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9. Abstract Data Types
Section 3.
C O M P U T E R S C I E N C E
S E D G E W I C K / W A Y N E
9. Abstract Data Types
• Overview
• Color
• Image processing
• String processing
C S. 9. A. A D T s. O v e r v i e w
Abstract data types
Primitive types
- values immediately map to
machine representations
operations immediately map to
machine instructions.
A data type is a set of values and a set of operations on those values.
An abstract data type is a data type whose representation is hidden from the client.
We want to write programs that process
other types of data.
Colors, pictures, strings,
- Complex numbers, vectors, matrices,
...
Object-oriented programming (OOP)
Object-oriented programming (OOP).
Create your own data types.
- Use them in your programs (manipulate objects ).
An abstract data type is a data type whose representation is hidden from the client.
Impact: We can use ADTs without knowing implementation details.
- This lecture: how to write client programs for several useful ADTs
Next lecture: how to implement your own ADTs
data type set of values examples of operations
Color three 8-bit integers get red component, brighten
Picture 2D array of colors get/set color of pixel (i, j)
String sequence of characters length, substring, compare
An object holds a data type value.
Variable names refer to objects.
Examples (stay tuned for details)
C A T A G C G C
Sound
5
We have already been using ADTs!
public class StdAudio
void play(double[] a)
play the given sound wave
void save(String file, double[] a) save to a .wav file
double[] read(String file) read from a .wav file
Values: Array of doubles.
Operations: specified in API.
Representation: Hidden from user (.wav and other formats needed by devices).
Sound ADT
Strings
6
publicpublic classclass StringString
String(String s)
create a string with the same value
int length()
string length
char charAt(int i) ith character
String substring(int i, int j) ith through (j-1)st characters
boolean contains(String sub) does string contain sub_?_
boolean startsWith(String pre) does string start with pre_?_
boolean endsWith(String post) does string end with post_?_
int indexOf(String p) index of 1st occurrence of p
int indexOf(String p, int i)
index of 1st occurrence of p after i
String concat(String t)
this string with t appended
int compareTo(String t) string comparison
String replaceAll(String a,String b) result of changing a s to b s
String[] split(String delim) strings between occurrences of delim
boolean equals(String t) is this string's value the same as t 's
Operations (API)
Java's String ADT allows us to write Java
programs that manipulate strings.
A String is a sequence of Unicode characters.
defined in terms of its ADT values (typical)
stay tuned for more complete API later in this lecture
We have already been using ADTs!
Using a data type: constructors and methods
To use a data type, you need to know:
- Its name (capitalized, in Java).
How to construct new objects.
- How to apply operations to a given object.
To construct a new object
- Use the keyword new to invoke a constructor.
Use data type name to specify type of object.
To apply an operation (invoke a method)
Use object name to specify which object.
- Use the dot operator to indicate that an
operation is to be applied.
- Use a method name to specify which operation.
String s;
s = new String ("Hello, World);
StdOut.println( s.substring(0, 5) );
new Building()
Pop quiz on ADTs
Q. What is an abstract data type?
Q. What is a data type?
A. A set of values and a set of operations on those values.
13
Color client example: Albers squares
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 = 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);
create first color
create second color
draw first pair
of squares
draw second pair
of squares
% java AlbersSquares 0 64 128 105 105 105
Computing with color: monochrome luminance
14
Def. The monochrome luminance of a color quantifies its effective brightness.
examplesexamplesexamplesexamplesexamplesexamplesexamplesexamples
red intensity
green intensity
blue intensity
color
luminance
Applications (next)
Choose colors for displayed text.
- Convert colors to grayscale.
NTSC standard formula for luminance: 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 .299r + .587g + .114*b;
public static void main(String[] args)
int r = Integer.parseInt(args[0]);
int g = Integer.parseInt(args[1]);
int b = Integer.parseInt(args[2]);
Color c = new Color(r, g, b);
StdOut.println(Math.round(lum(c)));
% java Luminance 0 64 128
52
Computing with color: compatability
Q. Which font colors will be most readable with which background colors on a display?
Rule of thumb. Absolute value of difference in luminosity should be > 128.
public static boolean compatible(Color a, Color b)
return Math.abs(lum(a) - lum(b)) > 128.0;
255 76 179 24
76 255 255 52
179 255 255 203
24 52 203 255
Computing with color: grayscale
Goal. Convert a colors to grayscale values.
Fact. When all three R, G, and B values are the same,
resulting color is on grayscale from 0 (black) to 255 (white).
Q. What value for a given color?
A. Its luminance!
public static Color toGray(Color c)
int y = (int) Math.round(lum(c));
Color gray = new Color(y, y, y);
return gray;
examplesexamplesexamplesexamplesexamplesexamplesexamplesexamples
red intensity
green intensity
blue intensity
color
luminance
grayscale
method for Luminance library
OOP context for color
17
Q. How does Java represent color? Three int values? Packed into one int value?
A. We don't know. The representation is hidden. It is an abstract data type.
An object reference is analogous to a variable name.
- It is not the value but it refers to the value.
We can manipulate the value in the object it refers to.
- We can pass it to (or return it from) a method.
Possible memory representation of red = new Color(255, 0, 0)
and gray = new Color(105, 105, 105);
105 105 105 x y 255 0 0
y red
memory
address
gray x
We also use object references to
invoke methods (with the. operator)
References and abstraction
18
René Magritte. This is not a pipe.
Java. These are not colors.
public static Color toGray(Color c)
int y = (int) Math.round(lum(c));
Color gray = new Color(y, y, y);
return gray;
Object-oriented programming. A natural vehicle for studying abstract models of the real world.
It is a picture of a painting of a pipe.
"This is not a pipe."
Surrealist computer scientist:
Neither is this.
% java RandomSeq 10000 | java Average
Yes it is! He's referring to the physical object he's holding.
Joke would be better if he were holding a picture of a pipe.
This is not a pipe.
C O M P U T E R S C I E N C E
S E D G E W I C K / W A Y N E
- Abstract Data Types
- Overview
- Color
- Image processing
- String processing
C S. 9. C. A D T s. I m a g e s
Pop quiz 1b on image processing
Q. What is the effect of the following code (not-so-easy question)?
25
Picture pic = new Picture(args[0]);
for (int i = 0; i < pic.width(); i++)
for (int j = 0; j < pic.height(); j++)
pic.set(i, pic.height()-j-1, pic.get(i, j));
pic.show();
Pop quiz 1c on image processing
Q. What is the effect of the following code?
26
Picture source = new Picture(args[0]);
int width = source.width();
int height = source.height();
Picture target = new Picture(width, height);
for (int i = 0; i < width; i++)
for (int j = 0; j < height; j++)
target.set(i, height-j-1, source.get(i, j));
target.show();
Picture client example: Scaling filter
Goal. Write a Java program to scale an image (arbitrarily and independently on x and y ).
Source: mandrill.jpg
300x
% java Scale mandrill.jpg 500 500
500x
% java Scale mandrill.jpg 600 200
600x
% java Scale mandrill.jpg 200 400
200x
99x
% java Scale mandrill.jpg 99 99
Picture client example: Scaling filter
Goal. Write a Java program to scale an image (arbitrarily and independently on x and y ).
Ex. Downscaling by halving.
Shrink in half by deleting
alternate rows and columns.
Ex. Upscaling by doubling.
Double in size by replacing
each pixel with four copies.
Picture client example: Scaling filter
29
Goal. Write a Java program to scale an image (arbitrarily and independently on x and y ).
A uniform strategy to scale from ws-by-hs to wt-by-ht.
Scale column index by ws/wt.
- Scale row index by hs/ht.
ws
hs
wt
ht
row tj
column ti
Approach. Arrange computation to compute exactly one value for each target pixel.
row tj x hs/ht
column ti x ws/wt
30
Picture client example: 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);
target.show();
% java Scale mandrill.jpg 300 900
More image-processing effects
wave filter glass filter Sobel edge detection
RGB color separation
swirl filter
C O M P U T E R S C I E N C E
S E D G E W I C K / W A Y N E
- Abstract Data Types
- Overview
- Color
- Image processing
- String processing
C S. 9. D. A D T s. S t r i n g s
37
String client example: Gene finding
public class GeneFind
public static void main(String[] args)
String start = args[0];
String stop = args[1];
String genome = StdIn.readAll();
int beg = -1;
for (int i = 0; i < genome.length() - 2; i++)
String codon = genome.substring(i, i+3);
if (codon.equals(start)) beg = i;
if (codon.equals(stop) && beg != -1 && beg+3 < i)
String gene = genome.substring(beg+3, i);
if (gene.length() % 3 == 0)
StdOut.println(gene);
beg = -1;
% more genomeTiny.txt
ATAGATGCATAGCGCATAGCTAGATGTGCTAGC
% java GeneFind ATG TAG < genomeTiny.txt
CATAGCGCA
TGC
Fixes bug in Program 3.1.
Pop quiz 1: What's the bug?
Pop quiz 2: Give input that causes
Program 3.1.8 to crash
OOP context for strings
38
Possible memory representation of
x 15 x+9 4 x+1 4
x
a a c a a g t t t a c a a g c
genome
memory
address
length
t s
Implications
- s and t are different strings that share the same value "acaa".
(s == t) is false (because it compares addresses).
- (s.equals(t)) is true (because it compares character sequences).
Java String interface is more complicated than the API (and not really an ADT).
String genome = "aacaagtttacaagc";
String s = genome.substring(1, 5);
String t = genome.substring(9, 13);
Object-oriented programming: summary
In Java, programs manipulate references to objects.
- String, Picture, Color, arrays, (and everything else) are reference types.
Exceptions: boolean, int, double and other primitive types.
- OOP purist: Languages should not have separate primitive types.
Practical programmer: Primitive types provide needed efficiency.
This lecture: You can write programs to manipulate sounds, colors, pictures, and strings.
Next lecture: You can define your own abstractions and write programs that manipulate them.
Object-oriented programming.
Create your own data types (sets of values and ops on them).
- Use them in your programs (manipulate objects ).
An object holds a data type value.
Variable names refer to objects.
T A G A T G T G C T A G C
C O M P U T E R S C I E N C E
S E D G E W I C K / W A Y N E
h t t p : / / i n t r o c s. c s. p r i n c e t o n. e d u
9. Abstract Data Types
Section 3.
