3.1 Data Types: Using Objects

1. 3.1 Data Types: Using Objects

2. Data Types Data type. Set of values and operations on those values. Primitive types. • “Built-in” for a language. Basic “building blocks” • Usually single values. • Operations directly translate to machine instructions. OK, but we want to write programs that process other types of data. • Colors, pictures, strings, input streams, … • Complex numbers, vectors, matrices, polynomials, … • Points, polygons, charged particles, celestial bodies, … Data Type Set of Values Operations boolean true, false not, and, or, xor int -231 to 231 -1 add, subtract, multiply double any of 264 possible reals add, subtract, multiply

3. Data Types and Abstraction Remember Abstraction? Hiding details…. One principle of “computational thinking.” When we use data types, keep these things in mind: • We don’t have to understand exactly how bits are stored in the machine. • We have an “information model” (or set of values), and… • We have operations to use -- even if we don’t know exactly how they’re implemented! Question for you: • If operations hide details and are a form of abstraction, can you think of something else we’ve done in Java that is also an example of this “procedural abstraction”?

4. “Non-primitive” types Reference type. • In Java, not a simple, basic data type. • Arrays are one kind of reference type. • Object types are another. • Examples: String, In • “Class” means data type that’s an object • Class and type. Object and variable. What’s important about reference types? • Object types are not part of the “core” language (perhaps part of standard libraries). Arrays are. • Object types can be defined by someone or by us. • The variable “refers to” or “points to” a separate chunk of memory. • We can access things inside what’s pointed to. • Example: int[] a = new int[10]; a.length • Problem: int[] b; b.length

5. Objects Object. Holds a value of a particular data-type. A variable name refers to an object. Class. The data type Impact. Enables us to create our own data types; define operations on them; and integrate into our programs. Data Type Set of Values Operations Color 24 bits get red component, brighten Picture 2D array of colors get/set color of pixel (i, j) String sequence of characters length, substring, compare

6. Example Data Type (Class): Charge A Charge class. Represents a charged particle in a 2-dim space. Can use this to study “electric potential.” Data Model. • Location in a 2-dim space, so x and y values • The charge at that point Operations. • Calculate potential due to this particle as some other (x,y) • Convert this to a String for output An API defines functions (known as methods) for each class: • Charge (double x0, double y0, double q0) // constructor • double potentialAt( double x, double y) • String toString()

7. Methods for a Class Note! No operators like +, *, etc. Methods / Functions. • These are slightly different than previous methods • These are “instance methods” • Previously we wrote “static” methods Instance methods are called “on” an object (a variable) • System.out.println(); // call println on System.out • String s = aCharge.toString(); // call toString on aCharge • In fileIn = new In(“file.txt”); fileIn.readInt(); Ways to think about this: • Send a message to an object. • Tell an object to do something.

8. Constructors In Java, remember: Variables must be initialized when created. We must have a way to make this happen for “new” types. One special instance method in the API: • Charge (double x0, double y0, double q0) // constructor • This is a method called when you invoke “new” to create a new object! • You can pass parameters to control initialization. • The class’ author defines the logic for this method to carry out proper initialization.

9. Example: Constructors and Methods To construct a new object: Use keyword new and name of data type. To apply an operation: Use name of object, the dot operator, andthe name of the method.

10. String Data Type String data type. Basis for text processing. Set of values. Sequence of Unicode characters. API. … http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html

11. Typical String Processing Code

13. Image Processing

14. Color Data Type Color. A sensation in the eye from electromagnetic radiation. Set of values. [RGB representation]2563 possible values, which quantify the amount of red, green, and blue, each on a scale of 0 to 255. R G B Color 255 0 0 0 255 0 0 0 255 255 255 255 0 0 0 255 0 255 105 105 105

15. Color Data Type Color. A sensation in the eye from electromagnetic radiation. Set of values. [RGB representation]2563 possible values, which quantify the amount of red, green, and blue, each on a scale of 0 to 255. API. Application Programming Interface. http://java.sun.com/j2se/1.5.0/docs/api/java/awt/Color.html

16. Albers Squares Josef Albers. Revolutionized the way people think about color. Homage to the Square by Josef Albers (1949-1975)

17. Albers Squares Josef Albers. Revolutionized the way people think about color. % java AlbersSquares 9 90 166 100 100 100

18. Using Colors in Java import java.awt.Color; publicclass AlbersSquares { publicstaticvoid main(String[] args) { int r1 = Integer.parseInt(args[0]); int g1 = Integer.parseInt(args[1]); int b1 = Integer.parseInt(args[2]); Color c1 =new Color(r1, g1, b1); int r2 = Integer.parseInt(args[3]); int g2 = Integer.parseInt(args[4]); int b2 = Integer.parseInt(args[5]); Color c2 =new Color(r2, g2, b2); StdDraw.setPenColor(c1); StdDraw.filledSquare(.25,.5,.2); StdDraw.setPenColor(c2); StdDraw.filledSquare(.25,.5,.1); StdDraw.setPenColor(c2); StdDraw.filledSquare(.75,.5,.2); StdDraw.setPenColor(c1); StdDraw.filledSquare(.75,.5,.1); } } to access Color library first color second color first square second square

19. Monochrome Luminance Monochrome luminance. Effective brightness of a color. NTSC formula. Y = 0.299r + 0.587g + 0.114b. import java.awt.Color; publicclass Luminance { publicstaticdouble lum(Color c) { int r = c.getRed(); int g = c.getGreen(); int b = c.getBlue(); return.299*r +.587*g +.114*b; } }

20. Color Compatibility Q. Which font colors will be most readable with which background colors on computer monitors and cell phone screens? A. Rule of thumb: difference in luminance should be  128. 256 208 105 47 28 14 publicstatic boolean compatible(Color a, Color b) { return Math.abs(lum(a)- lum(b)) >= 128.0; }

21. Grayscale Grayscale. When all three R, G, and B values are the same,resulting color is on grayscale from 0 (black) to 255 (white). Convert to grayscale. Use luminance to determine value. Bottom line. We are writing programs that manipulate color. publicstatic Color toGray(Color c) { int y =(int) Math.round(lum(c)); Color gray =new Color(y, y, y); return gray; } round double to nearest int

22. Picture Data Type j (0,0) Raster graphics. Basis for image processing. Set of values. 2D array of Color objects (pixels). API. i

23. Image Processing: Grayscale Filter Goal. Convert color image to grayscale according to luminance formula. import java.awt.Color; publicclass Grayscale { publicstaticvoid main(String[] args) { Picture pic =new Picture(args[0]); for(int i =0; i < pic.width(); i++) { for(int j =0; j < pic.height(); j++) { Color color = pic.get(i, j); Color gray = Luminance.toGray(color); pic.set(i, j, gray); } } pic.show(); } }

24. Image Processing: Grayscale Filter Goal. Convert color image to grayscale according to luminance formula. mandrill.jpg % java Grayscale mandrill.jpg

25. Horton got this far before Exam 2 in Spring 2008. • For exam 2, no need to look at slides after this. • Slides after this point will be covered after Exam 2.

26. Image Processing: Scaling Filter Goal. Shrink or enlarge an image to desired size. Downscaling. To shrink, delete half the rows and columns. Upscaling. To enlarge, replace each pixel by 4 copies.

27. Image Processing: Scaling Filter Goal. Shrink or enlarge an image to desired size. Uniform strategy. To convert from ws-by-hs to wt-by-ht: • Scale row index by ws/wt . • Scale column index by hs/ht . • Set color of pixel (i, j) in target image to color of pixel(iws/wt , j hs/ht) in source image. j hs/ht j i ? iws/wt target image(wt-by-ht) source image(ws-by-hs)

28. Image Processing: Scaling Filter import java.awt.Color; public class Scale { public static void main(String args[]) { String filename = args[0]; int w = Integer.parseInt(args[1]); int h = Integer.parseInt(args[2]); Picture source = new Picture(filename); Picture target = new Picture(w, h); for (int ti = 0; ti < w; ti++) { for (int tj = 0; tj < h; tj++) { int si = ti * source.width() / w; int sj = tj * source.height() / h; Color color = source.get(si, sj); target.set(ti, tj, color); } } source.show(); target.show(); } }

29. Image Processing: Scaling Filter Scaling filter. Creates two Picture objects and two windows. % java Scale 400 200 mandrill.jpg mandrill.jpg

30. More Image Processing Effects(See book and book-site) RGB color separation swirl filter wave filter glass filter Sobel edge detection

31. Objects and Reference Types: Important Issues • See textbook: pp. 352-357

32. Important Issues about Objects and Reference Types Study textbook: pages 352-357 References are not objects! Aliases Comparing objects. Don’t use ==. Use .equals() Orphan objects, Memory Management

33. OOP Context for Color Possible memory representation. Object reference is analogous to variable name. • We can manipulate the value that it holds. • We can pass it to (or return it from) a method. D0 D1 D2 D3 D4 D5 D6 D7 D8 255 0 255 0 0 0 105 105 105 B0 A0 gray magenta D6 D0 memory address("pointer")

34. OOP Context Reference. Variable that stores the name of a thing. Some consequences. • Assignment statements copy references (not objects). • The == operator tests if two references refer to same object. • Pass copies of references (not objects) to functions. • efficient since no copying of data • function can change the object Thing Name Web page www.princeton.edu Bank account 45-234-23310076 Word of TOY memory 1C Byte of computer memory 00FACADE Home 35 Olden Street

35. References René Magritte. "This is not a pipe." Java. This is not a color. (Where “this” means c or sienna below.) OOP. Natural vehicle for studying abstract models of the real world. Color sienna =new Color(160,82,45); Color c = sienna.darker();

36. Reference variables and equality Remember arrays? Can’t use == to see if two have the same content. Objects. Same thing! Never use == if you want to know if they have the same content. Use instance method equals(). All Java classes should provide a working version of equals() that takes a 2nd object as an argument. String s1 = new String("hello”); String s2 = new String("hello”); System.out.println( s1.equals(s2) ); true System.out.println( s1==s2 ); false System.out.println( s1==s1 ); true String s3 = s1; System.out.println( s1==s3 ); true

37. Some things can’t be changed: Color objects, for example Immutability. Can't change a Color object's value once created. Consequence. Don't need to worry about aliases. no relevant methods in Color API Color a =new Color(160,82,45); Color b = a; a =new Color(0,0,0); makes a point to a different Color, but does not change b

38. Java objects: Some are Immutable, Some are Not Immutability. Can't change a Color object's value once created. We can create a new color from an old one. We can change the Color object the reference points to. The String class is immutable too. Mutability. Can change a Picture object's value. Java has class StringBuffer that is like String but mutable. It has an append() method, for example. no relevant methods in Color API D0 Color red =new Color(255,0,0); pic.set(0,3, red); D4 D8 DC

39. Code Example public static void main (String[] args) { String s1 = "Defined in main."; changeString(s1); System.out.println("String obj is now: " + s1); // see? reference still points to original object StringBuffer sb1 = new StringBuffer("Defined in main."); changeStringBuffer(sb1); System.out.println("StringBuffer obj is now: " + sb1); // see? object pointed to has be changed! } public static void changeString(String s) { s = "Changed in method"; // change is only local } public static void changeStringBuffer(StringBuffer sb) { sb.append(" -- Changed in method"); }

40. Conclusion from Example Arguments that are object-references: • Code inside a method can change an argument passed to it. Only if that argument allows itself to be changed (i.e. is mutable). • Cannot make the object-reference point to a new object. In other words, inside the method: The thing “pointed to” can be changed. The “pointer” cannot be changed. This rule is same as for arrays!

41. OOP Summary Object. Holds a data type value; variable name refers to object. In Java, programs manipulate references to objects. • Exception: primitive types, e.g., boolean, int, double. • Reference types: String, Picture, Color, arrays, everything else. Bottom line. We wrote programs that manipulate colors, pictures,and strings. Next. We'll write programs that manipulate our own abstractions.

42. Extra Slides

43. Spectrum import java.awt.Color; publicclass Spectrum { publicstaticvoid main(String[] args) { int N = Integer.parseInt(args[0]); Picture pic =new Picture(N, N); for(int i =0; i < N; i++) { for(int j =0; j < N; j++) { int r =(i *256)/ N; int g =128; int b =(j *256)/ N; Color c =new Color(r, g, b); pic.set(i, j, c); } } pic.show(); pic.save("spectrum.png"); } }

44. Java Function Involving Colors We can write Java functions that manipulate colors. • Ex: return convex combination of two colors. import java.awt.Color; publicclass ColorCombiner { publicstatic Color combine(Color c1, Color c2,double beta) { int r =(int)(beta*c1.getRed()+(1-beta)* c2.getRed()); int g =(int)(beta*c1.getGreen()+(1-beta)* c2.getGreen()); int b =(int)(beta*c1.getBlue()+(1-beta)* c2.getBlue()); returnnew Color(r, g, b); } publicstaticvoid main(String args[]) { Color c1 =new Color(127,255,255); Color c2 =new Color(0,127,127); Color c3 = combine(c1, c2,0.15); System.out.println("c3 = "+ c3); } } % java ColorCombiner c3 = java.awt.Color[r=19,g=146,b=146]

45. Color Separation import java.awt.Color; publicclass ColorSeparation { publicstaticvoid main(String args[]) { Picture pic =new Picture(args[0]); int width = pic.width(); int height = pic.height(); Picture R =new Picture(width, height); Picture G =new Picture(width, height); Picture B =new Picture(width, height); for(int i =0; i < width; i++) { for(int j =0; j < height; j++) { Color c = pic.get(i, j); int r = c.getRed(); int g = c.getGreen(); int b = c.getBlue(); R.set(i, j,new Color(r,0,0)); G.set(i, j,new Color(0, g,0)); B.set(i, j,new Color(0,0, b)); } } R.show(); G.show(); B.show(); } }

46. Color Separation ColorSeparation.java. Creates three Picture objects and windows.

47. Image Processing: Swirl Filter Swirl.java. Creates two Picture objects and windows.

48. Image Processing: Swirl Filter Goal. Create swirl special effect by setting color of output pixel (i, j) to color of some other input pixel (ii, jj). double i0 =0.5* width; double j0 =0.5* height; for(int i =0; i < width; i++) { for(int j =0; j < height; j++) { double di = i - i0; double dj = j - j0; double r = Math.sqrt(di*di +dj*dj); double a = Math.PI /256* r; int ii =(int)(-di*Math.cos(a)+ dj*Math.sin(a)+ i0); int jj =(int)( di*Math.sin(a)+ dj*Math.cos(a)+ j0); if(ii >=0&& ii < width && jj >=0&& jj < height) pic2.set(i, j, pic1.get(ii, jj)); } }

49. Memory Management Value types. • Allocate memory when variable is declared. • Can reclaim memory when variable goes out of scope. Reference types. • Allocate memory when object is created with new. • Can reclaim memory when last reference goes out of scope. • Significantly more challenging if several references to same object. Garbage collector. System automatically reclaims memory;programmer relieved of tedious and error-prone activity.