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This program will have an infinite canvas (ie as long as the user scrolls, it becomes bigger) with a tiled background image, and you can drag and drop blocks and draw arrows between blocks. Obviously I won't use a layout manager for placing blocks and lines, since they will be absolutely positioned (any link on this, possibily with a snapping feature?). The problem arises with blocks and lines. Basically I'll have two options:

  1. Using a simple layout for each building block. This is the simplest and clearest approach, but does it scale well when you have hundreds of objects? This may not be uncommon, just imagine a database with 50 tables and dozens of relationships
  2. Drawing everything with primitives (rectangles, bitmaps, etc). This seems too complicated (especially things like text padding and alignment) but may be more scalable if you have a large number of objects. Also there won't be any event handler

Please give me some hints based on your experience. I have never drawn with Java before - well I did something rather basic with PHP and on Android. Here is a simple preview

enter image description here

DISCLAIMER

You are not forced to answer this. I am looking for someone who did something like this before, what's the use of writing I can check an open source project? Do you know how difficult it is to understand someone else's code? I'm talking about implementations details here... Moreover, there is no guarantee that he's right. This project is just for study and will be funny, I don't want to sell it or anything and I don't need your authorization to start it.

Glorfindel
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Raffaele
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  • See this [List of Unified Modeling Language Tools](http://en.wikipedia.org/wiki/List_of_Unified_Modeling_Language_tools), which includes several open-source Java implementations. – trashgod Oct 20 '11 at 22:33
  • The point is, you're asking for something that's already been done so many times that it's basically a solved problem. A boring one for most people, at that. There are a bunch of UML tools out there; if you intend to ever do a thing with open source, you better get used to reading other people's code...and that'd be a good place to start. – cHao Oct 20 '11 at 22:58
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    All I want to do is a drawing application, UML is just an accident. I think I am free to implement a drawing tool as an exercise, and I was asking what's the best way to do it. If you have any hint write it down, otherwise there are plenty of unanswered questions on stackoverflow... Men, we are here to help each other and you really aren't helping me. One can write an HTTP server in his spare time even if there are a number out there, can write a Mario clone, can develop anything... – Raffaele Oct 20 '11 at 23:08
  • One *can*, sure. But "How do i write an HTTP server?" or "How do i write a Mario clone?" is gonna get close votes too. Easy as it is, understood as it is, it's still too big, too vague, and too subjective a question. – cHao Oct 21 '11 at 04:09

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Measuring and drawing text isn't such a pain, since java has built in classes for doing that. you may want to take a look at the 2D Text Tutorial for more information. In fact, I did some text drawing computations with a different graphics engine which is much more primitive, and in the end it was rather easy (at least for the single-line drawing, for going multiline see the previous link).

For the infinite canvas problem, that's also something I always wanted to be able to do. A quick search here at stackoverflow gives this which sounds nice, althought I'm not sure I like it. What you can do, is use the way GIMP has a scroll area that can extend as you move - catch the click of the middle mouse button for marking the initial intention to move the viewport. Then, when the mouse is dragged (while the button is clicked) move the viewport of the jscrollpane by the offset between the initial click and the current position. If we moved outside the bounds of the canvas, then you should simply enlarge the canvas.

In case you are still afraid of some of the manual drawing, you can actually have a JPanel as your canvas, with a fixed layout. Then you can override it's paint method for drawing the connectors, while having child components (such as buttongs and text areas) for other interaction (and each component may override it's own paint method in case it wants to have a custom-painted rect).

In my last drawing test in java, I made an application for drawing bezier curves (which are basically curves made of several control points). It was a JPanel with overidden paint method that drew the curve itself, and buttons with custom painting placed on the location of the control points. Clicking on the control point actually was clicking on a button, so it was easy to detect the matching control point (since each button had one control point associated with it). This is bad in terms of efficiency (manual hit detection may be faster) but it was easy in terms of programming. Anyway, This idea can be extended by having one child JPanel for each class rectangle - this will provide easy click detection and custom painting, while the parent will draw the connectors.

So in short - go for nested JPanels with custom drawing, so that you can also place "on-canvas" widgets (and use real swing widgets such as text labels to do some ready drawing) while also having custom drawing (by overriding the paint method of the panels). Note that the con of this method is that some swing look-and-feel's may interfere with your drawing, so may need to mess a bit with that (as far as I remember, the metal and nimbus look-and-feel's were ok, and they are both cross-platform).

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Barak Itkin
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  • I think I will go with two custom JComponents for the block and the arrow, so pluggable LAF won't interfere with my drawing – Raffaele Oct 21 '11 at 09:07
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    @Raffaele: note that if you do so, since the bounding rectangles of the arrows may intersect with the bounding rectangles of the regular UML rectangles (even if the arrows themselves don't intersect the rectangles), you may need to use a JLayeredPane as the root container (instead of a regular JPanel) to make sure that drawing will catch even in the "intersecting" parts. Also, you **must** use a JLayeredPane if the actual drawings will intersect – Barak Itkin Oct 21 '11 at 09:28
  • Thank you very much, I didn't know of this. Anyway, the Java swing tutorial says that Swing top level containers' root panes automatically use a JLayeredPane, so maybe I don't really need to specify it. Good to know, though :) – Raffaele Oct 21 '11 at 19:04