Download Computer Science A, 1

Datalogi A 16: 21/11
Java
Play sounds in applications
Compiling to jar-files
Compiling to exe-files
Exceptions
Sorting, reading from a file, writing to a file
A short overview of java
Sound
<uses new version of JCanvas>
import javax.sound.sampled.Clip;
public class PlaySound{
public static void main(String args[]){
Clip crash =
JCanvas.loadClip("crash.au");
JCanvas.playClip(crash);
JCanvas.sleep(4000);
}
}
Sound
playClip(Clip clip) starts the sound, and
returns
loadClip(String filename)
format: .au, .wav, .aiff
stopClip(Clip clip)
loopClip(Clip clip,int n)
Jar files
Compress and collect several files in an archive
(zip like)
>jar cf HelloWorld.jar HelloWorld.class
Run it:
>java –cp HelloWorld.jar HelloWorld
cp: classpath: HelloWorld.jar,
Main class: HelloWorld
Access to resources
BufferedImage image
= JCanvas.loadImage(”pict.jpg”);
Clip sound
= JCanvas.loadClip(”sound.wav”);
Find it in local directory – same as the class
file.
More robust approach:
Access to resources
Access from a URL:
BufferedImage image
= JCanvas.loadImage(
PlaySound.class.getResource(”pict.jpg”));
Clip sound
= JCanvas.loadClip(
PlaySound.class.getResource(”sound.wav”));
Find the main class (PlaySound), from that class
create a URL of a ressource local to that class.
Jar files with resources
Put class files an resources in a jar file:
> jar cf PlaySound.jar *.class *.au
> java –cp PlaySound.jar PlaySound
Works if you access sounds using
”getResource”.
JSmooth
Java program launcher:
JSmooth: takes a .jar file, an .ico file and
generates an exe file.
The exe file locates a java runtime
environment on the machine and run the
jar-file
Running JSmooth
Skeleton: Windowed wrapper
Executable:
Executable Binary: full name of exe file
Executable Icon: full name of icon file
Current Directory: full name of directory
Application
Main Class: main class
Embedded jar: yes, jar file
System: save as
Project: compile
Jar to exe file
PlaySound.exe can be played on other
computers with jre (java runtime
environment)
Java: reading from a file
How to open a file for reading:
Scanner in=null;
try{
in=new Scanner(new File("text.txt"));
}catch(IOException e){
System.out.println("Cannot open file text.txt");
System.exit(0);
}
Scanner can now be used as with keyboard
input
Java: exceptions
try{
//try block
}catch(Exception e){
// handle exceptions from try-block
}
Examples of exceptions: null pointer dereference, class cast, array index,
Arithmetic exception (division by zero).
Read lines from a file
Scanner in=null;
try{
in=new Scanner(new File("text.txt"));
}catch(IOException e){
System.out.println("Cannot open file text.txt");
System.exit(0);
}
ArrayList<String> data=new ArrayList<String>();
while(in.hasNextLine()){
data.add(in.nextLine());
}
in.close();
Write to a file
import java.io.*;
…
PrintWriter out= null;
try{
out=new PrintWriter(new FileWriter("text1.txt"));
}catch(IOException e){
System.out.println("Cannot write to file text1.txt");
System.exit(0);
}
for(String s:data)
out.println(s);
out.close();
Java I/O
import java.io.*;
class File, class PrintWriter
File: make directory listings:
boolean isDirectory()
Long length
String[] list()
is the file a directory
size of a file(bytes)
(list of files in a dir.)
Sorting
The easy principle:
Find the smallest and place at index 0,
Find the smallest of the remaining and place
at index 1,…
Sorting
Sorting algorithm:
int[] data=new int[10];
for(int i=0;i<data.length;i++)
data[i]=random(1000);
for(int i=0;i<data.length;i++){
int j=indexSmallest(data,i);
swap(data,i,j);
}
Index of smallest in array
static int indexSmallest(int[] data,int from){
int index=from;
for(int i=from+1;i<data.length;i++)
if(data[i]<data[index])index=i;
return index;
}
static void swap(int[] data,int i1, int i2){
int h=data[i1];
data[i1]=data[i2];
data[i2]=h;
}
Timing the sorting algorithm
size
size
size
size
size
size
size
size
size
size
1000 time 0ms
2000 time 20ms
3000 time 20ms
4000 time 40ms
5000 time 70ms
6000 time 90ms
7000 time 130ms
8000 time 161ms
9000 time 200ms
10000 time 260ms
Java: overview
Variables:
a type and a name
Simple types:
int, float, double, boolean, String, char
Objects:
a class name
Arrays:
a type + ”[]”
Java: variables
Local variable:
void method(){
int i =64;
while(i>0){
System.out.println(i);
i=i/2;
}
}
Java: parameters
Like local variables – but initialised at the call
int sqare(int x){return x*x;}
You may change the value of a parameter in
the method – but it will not change the
argument in the call.
Java: fields in objects
class point{
int x,y;
Point(int x1,int y1){
x=x1; y=y1
}
}
public class Application{
public static void main(String args[]){
Point p=new Point(2,3);
}}
Java: static fields
One instance per class. You may use them from static
methods.
public class Program{
static int data[]=new int[10];
public static void main(String args[]){
data[0]=1;
….
}
}
Java: statements
ifif(x<0)x=-x;
While
while(x>1){y=y*2;x--;}
forfor(int i=0;i<10;i++)..
AssignmentsMethod calls- System.out.println(”hello”);
Blocks
Java: arrays
int[] data =new int[10]
Point[] points=new Points[20];
for(int i=0;i<data.length;i++)data[i]=i*i;
for(Point p:points)p.setY(0);
Java: classes
class content:
• fields,
• Static fields
• Methods
• Static methods
• Constructors