Download Using Data Types

3.1 Data Types: Using Objects
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.
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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
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, …
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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!
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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”?
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“Non-primitive” types
Reference type.
In Java, not a simple, basic data type.
Arrays are one kind of reference type.
Object types are another.
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Examples: String, In
“Class” means data type that’s an object
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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.
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Example: int[] a = new int[10];
a.length
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Problem:
b.length
int[] b;
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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
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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
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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()
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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
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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();
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Ways to think about this:
Send a message to an object.
Tell an object to do something.
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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
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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.
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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, and
the name of the method.
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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
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Typical String Processing Code
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Image Processing
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
255
0
0
0
255
0
0
0
255
255
255
255
0
0
0
255
0
255
105
105
105
Color
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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
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Albers Squares
Josef Albers. Revolutionized the way people think about color.
Homage to the Square by Josef Albers (1949-1975)
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Albers Squares
Josef Albers. Revolutionized the way people think about color.
% java AlbersSquares 9 90 166 100 100 100
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Using Colors in Java
import java.awt.Color;
to access Color library
public class AlbersSquares {
public static void 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 =
int g2 =
int b2 =
Color c2
Integer.parseInt(args[3]);
Integer.parseInt(args[4]);
Integer.parseInt(args[5]);
= 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);
first color
second color
first square
second square
}
}
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Monochrome Luminance
Monochrome luminance. Effective brightness of a color.
NTSC formula. Y = 0.299r + 0.587g + 0.114b.
import java.awt.Color;
public class Luminance {
public static double lum(Color c) {
int r = c.getRed();
int g = c.getGreen();
int b = c.getBlue();
return .299*r + .587*g + .114*b;
}
}
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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
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public static boolean compatible(Color a, Color b) {
return Math.abs(lum(a) - lum(b)) >= 128.0;
}
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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.
public static 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
Bottom line. We are writing programs that manipulate color.
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Picture Data Type
(0, 0)
j
Raster graphics. Basis for image processing.
Set of values. 2D array of Color objects (pixels).
i
API.
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Image Processing: Grayscale Filter
Goal. Convert color image to grayscale according to luminance formula.
import java.awt.Color;
public class Grayscale {
public static void 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();
}
}
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Image Processing: Grayscale Filter
Goal. Convert color image to grayscale according to luminance formula.
mandrill.jpg
% java Grayscale mandrill.jpg
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
Horton got this far before Exam 2 in
Spring 2008.
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For exam 2, no need to look at slides
after this.
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Slides after this point will be covered
after Exam 2.
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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.
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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.
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j  hs / ht
j
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i  ws / wt
source image
(ws-by-hs)
i
target image
(wt-by-ht)
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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();
}
}
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Image Processing: Scaling Filter
Scaling filter. Creates two Picture objects and two windows.
mandrill.jpg
% java Scale 400 200 mandrill.jpg
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More Image Processing Effects
(See book and book-site)
RGB color separation
swirl filter
wave filter
glass filter
Sobel edge detection
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Objects and Reference Types: Important Issues
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See textbook: pp. 352-357
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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
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OOP Context for Color
Possible memory representation.
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")
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.
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OOP Context
Reference. Variable that stores the name of a thing.
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
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
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References
René Magritte. "This is not a pipe."
Java. This is not a color. (Where “this” means c or sienna below.)
Color sienna = new Color(160, 82,
Color c = sienna.darker();
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OOP. Natural vehicle for studying abstract models of the real world.
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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
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Some things can’t be changed: Color objects, for example
Immutability. Can't change a Color object's value once created.
no relevant methods in Color API
Consequence. Don't need to worry about aliases.
Color a = new Color(160, 82,
Color b = a;
a = new Color(0, 0, 0);
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makes a point to a different Color, but does not change b
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Java objects: Some are Immutable, Some are Not
Immutability. Can't change a Color object's value once created.
no relevant methods in Color API
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.
Color red = new Color(255, 0,
pic.set(0, 3, red);
0);
D0
D4
D8
Java has class StringBuffer that is
like String but mutable.
It has an append() method, for example.
DC
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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");
}
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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!
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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.
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Bottom line. We wrote programs that manipulate colors, pictures,
and strings.
Next. We'll write programs that manipulate our own abstractions.
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Extra Slides
Spectrum
import java.awt.Color;
public class Spectrum {
public static void 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");
}
}
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Java Function Involving Colors
We can write Java functions that manipulate colors.
Ex: return convex combination of two colors.
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import java.awt.Color;
public class ColorCombiner {
public static Color combine(Color c1, Color c2,
int r = (int) (beta*c1.getRed()
+ (1-beta)
int g = (int) (beta*c1.getGreen() + (1-beta)
int b = (int) (beta*c1.getBlue() + (1-beta)
return new Color(r, g, b);
}
public static void 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
double beta) {
* c2.getRed());
* c2.getGreen());
* c2.getBlue());
c3 = java.awt.Color[r=19,g=146,b=146]
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Color Separation
import java.awt.Color;
public class ColorSeparation {
public static void 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();
}
}
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Color Separation
ColorSeparation.java. Creates three Picture objects and windows.
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Image Processing: Swirl Filter
Swirl.java. Creates two Picture objects and windows.
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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));
}
}
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Memory Management
Value types.
Allocate memory when variable is declared.
Can reclaim memory when variable goes out of scope.
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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.
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Garbage collector. System automatically reclaims memory;
programmer relieved of tedious and error-prone activity.
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