Tag: java

Spelling the feedback: The LED bar

Our fully automated project ecosystem provides us with feedback of very different type and granularity. We felt it was impossible to render every single notable event into its own extreme feedback device (XFD). Instead, we implemented an universal feedback source: the LED bar.

ledbar-alone

You know the LED bar already from a shop window of your town. It tells you about the latest special bargain, the opening hours of the shop or just something you didn’t want to know. But you’ve read it, because it is flashing and moving. You just can’t pass that shop window without noticing the text on the LED bar.

Our LED bar sells details to us. The most important issues are already handled by the ONOZ Lamp and the Audio feedback, as both are very intrusive. The LED bar is responsible to spell the news, rather than to tell it.

A very comforting news might be “All projects sane”, which happen to be our regular state. You might be told that you rendered “project X BROKEN”, but you already know this, as the ONOZ Lamp lit up and you were the one to check in directly before. It’s better to be informed that “project X sane” was the build’s outcome. After a while, the text returns to the regular state or blanks out.

Setting up the LED bar

We aren’t the only ones out there with a LED bar on the wall. Dirk Ziegelmeier for example installed his at the same time, but blogged much earlier about it. He even gives you detailed information about the communication protocol used by the device and a C# implementation for it. The lack of protocol documentation was a bugger for us, too. We reverse engineered it independently and confirm his information. We wrote a complete Java API for the device (in our case a LSB-100R), which we might open source on request. Just drop us a note if you are interested.

Basically, we wrote an IRC bot that understands commands given to it and transforms it into API calls. The API then deals with the low-level transformation and the device handshake. This way, software modules that want to display text on the LED bar from anywhere on the internal net only need to talk on IRC.

The idea of connecting an IRC channel and the led bar isn’t unique to us, either. The F-Secure Linux Team blogged about their setup, which is disturbingly equal to ours. Kudos to you guys for being cool, too.

Effects of the LED bar

The LED bar is the perfect place to indicate project news. Its non-intrusive if you hold back those “funny” displaying effects but versatile enough to provide more than simple binary (on/off) information. Its the central place to look up to if you want to know what’s the news.

We even found out that our company logo (created by Hannafaktur) is scalable down to 7×7 pixels, which exactly fits the LED bar in height:

logo_on_led

Try this with your company’s logo!

Read more about our Extreme Feedback Devices:

== or equals with Java enum

When you compare objects in Java you should prefer the equals()-method to == in general. The reason is that you get reference equality (like with ==) by default but you are able to change that behaviour. You can override equals() (DO NOT FORGET TO OVERRIDE hashCode() too because otherwise you will break the general class contract) to reflect logical equality which is often what you want, e.g when comparing some string constant with user input.

With primitive types like double and int you are more or less limited to == which is fine for those immutable value types.

But what is the right thing to to with the enum type introduced in Java 5?
Since enums look like a class with methods, fields and the like you might want to use equals() instead of ==. Now this is a special case where using reference equality is actually safer and thus better than logical equality.

Above (please mind the stupid example) we can see that comparing the EState enum with an ILamp using equals() is accepted perfectly by the compiler even though the condition never can be true in practice. Using == the compiler screams and tells us that we are comparing apples with oranges.

Using Flying Saucer PDF offline

Flying saucer is a nice tool for quick PDF generation from a (X)HTML page. Everything worked fine when we tested it at home but when we had a demo at a client’s site, no PDF could be generated. The problem was caused by a little snippet in the header of the HTML:

<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

The DTD declaration! So we took a look at the flying saucer issue database and found someone who had the same problem.
But another solution is even simpler:
Xerces has a default setting which tells the parser to load external dtds.
Turning this off solved our offline problems:

  DocumentBuilderFactory builderFactory =
    DocumentBuilderFactory.newInstance();
  builderFactory.setFeature(
    "http://apache.org/xml/features/nonvalidating/load-external-dtd",
    false);