Re: Could not find the main class. Program will exit.

Lew <>
Sat, 30 Jan 2010 15:05:16 -0500
Knute Johnson wrote:

I'm curious on a Windows system how you use two JDKs or JREs on the
same computer?

Essentially the same way as on any other platform - you install them into
different directories.

markspace wrote:

First, Sun designs their installer and JRE/JDKs to co-exist peacefully,
so it doesn't happen on accident. I only use the auto-update mechanism

Tell each installation a different directory for its Java.

It is probably best to install them in version order, save that you would want
to install your preferred version last.

from Sun, and my Program Files/Java directory looks like this:

Brenden@Homer /cygdrive/c/Program Files/Java
$ ls
jdk1.5.0_12 jdk1.6.0_17 jre1.6.0 jre1.6.0_03 jre1.6.0_07
jdk1.5.0_22 jre1.5.0_12 jre1.6.0_01 jre1.6.0_04 jre1.6.0_16
jdk1.6.0 jre1.5.0_22 jre1.6.0_02 jre1.6.0_05 jre6

(I'm running Cygwin, btw.)

However, the technique is a Windows technique, not a Cygwin technique.

In NetBeans, there's a little drop down that lets you choose your target
environment. I can switch between 1.5 and 1.6 on my copy of NetBeans.
This requires the runtimes be available, which is why I need both.

Eclipse has a JRE platforms option in its preferences dialog that serves the
same purpose.

In both IDEs, each project can target a different platform.

For deploying applications, you can specify Specification-Version in the
manifest to set the minimum compatible version of JRE required. I think
at least for applets, you can set the maximum version too, allowing to
to specify that only 1.5 would be used even if 1.6 is available, for

For the latter reason, the OP may need both JREs on his system.


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