Question : why JDK and JRE must be same version Can we mix and match

I like also to understand why there is a conflict when  jdk1.5.0_10 and jre1.6 are on the same directory,without  jre1.5 not  in the directory. There is a message like "multiple vm process run at the same time" displays when some application that need runtime java is run

Answer : why JDK and JRE must be same version Can we mix and match

One thing that may not be clear to you is that a JRE is a proper subset of a JDK.
This means that all of the functionality (and files) that exist in a JRE also exist in
a JDK.  The JDK has the additional files necessary to compile your Java Applications.

If you want to install a 1.6 JRE, or JDK you can do so, just not to the same directory that exists and contains your 1.5 JDK or JRE.

Install it to a separate directory, and you should be just fine.

Right now, I have a 1.4.2 JDK directory, and a 1.5 JDK directory, and a 1.6 JDK
directory all installed (and usable) on my machine.  To use one, I just set up
my environment variables to point to the appropriate directory structure.

Hopefully this helps.