not changed. public Complex plus(complex b) { double real = re + b.re; double imag = im + b.im; return new Complex(real, imag); 21 Complex.java public class Complex { double re; double im; public Complex(double real, double imag) { re = real; im = imag; 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); 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); instance variables constructor refers to b's instance variable creates a Complex object, and returns a reference to it methods 22 Immutability: Advantages and Disadvantages A Simple Client 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. public static void main(string[] args) { Complex a = new Complex( 3.0, 4.0); Complex b = new Complex(-2.0, 3.0); Complex c = new Complex( 2.0, 3.0); Complex d = a.plus(b).plus(c); System.out.println("a = " + a.tostring() ); System.out.println("b = " + b.tostring() ); System.out.println("c = " + c.tostring() ); System.out.println( d = " + d.tostring() ); % java TestClient a = i b = i c = i d = i 24 4

5 A Complex Client: Mandelbrot Set Mandelbrot set. A set of complex numbers. Plot. Plot (x, y) black if z = x + y i is in the set, and white otherwise. A Complex Client: Mandelbrot Set Mandelbrot set. Is complex number z 0 in the set? Iterate z t + 1 = (z t ) 2 + z 0. If z t diverges to infinity, then z 0 is not in set; otherwise z 0 is in set. t z t t z t 0-1/2 + 0i i 1-1/4 + 0i i 2-7/16 + 0i i Can be used to model complex rugged shapes such as uneven clouds, contours of mountains, winding riverbeds, arts, No simple formula describes which complex numbers are in set. - Instead, described using an algorithm / i / i / i z = -1/2 is in Mandelbrot set i i i z = 1 + i not in Mandelbrot set Testing Point in Mandelbrot Set Practical issues. Cannot iterate infinitely many times. Approximate solution. Fact: if z t > 2 for any t, then z not in Mandelbrot set. Pseudo-fact: if z then z "likely" in Mandelbrot set. Testing Point in Mandelbrot Set Our Mandelbrot test: Returns the number of iterations to check if z0 is in Mandelbrot public static int mand(complex z0) { final int max = 255; Complex z = z0; for (int t = 0; t < max; t++) { if (z.abs() > 2.0) return t; z = z.times(z).plus(z0); return max; z = z2 + z Plotting Mandelbrot Set Plotting Mandelbrot Set (DrawingPanel) Practical issues. Cannot plot infinitely many points. Display technique. User specifies center, size Program maps the points on the N-by-N drawing panel to center, size x c + y c i (x c -size/2) + (y c -size/2) i Each grid has length size/n 29 Plot the Mandelbrot set in gray scale. public static void main(string[] args) { double xc = Double.parseDouble(args[0]); double yc = Double.parseDouble(args[1]); double size = Double.parseDouble(args[2]); DrawingPanel panel = new DrawingPanel(N, N); Graphics g = panel.getgrahics(); for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) { double x0 = xc - size/2 + size*i/n; double y0 = yc - size/2 + size*j/n; Complex z0 = new Complex(x0, y0); int gray = mand(z0); Color color = new Color(gray, gray, gray); g.setcolor( color ); g.drawline(i, N-1-j, i, N 1 - j); // end of for // end of for (0, 0) is upper left 30 5

6 Mandelbrot Set Mandelbrot Set % java MandelbrotDrawingPanel % java MandelbrotDrawingPanel % java MandelbrotDrawingPanel % java MandelbrotDrawingPanel Picture Data Type Raster graphics. Basis for image processing. Set of values. 2D array of Color objects (pixels). Plotting Mandelbrot Set (Picture) Plot the Mandelbrot set in gray scale. public static void main(string[] args) { double xc = Double.parseDouble(args[0]); double yc = Double.parseDouble(args[1]); double size = Double.parseDouble(args[2]); Picture pic = new Picture(N, N); // NxN picture for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) { double x0 = xc - size/2 + size*i/n; double y0 = yc - size/2 + size*j/n; Complex z0 = new Complex(x0, y0); int gray = mand(z0); scale to screen Color color = new Color(gray, gray, gray); coordinates pic.set(i, N-1-j, color); API. // end of for // end of for (0, 0) is upper left 34 Mandelbrot Set Plot the Mandelbrot set in color using a color mapping table. % java ColorMandelbrot Mandelbrot Set Plot the Mandelbrot set in color using a color mapping table. % java ColorMandelbrot

7 Mandelbrot Set (-1.5, -1) Mandelbrot Set Outline q See video v=cxm9j77stl0 q Admin and recap q Defining classes o Motivation and basic syntax o Examples Ø The encapsulation principle Two Views of an Object The Encapsulation Principle You can take one of two views of an object: external (API) - the interaction of the object with its clients An object should be an encapsulated entity (a black box), providing an API (i.e., a set of external visible state and services/behaviors) The user, or client, of an object should not be aware of the internal representation of state or details of algorithms internal (implementation) - the structure of its data, the algorithms used by its methods Client can see only the external API (state and behaviors) Client Methods 41 Client should not see the internal state or behaviors Data/state 42 7

8 Encapsulation Analogy Encapsulation Analogy You don't understand the inner details of iphone, and you don't need to. Apple does not want to commit to any internal details so that Apple can continuously update the internal Client API Implementation client needs to know how to use API implementation needs to know what API to implement Implementation and client need to agree on API ahead of time. Examples What are some risks of allowing others to view/access the internal state of a class? The Coin class The Ball class The Point class The BankAccount class Why Encapsulating Data Consistency: so that we make it impossible for others to "reach in" and directly alter another object's state Protect object from unwanted access Example: Can't fraudulently increase an BankAccount balance. Maintain state invariants Example: Only allow BankAccounts with non-negative balance. Example: Only allow Dates with a month from Flexibility: so that you can change the state representation later without worrying about breaking others code Example: Point could be rewritten in polar coordinates (r, θ) so long you provide translation in the interface, clients will not see difference. Accomplish Encapsulation: Access Modifiers In Java, we accomplish encapsulation through the appropriate use of access modifiers An access modifier is a Java reserved word that specifies the accessibility of a method, data field, or class we will discuss two access modifiers: public, private we will discuss the other modifier (protected) later The public and private Access Modifiers access modifiers enforce encapsulation public members (data and methods: can be accessed from anywhere private members: can be accessed from a method defined in the same class Members without an access modifier: default private accessibility, i.e., accessible in the same package; otherwise, not accessible

9 Using Access Modifiers to Implement Encapsulation: Methods The Effects of Public and Private Accessibility Only service methods should be made public Support or helper methods created simply to assist service methods should be declared private variables methods public violate Encapsulation Use Caution provide services to clients private enforce encapsulation support other methods in the class Examples: Set the Access Modifiers Class Diagram Coin Ball BankAccount Point Coin - face : int + flip() : void + isheads() : boolean + tostring() : String class name attributes methods 51 Above is a class diagram representing the Coin class. - indicates private data or method + indicates public data or method 52 Ask, Don't Touch Encapsulated data types. Don't touch data and do whatever you want. Instead, ask object to manipulate its data. "Ask, don't touch." Adele Goldberg Former president of ACM Co-developed Smalltalk Lesson. Limiting scope makes programs easier to maintain and understand. "principle of least privilege" 9