Ticklish Techs

Disclaimer: I’m absolutely not the person who can tell a lot about Java. I learned it a few years ago for university, but used it most of the time only for small example stuff. Never as deep as I’m working with .net. But maybe my newest discoveries are useful for more programmers with .net background.

I started using Java 1.4 and until last week I never took a look at Java 5 or 6. The only thing I was aware of were generics in Java.

iterators in Java

I was always annoyed in the missing of a foreach loop in Java. There is a concept of iterators in Java comparable to the .net Enumerators. But to loop over a list you will find in millions of Java programs the following code:

Iterator<Integer> i = list.iterator();
while(i.hasNext()) {
    Integer current = i.next();

    // do something with the current element
}

Sometimes build into a regular for loop, but I don’t think that is more readable or convenient:

for(Iterator<Integer> i = list.iterator(); i.hasNext(); ) {
    Integer current = i.next();

    // just do something
}

By the way: Can anyone tell me why the method is called iterator() and not getIterator() as used everywhere else in Java?

foreach in Java

With Java 5 (or 1.5 – call it as you like) a foreach loop was introduced. This is about five years ago, so I’m very fast in getting news from the Java world.

for(Integer i : list) {
    // just use i as the current element
}

I like it. It’s working for everything that implements java.lang.Iterable and for regular arrays.

Imaging a small C# winform app with just one ListBox and the following code:

private void Form1_Load(object sender, EventArgs e)
{
    for (int i = 0; i < 5; i++)
    {
        Thread t = new Thread(() => AddToListbox(i));
        t.Start();
    }
}

private void AddToListbox(int i)
{
    if (this.InvokeRequired)
        this.Invoke(new Action<int>(AddToListbox), i);
    else
        this.listBox1.Items.Add(i);
}

A simple loop iterating over the numbers from 0 to 4 and adding these values asynchronous to a ListBox. What do you expect? I expected the numbers from 0 to 4 shown in the ListBox but in any random order since I do not control the threads in any way.

I didn’t expect any number to appear multiple times and I’m totally surprised to  see the number 5!

But ReSharper gave me a hint that I often saw but never understood:

So what’s going on?

I use the syntax of an Lambda expression instead of a regular function call (e.g. using the ThreadStart class and a delegate). This Lambda expression is not evaluated until the thread uses it. And by this time, the loop can be in its next iteration. If the loop is already finished i will be 5.

That is exactly what R# tries to tell me: “Hey, you are accessing here a variable but change it later on. Maybe that is not a good idea.”. – It isn’t.

The solution

Just make a copy of i before passing it into the expression. This copy must be a private copy that will not be changed later. The easiest way to do so is declaring a variable inside the body of the loop. In every iteration of the loop a new integer will be created on the stack and the Lambda expression will access this one.

for (int i = 0; i < 5; i++)
{
    int copy = i;
    Thread t = new Thread(() => AddToListbox(copy));
    t.Start();
}

Ok, what does this expression do or mean? First of all: never write something like this in your code. Nobody understands the meaning with a single look and that will lead to confusion and maybe to a bug in your software someday.

Now let’s take a deeper look. First you may think about the difference between x++ and ++x. If you write it as a single statement both expressions are equal, but if you use it as a part of a complex expression the first will evaluated to the value of x before increasing it; the second one will evaluate to the new value of x. So y = x++; leads to a different result for y as y = ++x;.

x+=a simply is a ‘shortcut’ for x=x+a.

Now let’s do it step by step. For example x is 5. Then first x will be increased by one to 6 but the old value will go into the formula that remains as x=x+5. Since x was increased before the result will be 11.

If you think that is all right, than please take a break and test it with your favorite compiler. If you are a C or C++ guy you will in fact receive 11 as an answer and everything is fine. But if you are a C# or java guy x will be 10. Why?

.Net as well as the Java Runtime are stack machines. The expression will be put on the stack step by step before evaluating the whole thing. At that time x is 5. x will be changed to 6 by the x++ part, but that only happens in the main memory. The old value (5) is still on the stack. After executing the whole expression the changed x will be overwritten by 10 (5+5).

And once again: NEVER write code like this!

I found this somewhere and I think it’s worth to write a short (only a very short) entry.

throw null;

If you use this expression in your C# code it will throw a NullReferenceException. That is because the throw-statement needs an object of type Exception as its single parameter. But this very object is null in my example.

I like this way to produce some exception while testing code.

It was very silent here in the past month. That was mainly because I was working on my diploma thesis until the end of September. After that I did my final exam and started working in Bonn. Now it is December and almost 2009. I’m sitting in a plane to Australia for holiday and have much time to write articles. I will try to keep this blog alive and will write articles more frequently.

Today I will introduce Flow. With Flow you can model the behavior of nodes in a wireless sensor network (WSN) in a data driven and event driven manor. I developed Flow as part of my diploma thesis.

Flow is build as a Visual Studio Addon (a VS Package) on top of the DSL-Tools containing three different domain specific languages to describe different parts of the software running on a sensor node. These different DSLs are working together and using most of the techniques I described earlier in this blog. E.g. my library JaDAL was initially build to support this diploma thesis.

You will find screenshots, more explanations and Flow itself on http://flow.irgendwie.net. While the webpage and the software are available in English the thesis itself is a pdf-document and can only be downloaded in a German version.

The software is fully working and if no wireless sensor network is available you can use a node simulator to evaluate Flow. The code is released under the new BSD license and also available for download.

Screenshots:

Dataflows modeled with Flow looks like that:

The simulated sensor nodes are represented as Windows forms programmed by such dataflows:

For more information just visit http://flow.irgendwie.net.

From 11/19 to 11/21/2008 I will be at the Microsoft Technical Summit in Berlin. The Technical Summit itself starts on 20th; on the 19th I will attend the 4th Academic Day.

If anyone will be at these conferences and likes to meet please drop me a line.

See you in Berlin. 🙂

Lately I was looking for a way to translate geo coordinates (latitude and longitude form my gps device) to some sort of text or names. As you know I’m trying to geotag my photos using a gps logger and PhotoTagStudio. This data can be used to visualize the place a photo was taken on a map (e.g. Google Maps or flickr!). The next step can be to use this data to describe the photo as well.

I think one should be able to translate a geo coordinate into at least the name of the county, the state and the city; maybe to the district or a street name, too. To achieve this I found the geonames.org webpage. This page collects exactly the data I need and allows searching via the website and a webservice.

The webservice itself consists of approximate 30 REST methods one can call by querying an URL. Most of these calls return a XML document (or optional a JSON document). Unfortunately  there is no SOAP / WSDL version of this webservice. For a .net and C# developer a SOAP webservice is very easy to consume but for such a REST / XML webservice you have to create the URLs yourself and parse the resulting XML document.

I started to create a library to query the webservice. This library provides a few static methods mapped to the most useable webservice-methods and returns good shaped .Net objects. These objects carry all the data coming from the webservice. At the moment this library is in an early beta status but you can use it in your own projects. It can be found on Codeplex: GeonamesDotOrgDotNet, there you will find a small example and a demo application. When using this library you should take a look at the documentation of the geonames.org webservices since the interfaces are nearly the same.

I hope this is useful for someone. I will extend the library in the next weeks. Feature requests are welcome, so is help on the coding side.

Currently the following webservice calls are supported:

• findNearby
• findNearbyPlaceName
• findNearByWeather
• findNearbyWikipedia
• get
• hierarchy
• children
• gtopo30
• srtm3

At the end a few random screenshots and a code snippet. For more information please take a look at the Codeplex project page.

// finding some place in Berlin, Germany
// (using prefix m for decimal literals)
Geoname Place =
GeoNamesOrgWebservice.FindNearbyPlaceName(52.51m, 13.4m);

// getting all parents
IEnumerable ParentPlaces = Place.Hierarchy();

// display the partents
// (will print out something like:
//  “Globe > Europe > Germany > Land Berlin > Berlin >”)
foreach (Geoname x in ParentPlaces)
Console.Write(x.Name + ” > “);

The Classes of the Library:

A screenshot of the demo application to test the library:

I created two DirectiveProcessor for Visual Studio to use in the T4 system. I added the classes to JaDAL hoping someone can use them in her project or use the code as an example when creating new DirectiveProcessors.

Let’s start with a short introduction to DirectiveProcessors. DirectiveProcessors are used in .tt-files to provide data to the template. The DirectiveProcessor can take parameters from the declaration in the .tt-file and adds some code to the compiled template. This code (often properties) can be used from within the template. For example the DSL Tools are generating one DirectiveProcessor for each DSL model. In a template form the DSL Tools you will see a line like the following:

<#@ Language1 processor=“Language1DirectiveProcessor”
requires=“fileName=’Sample.mydsl1′” #>

This line uses the Language1DirectiveProcessor and provides it with one parameter (fileName). The DirectiveProcessor adds code to the template to open the given file and creates (in this case) a ExampleModel-property to use in the template code.

I created two DirectiveProcessors to use the data of simple Xml-files and Visual Studio Project files in the templates. The code of the two DirectiveProcessors is very straight forward and maybe one could advance it (e.g. make it compatible with templates written in Visual Basic).

Using the XmlFileDirectiveProcessor

Add a line like the following to your .tt-file:

<#@ XmlFile processor=“XmlFileDirectiveProcessor” FileName=“example.xml” #>

Inside this template you can access the (full qualified) filename via the this.XmlFileName-Property and the Content of this file (as a XDocument) via the this.XmlFile-Property.

Using the VsProjectFileDirectiveProcessor

Add this line to your template:

<#@ ProjectFile processor=“VsProjectFileDirectiveProcessor” FileName=“x.csproj” #>

A property named this.ProjectFile will be added to the template. This property provides you with an instance of the VsProjectFile-class containing the project file contents. This class contains only very little functionality (feel free to add some more and submit it back to me!). Just take a look at the source code of this class. The method GetAllFiles() returns a string array of all files found in the project (supporting C++ and C# project files – .vcproj and .csproj).

Setup

An entry in the registry is needed to allow the T4 system to find custom DirectiveProcessors. Just add the following entries to your registry and don’t forget to point to the right location of the JaDAL.dll.

[HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\9.0Exp\
Configuration\TextTemplating\DirectiveProcessors\VsProjectFileDirectiveProcessor]
“Class”=”BenjaminSchroeter.Dsl.DirectiveProcessors.VsProjectFileDirectiveProcessor”
“CodeBase”=”D:\\JaDAL\\bin\\Debug\\JaDAL.dll”

[HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\9.0Exp\
Configuration\TextTemplating\DirectiveProcessors\XmlFileDirectiveProcessor]
“Class”=”BenjaminSchroeter.Dsl.DirectiveProcessors.XmlFileDirectiveProcessor”
“CodeBase”=”D:\\JaDAL\\bin\\Debug\\JaDAL.dll”

These registry keys are for the Experimental Hive of Visual Studio. To register the DirectiveProcessors for the normal Visual Studio instance the registry key starts with HKEY_LOCAL_MACHINE\Software\Microsoft\VisualStudio\9.0
\TextTemplating\DirectiveProcessors\

There was not that much going on here lately. Wolfram and I are very busy these days, but we didn’t forgot about the blog. There are many ideas for articles but to few spare time to write them. Fortunately this will become a little bit better in a few weeks.

For the meantime I build a nice tagcloud to remember you what’s usually going on her:

Produced with Wordle. Images of Wordles are licensed .

Stay tuned!

Every DSL you create with DSL Tools has a model explorer. This model explorer is a tool window in Visual Studio displaying the elements of your model in a hierarchical way. This is often a nice feature but sometimes a hierarchical view of your data is not appropriate. So I came to the question: How to remove the explorer from the generated code?

I could not find any option in the DSL design to remove the model explorer, but if you look at some of the .tt files you will find somewhere a query for this.Dsl.Explorer != null. For example in the package.tt file that generates the package.cs which is responsible for registering the tool window for the model explorer:

<#
    if(this.Dsl.Explorer != null)
    {
#>
    [VSShell::ProvideToolWindow(         typeof(<#= dslName #>ExplorerToolWindow),          MultiInstances = false,          Style = VSShell::VsDockStyle.Tabbed,          Orientation = VSShell::ToolWindowOrientation.Right,          Window = "{3AE79031-E1BC-11D0-8F78-00A0C9110057}")]
    [VSShell::ProvideToolWindowVisibility(         typeof(<#= dslName #>ExplorerToolWindow),          Constants.<#= dslName #>EditorFactoryId)]
<#
    }
#>

Even if I did not find any option to set the Dsl.Explorer property of the model to null (you did not see the Explorer property anywhere in the DSL diagram) it seems that the developers of the DSL Tools had this use case in mind.

To remove the Dsl.Explorer from your DSL model open the .dsl file with a text editor and go to the end. There you will find some XML tags like the following:

<Explorer ExplorerGuid="6c276297-6acd-4e9a-8740-b61ba834004b"  Title="HardwareDescription Explorer">
    <ExplorerBehaviorMoniker      Name="HardwareDescription/HardwareDescriptionExplorer" />
</Explorer>

Just delete these three lines and generate your code once again. Maybe you should reset your Experimental Hive, too.

I tested a DSL with a removed Model Explorer without any problems. It seems the developers of the DSL Tools did a very good job on the code generation templates . The generated ModelExplorer.cs file contains only a single line:

// This source file is empty because    this DSL does not define a model explorer.

That should be a good proof that a DSL can run without the model explorer even if it might be not supported by the DSL Tools. 🙂