Transcript 1-1 - People.cs.uchicago.edu

Introduction to Java
Java: A simple, object-oriented, network-savvy, interpreted,
robust, secure, architecture neutral, portable, highperformance, multithreaded, dynamic language.
CS 102-02
Lecture 1-2
April 1, 1998
CS102-02
Lecture 1-2
A History of the Java-Speaking Peoples
• Video on demand needs set-top boxes
– Set-top boxes need a small, network-friendly
programming language
– Do you have video on demand? Neither does
anyone else.
• Web catches on, needs small, networkfriendly programming language
• And the rest is history...
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What is Java?
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• Programming language
– Computers are stupid
– Step-by-step instructions
• Human languages allow for too much
ambiguity
“The policeman hit the boy with the stick.”
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Object-oriented
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• A world full of objects
– Programming a computer means translating
those objects
• OO languages make it easy to specify
objects and their relationships
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Writing for Networks
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• Network “rules of the road”
– Protocols: TCP/IP, HTTP & FTP
– Understand URLs
• Distributed programming
– Get other computers to do your work for you
– Example: RC5 code-breaking
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Interpreted
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• Computers don’t speak Java as a native
language
• Two ways to translate a book
– One line at a time for each person who reads it
– Do the whole thing at once
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Run Anywhere
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• Different strokes for different folks
– Should you break eggs at the little end or the
big end?
– “Look & feel”
• Byte-codes are universal
• Virtual machines are virtually the same
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My Platform is Better than Yours
His Majesty desired I would take some other opportunity of
bringing all the rest of his enemy's ships into his ports. And
so unmeasurable is the ambition of princes, that he seemed
to think of nothing less than reducing the whole empire of
Blefuscu into a province, and governing it by a Viceroy; of
destroying the Big-Endian exiles, and compelling that people
to break the smaller end of their eggs, by which he would
remain the sole monarch of the whole world.
-- Gulliver’s Travels by Jonathan Swift
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How to Do More than
One Thing at a Time
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• One processor means one thing at a time
• Switching among many tasks quickly is
tough
– Lots to track
– Use the computer to track it all
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Simple?! (Maybe Similar)
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• Looks like C/C++
– Good for existing programmers
– Scheme is simple, but Java isn’t
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Simple = Harder to Hurt Yourself
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• Removed some of C/C++’s foot-shooting
ability
– Useful but difficult to use well:
• Memory management
• Multiple inheritance
• Operator overloading
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“Do You Know Where Your Code Has
Been?”
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• Robust means it’s hard to break things
• Don’t talk to strangers
– How do you know who’s a stranger over a
network?
– Use a secret code!
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The Dynamic Uno
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• Don’t do today what you can put off until
tomorrow:
Either always carry suntan lotion and an
umbrella, OR wait for the sun to come out
• “Waiting” is dynamic, or run-time, lookup
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High-Octane Programming
Java: A simple, object-oriented, network-savvy, interpreted, robust,
secure, architecture neutral, portable, high-performance, multithreaded,
dynamic language.
• Java runs more slowly for two reasons
– Dynamic means waiting 'til a program runs to
do some housekeeping
– Interpreted means translating each line every
time
• Just-in-time compilation
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A First Java Program
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
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Run It, and Ta-Da!
Prints out the message
“Welcome to Java
Programming!” at a
specified location
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Got Something to Say?
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
Comments are key
The life you save might be your own...
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It Ain’t International Trade
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
import puts your code on a first-name basis with existing
class(es)
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Everything’s a Class
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
In Java, writing a program means creating a class
“Call me Welcome” -- Moby Java
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Using Someone Else’s Wheel
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
Inheritance in action
A “Welcome” is a kind of “Applet”
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Why Bother?
“Put away that checkbook! Look at all you get with
the amazing new Applet class!”
destroy()
Called by the browser or applet viewer to inform this applet that it is
being reclaimed and that it should destroy any resources that it has
allocated.
getAppletContext()
Determines this applet's context, which allows the applet to query and
affect the environment in which it runs.
getAppletInfo()
Returns information about this applet.
getAudioClip(URL)
Returns the AudioClip object specified by the URL argument.
getAudioClip(URL, String)
Returns the AudioClip object specified by the URL and name arguments.
getCodeBase()
Gets the base URL.
getDocumentBase()
Gets the document URL.
getImage(URL)
Returns an Image object that can then be painted on the screen.
getImage(URL, String)
Returns an Image object that can then be painted on the screen.
getLocale()
Gets the Locale for the applet, if it has been set.
getParameter(String)
Returns the value of the named parameter in the HTML tag.
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getParameterInfo()
Returns information about the parameters than are understood by this applet.
init()
Called by the browser or applet viewer to inform this applet that it has been
loaded into the system.
isActive()
Determines if this applet is active.
play(URL)
Plays the audio clip at the specified absolute URL.
play(URL, String)
Plays the audio clip given the URL and a specifier that is relative to it.
resize(Dimension)
Requests that this applet be resized.
resize(int, int)
Requests that this applet be resized.
setStub(AppletStub)
Sets this applet's stub.
showStatus(String)
Requests that the argument string be displayed in the "status window".
start()
Called by the browser or applet viewer to inform this applet that it should start
its execution.
stop()
Called by the browser or applet viewer to inform this applet that it should stop
its execution.
Lecture 1-2
Applets Inherit Too
java.lang.Object
java.awt.Component
java.awt.Container
java.awt.Panel
java.applet.Applet
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Painting Pretty Pictures
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
When the browser wants to show an applet, it calls paint()
Welcome  Applet because Welcome has its own paint()
April 1, 1998
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Lecture 1-2
Is There an Artist in the House?
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
Browser says, “Paint this!” and passes a Graphics object
Call it g for short
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Scribbling Strings
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
Graphics objects “know” how to print strings
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Lecture 1-2
Pick a String, Any String
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
If you want to draw a string, you’ve gotta specify:
A string (Given in quotes here because we want a literal string)
and...
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Where do you want it?
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
Start in the upper-left hand corner and count 25 (pixels) over,
then 25 down
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Lecture 1-2
Graphics Know-It-All
// A first program in Java
// import Applet class
import java.applet.Applet;
// import Graphics class
import java.awt.Graphics;
public class Welcome extends Applet {
public void paint( Graphics g ) {
g.drawString( "Welcome to Java Programming!", 25, 25 );
g.drawOval(5, 5, 220, 35);
}
}
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New & Improved
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What is Java?
• Convenient language for building
networked applications
• Designed to be easy for current
programmers to learn
• Protects you from yourself
• Handy for creating dancing babies
April 1, 1998
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Lecture 1-2