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How to modify a specific class in jar file ?

843798Oct 24 2007
I've downloaded a jar file for applet, the jar file works fine... but when I extract a specific class file from the jar file and just recompie it from my IDE (Eclipse) without even making any change and then compress again jar class files including the new modified class file instead of the old one (without any modifications to other classes)... then I get

(NoSuchMethodError ) exception whenever methods of other classes are invoked from within the modified class !!
...The manifist file of the jar file reads the following line

Created-By: 1.4.0_01 (Sun Microsystems Inc.)

I thought it means that jar class files were built using JDK 1.4.0_01 ...I used JDK 1.5.0_01 in my IDE...
I thought JRE incompatiblity was the reason of the problem so I downloaded JRE 1.4.0_01 and used it to build this class file... but i got the same exception..
so what is the main reason of the problem ? ...should I make changes to IDX files accompanying applet jar files ??
If no, then how can I overcome this problem and be able to change and rebuild a specific class from this jar file ?

(I cannot rebuild all classes in jar because there are errors I cannot resolve !!)
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Locked on Nov 21 2007
Added on Oct 24 2007
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