How to modify a specific class in jar file ?

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 !!)

Could you please clarify: do you want to run your project or a project from an independent jar?
In the first case just select Run Project from the project context menu or select a class with main method and click Run File in the class context menu.
Regarding the second case:
- I don't think there is such a feature in the IDE (running third party jars is not an IDE function). Could you explain why you need this?

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How to modify a specific class in jar file ?

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