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Introduction to
Computational Modeling of Social Systems
Java Primer III
Prof. Lars-Erik Cederman
Center for Comparative and International Studies (CIS)
Seilergraben 49, Room G.2, [email protected]
Nils Weidmann, CIS Room E.3 [email protected]
http://www.icr.ethz.ch/teaching/compmodels
Lecture, November 23, 2004
Today’s agenda
• Packages
• Arrays
– Statistics example
• Collections
– SimpleList example
2
Creating a package
• Membership of a class in a package is defined by putting a
package command at the top of the source code:
package carPackage;
public class Vehicle {
(...)
public void drive() {
(...)
}
}
• Package hierarchy:
package topPackage.subPackage1.subPackage2;
3
Using packages
• By providing the fully-qualified package name:
public class VehicleTest {
carPackage.Vehicle v;
public void driveTest() {
v.drive();
}
}
• By making the packaged classes known to the compiler:
import carPackage.Vehicle;
import carPackage.*;
// makes class „Vehicle“ visible
// makes all classes in package
// „carPackage“ visible
4
Java‘s built-in packages
• Java comes with a lot of ready-to-use
classes for different purposes
• These classes are organized in
packages
• For each package, a documentation
is available
(see java.sun.com/j2se/1.4.2/api)
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Example: java.lang
• Wrappers for the primitive data types, example:
java.lang.Double
• „mother of all objects“: class Object
• Standard input and output streams: class System.
Remember:
public class Model {
public static void main(String[] args) {
System.out.println((int)sedan.getPersonMile() + " " +
(int)roadster.getPersonMile() + " " +
(int)suv.getPersonMile());
}
}
6
Managing many agents
•
•
•
•
Arrays and collections help the user handle many objects
Both data types are objects
Arrays are simpler and less powerful
Collections are more complex but also more flexible
7
Arrays in Java
8
• An array holds a vector or matrix of data
• Arrays must be dynamically created:
int a[] = new int[10];
Car parking[][] = new Car[n][m];
• May also declare default values:
int a[]={3,8,2,5,7,7,1,4,9,2}
3
8
a[0]=3
2 5
7
7 1
a[4]=7
4
9
2
a[9]=2
Multidimensional arrays
9
int num = new int[3][3];
for(int i=0;i<3;i++)
for(int j=0;j<3;j++)
num[i][j]=i*3+j;
Or:
int num[][]={{0,1,2},
{3,4,5},
{6,7,8}};
num[2][1]=7
0
0 0
1
1
2
2
1 3
4
5
2 6
7
8
Statistics: Code
10
public class Statistics {
public static void main(String[] args) {
int n = 10;
int maxValue = 100;
int x[] = new int[n];
int max = 0;
int sum = 0;
for (int i=0; i<n; i++) {
x[i] = (int)(Math.random() * maxValue);
System.out.print(x[i]+" ");
}
for (int i = 0; i<n; i++) {
sum = sum + x[i];
if (x[i] > max)
max = x[i];
}
System.out.println();
System.out.println("mean = " +
(double)sum/n + " max = " + max);
}
}
Output:
10 26 38 16 27 30 94 40 54 89
mean = 42.4 max = 94
Collections
• Java features collections
• Unlike arrays, collections are dynamic data structures holding objects*
that can change during run-time
• Collections can be of type ArrayList, LinkedList, Set etc. (old
type: Vector)
• Read more: chap. in Eckel book or
http://java.sun.com/docs/books/tutorial/collections/
*) Simple types have to be wrapped: Integer etc.
11
Collections: Data types
12
Collection
Set
List
ArrayList
LinkedList
Arrays and Linked Lists
13
Arrays: fast access
0
3
1
8
2
2
3
5
4
7
5
7
6
1
7
4
8
9
9
2
Linked Lists: flexibility
3
8
5
2
7
Lists in Java
14
LinkedList list = new LinkedList();
Iterator it =
list.iterator();
agent=(Agent)it.next();
list.add(a4);
a1
a2
a3
a4
Collections
15
// Basic operations
// Iterator
int size();
boolean hasNext();
boolean isEmpty();
Object next();
boolean contains(Object o);
void remove();
void add(Object o);
// ... ListIterator also:
void remove(Object o);
boolean hasPrevious();
Object is a
Iterator iterator();
Object previous;
generic root
// ... LinkedList also:
class
Object get(int i);
// Built-in algortihms:
void set(int i, Object o);
Collections.sort(List l);
// etc.
Collections.shuffle(List l);
// Bulk operations
Collections.reverse(List l);
boolean containsAll(Collection c);
addAll(Collection c);
removeAll(Collection c);
retainAll(Collection c);
Static methods, like Collections.shuffle()
clear();
and Math.random()belong to a class rather
than to an instance of a class; see Eckel!
Example: SimpleList
•
•
•
•
•
•
Create agents with different height and gender
Find average and max heights
Sort list
Divide it into girls and boys
Concatenate the two sublists
Scramble list
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SimpleList: Sample output
0 4.53 false
1 4.58 true
2 5.67 true
3 4.67 false
4 4.97 true
5 4.07 true
6 5.69 false
7 5.48 false
8 4.73 true
9 4.03 true
Average height: 4.84
Max height: 5.69
Sorted list: [4.03, 4.07, 4.53, 4.58, 4.67, 4.73, 4.97, 5.48, 5.67, 5.69]
Girls: [4.03, 4.07, 4.58, 4.73, 4.97, 5.67]
Boys: [4.53, 4.67, 5.48, 5.69]
Boys and girls: [4.53, 4.67, 5.48, 5.69, 4.03, 4.07, 4.58, 4.73, 4.97, 5.67]
Scrambled list: [4.67, 4.58, 4.03, 4.97, 4.53, 5.69, 5.48, 4.73, 4.07, 5.67]
17
SimpleList (Agent code)
18
package simplelist;
import java.text.*;
public class Agent{
int id;
boolean female;
double height;
public Agent(int i, boolean f, double h) {
id = i;
female = f;
height = h;
}
toString() method can
be defined for printing
all Java Objects,
public String toString() {
return new DecimalFormat("###.00").format(height);
}
}
SimpleList (Model code 1)
package simplelist;
import java.util.*;
import java.text.*;
import statements necessary
when invoking packages
that are not in the Java core
public class Model {
public static void main(String[] args) {
int n = 10;
boolean female;
double height;
// Creating list
ArrayList list = new ArrayList();
...
19
SimpleList (Model code 2)
// Initializing list
for (int i=0; i<n; i++) {
if (Math.random()<0.5) {
female = true;
height = 4.0 + Math.random()*2.0;
} else {
female = false;
height = 4.5 + Math.random()*2.5;
}
Agent a = new Agent(i,female,height);
list.add(a);
System.out.println(i+" "+
(new DecimalFormat("###.00").format(height))+
" "+female);
}
...
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SimpleList (Model code 3)
// Calculating average height
double s = 0.0;
Iterator it = list.iterator();
while (it.hasNext()) {
Agent a = (Agent)it.next();
Warning: Casting
s = s + a.height;
necessary
}
System.out.println(„Average height: "+
(new DecimalFormat("###.00").format(s/(double)n)));
// Calculating max height
double max = 0.0;
for (int i=0; i<list.size(); i++) {
Warning:
Agent a = (Agent)list.get(i);
get() and set() can be
slow in a LinkedList
if (a.height > max)
max = a.height;
}
System.out.println("Max height: "+
(new DecimalFormat("###.00").format(max)));
21
SimpleList (Model code 4)
// Sorting list
ArrayList temp = new ArrayList();
while (!list.isEmpty()) {
Agent shortest = (Agent)list.get(0);
for (int i=0; i<list.size(); i++) {
Agent a = (Agent)list.get(i);
if (a.height < shortest.height)
shortest = a;
Collections are
}
printed as lists
list.remove(shortest);
with the elements
temp.add(shortest);
defined by
}
toString()
list = temp;
System.out.println("Sorted list: "+list);
...
Note: There is a pre-defined, efficient
routine: Collections.sort()
22
SimpleList (Model code 5)
// Creating list of girls
ArrayList girls = new ArrayList(list);
it = girls.iterator();
while (it.hasNext()) {
Agent a = (Agent)it.next();
if (!a.female)
it.remove();
}
System.out.println("Girls: "+girls);
// Creating list of boys
ArrayList boys = new ArrayList();
it = list.iterator();
while (it.hasNext()) {
Agent a = (Agent)it.next();
if (!a.female)
boys.add(a);
}
System.out.println("Boys: "+boys);
Warning: Be careful
when removing objects
while iterating: remove()
can only be called
once per next()
23
SimpleList (Model code 6)
// Concatenating the two lists
ArrayList boysAndGirls = new ArrayList(boys);
boysAndGirls.addAll(girls);
System.out.println("Boys and girls: "+boysAndGirls);
// Shuffling the original list
Collections.shuffle(list);
System.out.println("Scrambled list: "+list);
24