So I just installed forge for 1.7.10 and 1.12.2 AND optifine for 1.14, but every time I try to launch minecraft it gives out this error:

OpenJDK 64-Bit Server VM warning: Option UseConcMarkSweepGC was deprecated in version 9.0 and will likely be removed in a future release.
Exception in thread "main" java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap')
    at net.minecraft.launchwrapper.Launch.<init>(Launch.java:34)
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

MinecraftForge needs java 8 and you have 9 or higher. Try installing java 8 and use it. If you don't get it work with the default launcher i'd recommend MultiMC. There you can set the java installation in preferences/java and installation of forge is much easier.

| improve this answer | |
  • thanks! it worked. – Daniel Cunha May 27 '19 at 16:35
  • FYI: Currently, MultiMC doesn't support Forge past 1.13. – Avernaism May 3 at 1:05
  • @Avernaism it actually does since 29 of March. See here – noeppi noeppi May 3 at 17:04

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