Change Minecraft Code?

To
- in Mods
1

I would like to change the Minecraft code a bit like changing the size of zombies and for that I always download the latest version of the Mod Coder Pack, Eclipse, then I decompile Eclipse with the workspace in the Mod Coder Pack / then eclipse, but then there's always an exclamation mark next to Client (in Eclipse) (The problem is: Project 'Client' is missing required library: 'jars / libraries / com / mojang / realms / 1.10.16 / realms-1.10. 16.jar 'Client Build path Build Path Problem AND

The project can't be built until build path errors are resolved Client Unknown Java Problem

) and when I have changed the code and want to run the program I get an error message (Error: Could not create the Java Virtual Machine. Error: A fatal exception has occurred. Program will exit).

an
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