Eric RobertsProfessor of Computer Science

Stanford University

Google AtlantaOctober 1, 2010

Converting Java into JavaScript

What Got Me Started?• Back in the 1990s, I was frustrated by the fact that—unlike

the situation in most sciences—you can’t walk into your favorite bookstore and find a book about the great intellectual ideas in computer science.

Walk into any bookstore, and you’ll see how to Teach Yourself Java in 7 Days alongside endless variations offering to teach Visual Basic, Windows, the Internet, and so on in a few days or hours. I did the following power search at Amazon.com:

pubdate: after 1992 and title: days and (title: learn or title: teach yourself)

and got back 248 hits. The first 78 were computer books (number 79 was Learn Bengali in 30 days). I replaced “days” with “hours” and got remarkably similar results: 253 more books, with 77 computer books followed by Teach Yourself Grammar and Style in 24 Hours at number 78. Out of the top 200 total, 96% were computer books.

—Peter Norvig, “Learn Programming in Ten Years,” 2001.

• What is in the bookstore? Peter Norvig, Google’s Director of Research, tells the story better than anyone else:

Talking about Great Ideas• In response, I started both teaching a course and developing a

book on the theme of The Intellectual Excitement of Computer Science.

• Since that time, I have taught the course 12 times. I’m still working on the book.

• Since that time, I have taught the course 12 times. I’m still working on the book.

• It was clear to me that such a book would be effective with a general audience only if readers could play with the ideas. To that end, I developed a simplified teaching language called MiniJava along with Java-based applets that let readers work with different machine models and algorithmic ideas, such as:− Karel the Robot− Babbage machines− Turing machines− The Enigma machine− Finite state machines− Various classic computer science games and algorithms

Writing Books in Internet Time • Unfortunately, several of the design decisions in the book

have proven to be less than ideal with the passage of time:− Java has proven to be difficult to use at the introductory level.

The March of Progress

266 pages 274 pages

911 pages

1536 pages

A sobering thought: There are more public methods in the java and javax package hierarchies than there are words in Jensen and Wirth’s Pascal User Manual and Report. The amount of explanation once deemed sufficient to teach the standard introductory programming language is thus no longer sufficient for an index of the operations available today.

Writing Books in Internet Time • Unfortunately, several of the design decisions in the book

have proven to be less than ideal with the passage of time:− Java has proven to be difficult to use at the introductory level.− It was impossible to maintain the alignment between Java and

MiniJava as Java evolved.− Java applets are no longer well supported in all browsers,

having lost the battle for client-side programming to JavaScript.

• For a number of years—and four other books—I’ve been at something of an impasse in terms of knowing how to proceed.

• And then, on May 22, 2010 . . .

The Inspiration

Divergent Views of the Impact

The Challenge• I had all of these applets running in Java.• PacMan had shown that the level of interactivity I needed was

available in JavaScript.• Would it be possible to translate the Java applets I had into

JavaScript with minimal work?• I looked at Google’s work in the area and decided that the

learning curve might be too high.• I decided it would be fun to sit down and see how close I

could come without writing any difficult code.• The bottom line: I was able to get my applets working by

introducing some straightforward modifications to the Java code and then running those programs through what is essentially a sed-script translator.

The Easy• Declarations (changing types to var)

• Integer division (3 / 2 is 1.5 in JavaScript)• Object and method syntax

The Tricky• Inheritance• Method overloading• JavaScript’s broken iterator syntax for arrays

The Really Difficult• The lack of threads and any mechanism for waiting• Java packages that use that model

My solution:

• Rewrite the Java programs to use a pure-event model that makes translation easy

Demo Time

The End