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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
PA R T I : P R O G R A M M I N G I N J AVA
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
R O B E R T S E D G E W I C K
K E V I N W A Y N E
Computer Science
C
omputer
S
cience
An Interdisciplinary Approach
8. Abstract Data Types
Section 3.
8. Abstract Data Types
- Overview
- Color
- Image processing
- String processing
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
PA R T I : P R O G R A M M I N G I N J AVA
C S. 8. 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 ).
Best practice: Use abstract data types (representation is hidden from the client ).
Impact: Clients 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
Strings
5
public class String
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_?_
Operations (API)
Java's String ADT allows us to write Java programs that manipulate strings.
The exact representation is hidden (it could change and our programs would still work).
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
6
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.
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.
A. A data type whose representation is hidden from the client.
Color client example: Albers squares
13
Goal. Write a Java program to generate Albers
% java AlbersSquares 0 64 128 105 105 105
% java AlbersSquares 251 112 34 177 153 71
% java AlbersSquares 28 183 122 15 117 123
14
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 square
draw second square
% java AlbersSquares 0 64 128 105 105 105
Computing with color: monochrome luminance
Def. The monochrome luminance of a color quantifies its effective brightness.
examples
red intensity 255 0 0 0 255 0 119 105
green intensity
blue intensity 0 0 255 0 255 128 27 105
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: compatibility
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
255 179 255 255 203
52 24 52 203 255
Computing with color: grayscale
17
Goal. Convert 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;
examples
red intensity 255 0 0 0 255 0 119 105
green intensity
blue intensity 0 0 255 0 255 128 27 105
color
luminance
grayscale
method for Luminance library
OOP context for color
18
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
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.
import java.awt.Color;
public class Grayscale
public static void main(String[] args)
Picture pic = new Picture(args[0]);
for (int col = 0; col < pic.width(); col++)
for (int row = 0; row < pic.height(); row++)
Color color = pic.get(col, row);
Color gray = Luminance.toGray(color);
pic.set(col, row, gray);
pic.show();
25
Picture client example: Grayscale filter
create a new picture
fill in each pixel
% java Grayscale mandrill.jpg
Q. What is the effect of the following code (easy question)?
Pop quiz 1a on image processing
26
Picture pic = new Picture(args[0]);
for (int col = 0; col < pic.width(); col++)
for (int row = 0; row < pic.height(); row++)
pic.set(col, row, pic.get(col, row));
pic.show();
Q. What is the effect of the following code (easy question)?
Pop quiz 1a on image processing
A. None. Just shows the picture.
Picture pic = new Picture(args[0]);
for (int col = 0; col < pic.width(); col++)
for (int row = 0; row < pic.height(); row++)
pic.set(col, row, pic.get(col, row));
pic.show();
Pop quiz 1b on image processing
Q. What is the effect of the following code (not-so-easy question)?
Picture pic = new Picture(args[0]);
for (int col = 0; col < pic.width(); col++)
for (int row = 0; row < pic.height(); row++)
pic.set(col, pic.height()-row-1, pic.get(col, row));
pic.show();
Pop quiz 1b on image processing
Q. What is the effect of the following code (not-so-easy question)?
29
A. Tries to turn image upside down, but fails.
An instructive bug!.
Picture pic = new Picture(args[0]);
for (int col = 0; col < pic.width(); col++)
for (int row = 0; row < pic.height(); row++)
pic.set(col, pic.height()-row-1, pic.get(col, row));
pic.show();
Pop quiz 1c on image processing
Q. What is the effect of the following code?
30
Picture source = new Picture(args[0]);
int width = source.width();
int height = source.height();
Picture target = new Picture(width, height);
for (int col = 0; col < width; col++)
for (int row = 0; row < height; row++)
target.set(col, height-row-1, source.get(col, row));
target.show();
Pop quiz 1c on image processing
Q. What is the effect of the following code?
A. Makes an upside down copy of the image.
Picture source = new Picture(args[0]);
int width = source.width();
int height = source.height();
Picture target = new Picture(width, height);
for (int col = 0; col < width; col++)
for (int row = 0; row < height; row++)
target.set(col, height-row-1, source.get(col, row));
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
C O M P U T E R S C I E N C E
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PA R T I : P R O G R A M M I N G I N J AVA
C S. 8. C. A D T s. I m a g e s
- Abstract Data Types
- Overview
- Color
- Image processing
- String processing
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
PA R T I : P R O G R A M M I N G I N J AVA
C S. 8. D. A D T s. S t r i n g s
String ADT
A String is a sequence of Unicode characters.
public class String
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) i th 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 first occurrence of p
int indexOf(String p, int i) index of first 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(Object t)
is this string's value the same as t 's?
Operations (API)
Java's ADT allows us to
write Java programs
that manipulate strings.
defined in terms of its ADT values (typical)
Programming with strings: typical examples
public static boolean isPalindrome(String s)
int N = s.length();
for (int i = 0; i < N/2; i++)
if (s.charAt(i) != s.charAt(N-1-i))
return false;
return true;
Is the string a palindrome?
String query = args[0];
while (!StdIn.isEmpty())
String s = StdIn.readLine();
if (s.contains(query))
StdOut.println(s);
Find lines containing a specified string in StdIn
while (!StdIn.isEmpty())
String s = StdIn.readString();
if (s.startsWith("http://") && s.endsWith(".edu"))
StdOut.println(s);
*Search for .edu hyperlinks in the text file on StdIn
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
A T A G A T G C A T A G C G C A T A G C T A G A T G T G C T A G C
String client example: gene finding
41
Pre-genomics era. Sequence a human genome.
Post-genomics era. Analyze the data and understand structure.
Goal. Write a Java program to find genes in a given genome.
Genomics. Represent genome as a string over A C T G alphabet.
Gene. A substring of genome that represents a functional unit.
Made of codons (three A C T G nucleotides ).
- Preceded by ATG ( start codon).
Succeeded by TAG, TAA, or TGA ( stop codon).
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
A T A G A T G C A T A G C G C A T A G C T A G A T G T G C T A G C
start gene stop start
stop gene
42
String client warmup: Identifying a potential gene
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
A T G C A T A G C G C A T A G
start gene stop
Goal. Write a Java program to determine
whether a given string is a potential gene.
public class Gene
public static boolean isPotentialGene(String dna)
if (dna.length() % 3 != 0) return false;
if (!dna.startsWith("ATG")) return false;
for (int i = 0; i < dna.length() - 3; i+=3)
String codon = dna.substring(i, i+3);
if (codon.equals("TAA")) return false;
if (codon.equals("TAG")) return false;
if (codon.equals("TGA")) return false;
if (dna.endsWith("TAA")) return true;
if (dna.endsWith("TAG")) return true;
if (dna.endsWith("TGA")) return true;
return false;
public static void main(String[] args)
StdOut.println(isPotentialGene(args[0]));
% java Gene ATGCATAGCGCATAG
true
% java Gene ATGCGCTGCGTCTGTACTAG
false
% java Gene ATGCCGTGACGTCTGTACTAG
false
String client exercise: Gene finding
Algorithm. Scan left-to-right through dna.
If start codon ATG found, set beg to index i.
- If stop codon found and substring length is a multiple of 3, print gene and reset beg to -1.
i
codon
beg output remainder of input string
start stop
0 -1 A T A G A T G C A T A G C G C A T A G C T A G A T G T G C T A G C
1 TAG -1 T A G A T G C A T A G C G C A T A G C T A G A T G T G C T A G C
4 ATG 4 A T G C A T A G C G C A T A G C T A G A T G T G C T A G C
9 TAG 4 T A G C G C A T A G C T A G A T G T G C T A G C
16 TAG 4 C A T A G C G C A T A G C T A G A T G T G C T A G C
20 TAG -1 T A G A T G T G C T A G C
23 ATG 23 A T G T G C T A G C
29 TAG 23 T G C T A G C
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
A T A G A T G C A T A G C G C A T A G C T A G A T G T G C T A G C
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
A T A G A T G C A T A G C G C A T A G C T A G A T G T G C T A G C
start gene stop start
gene stop
Implementation. Entertaining programming exercise!
Goal. Write a Java program to find genes in a given genome.
OOP context for strings
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.
String genome = "aacaagtttacaagc";
String s = genome.substring(1, 5);
String t = genome.substring(9, 13);
