Blackdown java and offical sun java
 

Is the blackdown java JDK compatible with the offical sun microsystems version of java. ie I can develop applications using both of them. As at uni I use sun but at home I use blackdown. The sun micorsystems amd64 bit version doesn't come with a browser plugin.

Well for one thing Blackdown only has Java 1.4, while Sun uses Java 1.5; so you may want to be careful about that.

but as long as am careful about the functions am using then it's ok then right?

No, Java 1.5 uses a new bytecode format. If you compile on Java 1.5, the class files will not run on 1.4 and before; unless you avoid using any new Java 1.5 language features and then use "javac -target 1.4" or something like that to make it compile into 1.4 format. If you DO need to use new Java 1.5 language features, you can check out Retroweaver.

ah thanks alot thankfully i dont think i would need any 1,5 specifics. I will be able to avoid them in my case phew

As far as I know, Blackdown Java is not a different JVM implementation, but is actually Sun's code ported to Linux.
It was interesting before Sun released JREs/JDKs for Linux too, but these days I'm wondering if there is really a difference (outside differing subrelease version numbering, and Java 5.0 and 6.0 missing in Blackdown).