march 29, 2000cs102-01lecture 1.2 introduction to java cs 102-01 lecture 1.1 java: a simple,...

March 29, 2000 CS102-01 Lecture 1.2

Introduction to Java

CS 102-01

Lecture 1.1

Java: A simple, object-oriented, network-savvy, interpreted, robust, secure, architecture neutral, portable, high-performance, multithreaded, dynamic language.


What is Java?

• Programming language– Computers are stupid– Step-by-step instructions

• Human languages allow for too much ambiguity“The policeman hit the boy with the stick.”



Object-oriented

• A world full of objects– Programming a computer means translating

those objects

• OO languages make it easy to specify objects and their relationships



Writing for Networks

• Network “rules of the road”– Protocols: TCP/IP, HTTP & FTP– Understand URLs

• Distributed programming– Get other computers to do your work for you– SETI@home



Interpreted

• 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



Run Anywhere

• 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



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

My Platform is Better than Yours


How to Do More thanOne Thing at a Time

• 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



Simple?! (Maybe Similar)

• Looks like C/C++– Good for existing programmers– Scheme is simple, but Java isn’t



Simple = Harder to Hurt YourselfJava: 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


“Do You Know Where Your Code Has Been?”


• 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!


The Dynamic Uno


• 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


High-Octane Programming


• 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


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 );

}

}


Run It, and Ta-Da!

Prints out the message “Welcome to Java Programming!” at a specified location


Got Something to Say?// A first program in Java








}

}

Comments are key

The life you save might be your own...


It Ain’t International Trade// A first program in Java








}

}

import puts your code on a first-name basis with existing class(es)


Everything’s a Class// A first program in Java








}

}

In Java, writing a program means creating a class

“Call me Welcome” -- Moby Java


Using Someone Else’s Wheel// A first program in Java








}

}

Inheritance in action

A “Welcome” is a kind of “Applet”


Why Bother?

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.

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.

“Put away that checkbook! Look at all you get with the amazing new Applet class!”


Applets Inherit Too


Painting Pretty Pictures// A first program in Java








}

}

When the browser wants to show an applet, it calls paint()


Is There an Artist in the House?// A first program in Java








}

}

Browser says, “Paint this!” and passes a Graphics object

Call it g for short


Scribbling Strings// A first program in Java








}

}

Graphics objects “know” how to print strings


Pick a String, Any String// A first program in Java








}

}

If you want to draw a string, you’ve gotta specify:

A string (Given in quotes here because we want a literal string) and...


Where do you want it?// A first program in Java








}

}

Start in the upper-left hand corner and count 25 (pixels) over, then 25 down


Graphics Know-It-All// A first program in Java








g.drawOval(5, 5, 220, 35);

}

}


New & Improved


What is Java?

• Convenient language for building networked applications

• Designed to be easy for current programmers to learn

• Protects you from yourself

march 29, 2000cs102-01lecture 1.2 introduction to java cs 102-01 lecture 1.1 java: a simple,...

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