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Java Source Code Presenter

The JDisplay Java Applet displays the large program listings on this web page. JDisplay requires an up-to-date browser and Java version 1.8+, preferably 1.8.0_131. If you can’t see the listings, or if you just want to learn more about JDisplay, click  Help Use Firefox for best results.


This essay does not describe an existing computer program, just one that should exist. This essay is about a suggested student project in Java programming. This essay gives a rough overview of how it might work. I have no source, object, specifications, file layouts or anything else useful to implementing this project. Everything I have prepared to help you is right here.

This project outline is not like the artificial, tidy little problems you are spoon-fed in school, when all the facts you need are included, nothing extraneous is mentioned, the answer is fully specified, along with hints to nudge you toward a single expected canonical solution. This project is much more like the real world of messy problems where it is up to you to fully the define the end point, or a series of ever more difficult versions of this project and research the information yourself to solve them.

Everything I have to say to help you with this project is written below. I am not prepared to help you implement it; or give you any additional materials. I have too many other projects of my own.

Though I am a programmer by profession, I don’t do people’s homework for them. That just robs them of an education.

You have my full permission to implement this project in any way you please and to keep all the profits from your endeavour.

Please do not email me about this project without reading the disclaimer above.

The JDisplay Java Applet displays the large program listings on this web page. JDisplay requires Java version 1.5 or later, preferably 1.6.0_12. If you can’t see the listings, or if you just want to learn more about JDisplay, click here for help.

This converts Java source into HTML (Hypertext Markup Language) for public presentation, much like Jonathan Revusky’s JavaToHTML. It would colour various syntatic elements. It would use bold on a variable in the place it is defined. It would use variable sized () and {} depending on how deeply nested they are. You can see some examples of the sort of output I’m after by looking at the sample. It might use special markings to delimit methods and classes. Here is the output of a crude version I use now that generated HTML with style tags, that than the be use to control the fonts, colours, sizes etc of each component of the listing. I have chosen a rather conservative colour scheme with only subtle variations.

It would render the HTML inside the Javadoc comments (e.g <b>length</b> must be &lt; 100) in human form (e.g. length must be < 100) rather than displaying it literally as it appeared in the orginal source.

For importing ordinary Java source code, a parser such as JavaCC or ANTLR (Another Tool for Language Recognition) might be useful.


The tricky part is figuring out how to do decent variable left indenting in HTML. I know of five techniques:

  1. Use a dummy gif and stretch its width. This is slow, verbose and does not look all that good in all browsers. Remember to use border=0 to avoid a line around the dummy gif.The advantage is you can also control vertical spacing by stretching the gif height.
  2. Use <ul> to indent and </ul> to unindent. This has the unwanted side effect of extra blank lines. This is technically invalid and will set HTMLValidator screaming, However, it works in all the browsers I have tried. I get rude public postings about my use of it from purists though.
  3. Use <td colspan=n></td> <td colspan=max-n>real stuff</td>. You can’t control the indentation column widths with <TABLE CELLSPACING=30 because it affects vertical spacing as well. You hide the tableness of the whole thing by using <TABLE BORDER=0. This technique has the added advantage of properly indenting wrapped long lines that don’t quite fit on the screen in some browsers. See the HTML used to create the waterfall entry.
  4. Use the margin control in CSS (Cascading Style Sheets), e.g. with a <div class=group> and a style { margin-left : 10px;}
  5. Use <pre>. The advantage of this is you don’t interfere at all with the original indenting or line breaks. You just do what the programmer did originally. You can’t screw it up or do it a different way from the way he likes. The problem with <pre> stuff is that the tiniest jostle and it is turned to mush, with all the lines wrapped.
Ideally whatever you generate is pure CSS, so the look can be changed at whim with just the style sheet.

You might run this as a batch utility to prepare HTML. You might also create it as an Applet that provides a scrolling display. You pass the name of a *.java file in the Applet tag or provide the source code text snippet inline. The end user can click a button to download the raw *.java file.

The problem with generating HTML and embedding it in your HTML pages is you can’t edit it. It just looks like gibberish. Further, if I change my mind on how the rendering should be done, I have to manually regenerate all the HTML from the orgininal Java (perhaps now lost) and replace the HTML. I think it would be better to post the plain Java files and then have an Applet render them on the fly. Then people could download the pristine source without any HTML contamination effects, such an unwanted line breaks.

The other problem with generating HTML is html beautifiers break lines introducing improper white space into the program. Another problem is with <DIV> tags you can’t exactly match the indentation of the original. You must deduce the nesting depth. This gets complicated. You could do it by generating HTML with <PRE> tags or with an Applet that drew on a Canvas. To make cut and paste work, is very tricky since it would be just a bit image you are displaying. If you generated HTML and displayed it internally with Swing, at least cut and paste would come out in the wash so people could take copies of snippets of the program without fuss. The advantage of an Applet is it need not render the entire listing. The disadvantage is the browser search and search engines would not see the rendered text.

The HTML you generate can be done with <font or with <span class=operator> type tags. The second form lets you tune the rendering just by changing the style sheet. The first type will work in Swing components, whereas the second will not.

So most likely you will want to render from plain *.java files on the fly in the Applet and give the user the option to download the plain *.java file. Render using Swing components and old-fashioned <pre> and <font tags.

JDisplay Implementation

I have implemented this project in a quite different fashion than described above. I have convert my website to use it. I still have not got around to bundling it up for distribution. Here is the JApplet that displays predigested source code. view Here is the main drawing class:



How It Works

I did not use a standard Java version 1.5 parser. I first cooked up a simple-minded tokenizer and parser that breaks the text into chunks called tokens and identifies what sort of thing each token is, e.g. a method name, a method definition, a class, an interface, an operator, a keyword, a { etc.

Then I write out each snippet as a serialized array of objects, each of which knows how to render itself.

The Applet then just reads the array of objects and uses drawString to render each token in the appropriate colour, font, style and size.

The problem is you can’t copy/paste from my colourful bitmap, so when you click the display turns into a black and white TextField rendering that you can copy from.

I use the Graphics.getClipBounds to only bother rendering parts of the Java source program that are scrolled on-screen.

// Graphics g is passed to your paint as a parameter.
// get x, y, width and height of region we are actually painting
Rectangle r = g.getClipBounds();

I use a macro to generate the Applet tags and a download button for those who do not have Java to see the snippet as raw text.

Torture Test

Here is a torture test of my Java parser and presenter, a particularly difficult document to render since it contains syntax errors. I check that it renders precisely the same the same text I feed it. Here is the Java test program rendered by the JDisplay Applet: Here is the Java test program rendered in HTML : Here is a torture test of my bat parser as rendered by the JDisplay Applet: Here is the bat test program rendered in HTML :


To summarise, if you use the HTML-generation-the-embed approach, you don’t have to get it perfect. You can always manually tune the results afterward, as I do for jtoh. On the other hand, your HTML editors will inappropriately tidy your HTML inserting unwanted white space and removing it improperly in its desire to get all the lines the same length. For shorter programs this will be faster since you don’t have to fire up Java and an Applet.

If you go the dynamic generation Applet route, you can’t use style sheets, your program must be 100% bug free and your end users must have recent working Java Plug-Ins with Swing. In turn you have very little work to do to prepare each new program for display. Your editors won’t be able to see, much less mess with your generated HTML. For very long programs this may be faster, since you don’t have to download all the style tags.

What are the colour-size-font-bold-coding classes? keywords, operators, packages, classes, methods, variables, class definitions (the class name in the place where the class is defined, e.g. right after public class), method definitions. variable definitions, constants, literals, string literals, assignment (=), dot (.), parentheses () with size and colour coding for nest depth, brackets [] with size and colour coding for nest depth, braces {} with size and colour coding for nest depth, operators (+ - ++ == !=… perhaps in shades to indicate 14 levels of priority lightest computed first, darkest and biggest last). I also encode a name where it is defined in bold and plain where it is referenced.

There is a such a program part of the SwingSet2 demo that comes in the demos directory of the JDK (Java Development Kit), under Progress Bar, though I have not yet figured out what classes it uses or how to work it. It tends to create displays in type too small. For inspiration:

JDisplay Applet
troubleshooting JDisplay

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