Alex' Technology Blog

Effective Java, Joshua Bloch

alxtechblog — Mon, 21 Jun 2010 20:48:15 +0000

Effective Java, 2nd Edition (Joshua Bloch)

link to page

Chapter 2: Creating and Destroying Objects
- Item1: Statische Factory-Methoden anstelle von Konstruktoren
+ aussagekräftige Namen,
+ Rückgabe von Subtypen,
+ Wiederverwendung von existierenden Objekten
- Keine Vererbung von Klassen ohne public Konstruktor
– Item2: Builder-Objekte bei vielen Konstruktor-Parametern
(sonst Teleskop-Pattern oder Bean-Objekt)
+ simuliert benannte optionale Parameter
+ bei vielen ( >= 4, auch optionalen) -Konstruktor-Parametern sinnvoll
- Item3:   Singleton-Objekte mit privatem Konstruktor bzw. enum-Type (ab Java 1.6)
- Item4:   Privater Konstruktor bei Aggregationsklassen von statischen Methoden (“Utility-Klassen”)
! reicht nicht, da ererbte Klassen instanziiert werden können
- Item5:   Keine neuen Objekte erzeugen, wenn existierende wiederverwendet werden können
Primitive Datentypen gegenüber “boxed” Typen vorziehen (Autoboxing kann “teuer” werden)
- Item6: Unter Umständen ist manuelles Eliminieren von Objektreferenzen notwendig
! Im Normalfall sollte jedoch die Garbage-Machine reichen, nicht jedoch bei eigener programmatischen Speicherverwaltung
! Memory Leaks bei 1. eigener Speicherverwaltung, 2. caches, 3. listeners und callbacks
- Item7: Finalizer vermeiden, nie zeitkritisches reintun!
! bevorzuge explizite Terminierungsmethoden
+ verlangsamen Code
+ werden “irgendwann” aufgerufen
+ evtl. für Logging bzw. als Backup-Lösung

Data Visualization with the Google Charts API

alxtechblog — Tue, 08 Jun 2010 20:33:18 +0000

Whoever wants to include a graphical chart in a web page without having to design it in some other tool and export it as image, try the google charts api!
It’s just one line of html-code, looks nice, can be easily configured, and – with some basic java scipt – even interactive….
I liked it, so click here for more information about the chart tools api or here for the gallery. See below for my very first chart

QR Codes for quick & easy access to web pages on your smart phone

alxtechblog — Tue, 01 Jun 2010 21:24:17 +0000

You have a mobile phone equipped with a qr-tag reader? You want to open the same web page on your mobile phone like on your desktop pc but are tired of typing a looong url with the smaaall keys / touch buttons of your phone ?

Here’s the answer:

Create a bookmark to this link: javascript:window.location=’http://chart.apis.google.com/chart?cht=qr&chs=480×480&chl=’+window.location

Now whenever you are on an interesting web page, open this book mark and a qr code will apear containing the link to your current page. Take a picture with your mobile phone and find yourself on the same page on your mobile phone like on your desktop pc … nice and easy

Find below a sample qr tag containing a link to the main page of this blog:

Free little tool for learning vocabulary …

alxtechblog — Sun, 09 May 2010 17:19:30 +0000

Some years ago I spent a while in Finland. During that time I decided to try to learn basic finish, however this is for sure not a language easy to learn! To facilitate this (hopeless) effort a little bit, I developed a small piece of software which works like post-its: It gives you some word and you can think of the translation. This tool gives you some statistics of your learning progress and puts the words in a sensible order so that wrongly answered words come more often than correctly answered ones … Of course both words and answers need to be specified by the user before.

Download this little tool here (windows only), try it (some finnish word list as example included ) of course for free. Well admittedly I haven’t really succeeded in learning finnish but I am kinda sure it’s not because of this tool, it’s rather my langauge capabilites …

Here is the project page of this vocabulary software: vocabletester.weebly.com/

Resurrecting the good old digiFrame Connector

alxtechblog — Thu, 29 Apr 2010 21:41:37 +0000

A bit more than a year ago I had built a software running on a laptop which could receive pictures over the internet and display them. The fancy thing was that it was possible to send pictures to this “digital picture frame” just by email or by using a little web application which also let you amend these pictures with text and so on ( click here to see the digiFrame Connector Web page for more information). Only few seconds after the picture was send to the digiFrame it popped up and could be viewed, so you could see this picture almost instantly. For viewing all pictures that already had been sent to the digiFrame there were 2 modes: A slideshow mode which just showed random pictures and the “history mode”, wich showed the last 10 pictures sent to the digiFrame.

I built this digiFrame system because I thought it was interesting and also practical. Then I tried to spread the existence of the digiFrame among my friends and hey – suddenly I got pictures sent to me from Finland, Spain, Hamburg and other cities …. nice However, after a while I received less and less pictures, eventually I put the digiFrame display in the basement and forgot about it! But hey, in the last month I have spoken to 4 people who asked me how I liked their pics….”What pics?” – “Well the pics I sent to your digiFrame!” – “UUps!” …. So I ran down to the basement, put it onto my desk again and was really happy and suprised to see those 4 new pictures on my digiFrame.

And this is what this post is all about: The digiFrame is alive again! If you send me your pics, they won’t disappear straight in the basement! Instead I will enjoy them and give you feedback on your photos! Promise

…. again the link to the project page: digiframe.weebly.com

Firefox Addons which I am currently using …

alxtechblog — Tue, 27 Apr 2010 21:46:49 +0000

Today I want to present you 4 of the best firefox addons (at least according to my opinion…). Firefox addons – or generally spoken “browser addons” – are very important because they provide great extra functionality for browsing the internet which I wouldn’t want to miss anymore. Download following addons and many others from the firefox addons site. Trust me, just give them a try …

- Xmarks Bookmark and Password Sync

Xmarks Bookmarks allows you to sync and backup your bookmarks (and optionally passwords if you trust them ) on a xmark server in the internet. This means that you can download your bookmarks – even open taps from you last session – on any other computer with internet access … nice!

- Cooliris

Looking for images or movies and trying to get an overview of the results e.g. in google images search? Cooliris presents you a 3d-wall full of images from web pages. Zoom into these images, walk along the image wall … just enjoy this beautiful and quick way of viewing visual media of a web page.

- Firebug

Analyze java script, css, html, dom elements, …., of webpages which you are currently viewing and modify them on the fly. Everybody who’s designing or creating web pages probably uses this addon. Everybody else: Keep this wonderful addon in your mind! May be you’ll need it some day trying to find out why the page you are currently viewing just looks totally different in the internet explorer than in any other browser!

- AdBlock (Plus)

Tired of seamingly infinite add popups opening in the browser while surfing through the internet? Use this famous addon and you’ll be (almost) free of add popups. It works by applying filters according to pre-defined but always up-to-date black lists.

Databases: Indices and Partitioning

alxtechblog — Sun, 25 Apr 2010 21:38:31 +0000

If databases need to cope with high traffic it is very important to use them correctly in order to gain as much performance as possible. This means using indices (pointers on table attributes organized as b-trees) and partitioning. A very good and understandable survey can be found on Matthias Bank’s blog (indices, partitioning). Have fun reading …

Good Linux Tutorial

alxtechblog — Sun, 18 Apr 2010 21:05:27 +0000

For everybody who is new to Linux and wants to get started with a fair amount of background information, this online and free tutorial is a great start: http://www.linux-tutorial.info/modules.php?name=MContent&pageid=224 .

Using VirtualBox for OS virtualization

alxtechblog — Sun, 28 Mar 2010 14:42:29 +0000

Since I am Windows 7 user but quite often need unix tools I was looking for a way of running linux within my windows environment( I didn’t want to have a dual boot since I am too lazy for rebooting my computer for then 2nd os!). I decided to use virtual box from Sun as virtualization software.

In short, this is a great and free tool with good docs. It is very easy and straight forward to have another os running within your main os (in my case kubuntu 9.10 as guest in windows 7) .

It comes with some little add-ons called “guest additions” which give you some more nice integration features, i.e. “seamless integration” (add the kde-task line on top of the windows taks line) or “easy file sharing (“just mount the windows disks in your guest os”) .

In my case I wanted to use linux as web server running some web application so I was trying to access the port 8080 of my guest system with my browser from my host system. For doing this you have to manually make some config-modifications to create the port forwarding. In the virtual box installation directory on your host, enter following modifications.

Here are the commands which are needed if you want to have a ssh-daemon running on your guest-system and want to login from your host at port 2222:

VBoxManage setextradata “kubuntu” “VBoxInternal/Devices/pcnet/0/LUN#0/Config/myssh/HostPort” 22

VBoxManage setextradata “kubuntu” “VBoxInternal/Devices/pcnet/0/LUN#0/Config/myssh/GuestPort” 22

VBoxManage setextradata “kubuntu” “VBoxInternal/Devices/pcnet/0/LUN#0/Config/myssh/Protocol” TCP

Now you can do the same port forwarding for accessing you web server running on your guest!

If you want to remove this port forwarding use:

VBoxManage setextradata “kubuntu” “VBoxInternal/Devices/pcnet/0/LUN#0/Config/myssh/HostPort”

VBoxManage setextradata “kubuntu” “VBoxInternal/Devices/pcnet/0/LUN#0/Config/myssh/GuestPort”

VBoxManage setextradata “kubuntu” “VBoxInternal/Devices/pcnet/0/LUN#0/Config/myssh/Protocol”

In short, if someone needs virtualization right now, I’d recommend Sun’s Virtual Box very nice and free (unlike vmware!) option!