random() » learning Java

anonymous class not serializable

max — Thu, 19 Aug 2010 20:05:39 +0000

Spent a while tracking down a serializable error. Took me a while to discover that anonymous classes are not serializable, even if they are implementing an interface that extends serializable. Wasn’t really a problem from me but did take a while to find.

[Ljava.lang.Object cast errors

max — Sat, 14 Aug 2010 15:13:00 +0000

This turned out to be a really simple thing. I was trying to cast an object to something else. I had received an object in good faith from an xml rpc call. It was supposed to contain a List of stuff but I kept getting casting errors that talked of being unable to cast [Ljava.lang.Object to what ever it was I wanted. I went out of my mind trying to find out what was wrong. It turns out that [Ljava.lang.Object means an array of objects!

[Ljava.lang.Object is an array of objects not an object.

ConcurrentModificationException in a single thread

max — Sun, 14 Feb 2010 17:16:54 +0000

Today I came across a ConcurrentModificationException being thrown when no concurrent work was taking place. I learned (after a bit of hair tearing) that when you are looping over a list and you decide to remove something from it you will get this error.

for(Node n : nodes){
    //do some stuff
    nodes.remove(n);//This will throw the error
}

It makes perfect sense if you think about it. You are stepping over a list, no doubt under the hood somewhere a counter is being used. When you remove a element from a list, all the other elements are shuffled down so now your iterator is going to miss an element and shoot off the end of the list, so instead of that it fails fast and throws an exception. However, more importantly than why it happens, how can we stop it happening? Well we use a list iterator instead.

ListIterator<Node> i = nodes.listIterator();
while(li.hasNext()){
    Node n = i.next;
    li.remove(); //No error if you remove the element through the iterator
}

More cool images from my msc project

max — Sun, 23 Aug 2009 11:16:46 +0000

Just finished work on a particle view module for my project. Basically you have landmarks and trails. Particles get sent from landmark A to landmark B proportionally to the number of people who have traveled from A-B. Video to follow soon.

particle map of suffix tree data

This view on the data was partially inspired by this video.

Netbeans fails to start

max — Mon, 13 Apr 2009 09:04:51 +0000

I upgraded to leopard finally, and NetBeans failed to start. The symptoms were this: I double click the icon in the doc and it jumps up and down a couple of times and then nothing. I had a look in the logs and I found the following “…org.netbeans.ide[14238]) Exited with exit code: 139″ I also noticed it was always preceeded by … ReportCrash[14543] Formulating crash report for process java[14542].

The Java crash report contained the following

Process: java [15805]
Path: /System/Library/Frameworks/JavaVM.framework/Versions/1.5/Home/bin/java
Identifier: java
Version: ??? (???)
Code Type: X86-64 (Native)
Parent Process: bash [15667]

Date/Time: 2009-04-12 13:27:07.932 +0100
OS Version: Mac OS X 10.5.6 (9G66)
Report Version: 6

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: 0x000000000000000d, 0×0000000000000000
Crashed Thread: 0

Thread 0 Crashed:
0 ??? 0x00000001000111c1 0 + 4295037377
1 ??? 0x000000010001276f 0 + 4295042927
2 ??? 0x0000000100012a1c 0 + 4295043612
3 ??? 0x000000010001104d 0 + 4295037005

Thread 0 crashed with X86 Thread State (64-bit):
rax: 0×0000000000000041 rbx: 0x00007fff5fbfe550 rcx: 0×0000000000000000 rdx: 0×0000000000000014
rdi: 0×0000000100014708 rsi: 0x352e312f736e6f69 rbp: 0x00007fff5fbfe330 rsp: 0x00007fff5fbfe330
r8: 0x0000000092b5a5f2 r9: 0×0000000000000000 r10: 0×0000000000000000 r11: 0×0000000000000000
r12: 0×0000000000000014 r13: 0×0000000100014708 r14: 0×0000000000000015 r15: 0×0000000000000000
rip: 0x00000001000111c1 rfl: 0×0000000000010293 cr2: 0×0000000100014708

Binary Images:
0x7fff5fc00000 – 0x7fff5fc2e643 dyld 97.1 (???) /usr/lib/dyld

After much searching it seems it may be a problem with apples 64bit JVM, it all seems quite complicated and I don’t fully understand it but this thread here seems to say that that is the problem and that there is a strange workaround. To get NetBeans starting again, albeit only from the terminal, I added the following line to by .profile file in my home directory (you need to log out and back in again for it to take effect):

export JAVA_HOME=$JAVA_HOME:/no/path

I found I could then open NetBeans from the command line with the following command:

open /Applications/NetBeans/NetBeans\ 6.5.1.app

Not a proper solution but it gets you working again when you have an assignment due, and on that note!

!!UPDATE!!

Had I read more carefully I would have found this link to a better solution. Basically it tells you how to remove the 64bit part of the java excitable so NetBeans can only use the 32bit version. The steps are this:

Open Terminal.app, become root (sudo -s) and run the following commands.
(The ‘#’ is the shell prompt showing that we’re running as root)
# cp -p /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home/bin/java /tmp/java_original_binary
# lipo /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home/bin/java -remove x86_64 -output /tmp/java
# cat /tmp/java > /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home/bin/java

Thanks to Noah Williamson for the tip

*More update*

Apple software update ran this morning and installed a java update. This removed the fix and stopped netbeans opening again. Applying the fix was marginally more tricky this time – the final command failed because of permission issues. The work around I found was to go to /tmp/java and manually allow read and write to everyone and to do the same for /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home/bin/java . I’m not sure if you need to do the first one but you defiantly need to do the second. When done I set the permissions back to how they were. now I’m able to work again.

Visualize txt files with particles

max — Sun, 06 Jul 2008 21:41:01 +0000

learning Java rolls on. Building on my last experiment I have created an app that can read in a text file and then map the words to particles ordered by their number of occurrences within the text.

javagraphics

You need to pass a link to a txt file when launching from the command line like this

java -jar javagraphics.jar someText.txt

The bible is easy to get hold of as a txt file as was war and peace. This is still very much a badly written toy but I hope to develop it more.

Giving Java a go!

max — Sun, 29 Jun 2008 21:04:49 +0000

Thought I would give Java a go today, wanted to try something I might do with flash and compare to see how much harder/time consuming it might be Vs how much more power you can get from Java. Well first results are in, and I was surprised to find it didn’t take much longer and was MUCH more powerful. I was using NetBeans which is a very nice IDE. The piece is a particle field and I was originaly able to crank it up to 500,000 particles, the frame rate dropped but it was still under a frame a second on a 2.16 ghz Macbook Pro. I tried for a 1,000,000 partcles but it ran out of heap space, although I think there is probably a way around that. Well compared to flash this is amazing you could never get performance like that.
It seems to me that not everything is going to be as easy as in flash but that the performance will make up
for the difficulty. I will have to leave it there for now but hope to play some more soon!