You should be able to run 1.3 classes under 1.4. You MAY not be able to run 1.4 apps under 1.3 depending on what classes were used. You CAN have both JRE's implemented on the same machine, and just switch the classpath etc when you execute the apps. Make a .bat file and put your command to invoke the application in there. Include the classpath to the version of the JRE that you want as a parameter when you invoke the app. Then just run the bat file.
"JavaRanch, where the deer and the Certified play" - David O'Meara
Two gotchas--> 1- if you're on Windows, you might have to mess around with your registry to switch between versions. 2- if your JDK 1.3 app uses a method called .assert() you'll have issues as this is a new key word in 1.4
The assert thing is something that could cause a problem. However, I believe if you explicitly name your classpath in your execution statement, that it overrides the classpath parameter in the Windows Registry. This is a .bat file that we use to insure that the user in picking up the version of the JRE that we want: myApp.bat _______________________________________________ set E2_RUN_DIR=c:\E-Squared\ "c:\program files\javasoft\jre\1.3.1\bin\java" -classpath .;"c:\program files\javasoft\jre\1.3.1\lib\rt.jar";wleclient.jar;m3envobj.jar;jh.jar;evpa-live.jar;report11_pro.jar;report12_pro.jar;javaARM.jar -DWSS_DIR=%E2_RUN_DIR% com.myCompany.evpa.wss.Wss myApp.properties ------------------------------------------------ invoke the Wss.class file using the myApp.properties file as a parameter.