Hi i am trying to use eclipse cause i want to mess with minecraft's code and i wanna see what i can do but minecraft isnt launching it just says [14:07:11] [Client thread/INFO]: Setting user: Player447 and then nothing happens

You means modding ? If yes just follow https://mcforge.readthedocs.io/en/1.15.x/

If you means mcp, unzip the version that you want (http://www.modcoderpack.com/website/releases) then follow a tutorial

If you think it's eclipse, check if you're using JDK8 (not 13)

6 hours ago, oke said:

Hi i am trying to use eclipse cause i want to mess with minecraft's code and i wanna see what i can do but minecraft isnt launching it just says [14:07:11] [Client thread/INFO]: Setting user: Player447 and then nothing happens

You always need to send the crash-log

First when i run it in debug mode nothing happens and when i press resume i get this

Exception in thread "Client thread" [23:46:08] [Client thread/INFO]: [STDERR]: java.lang.Error: Unresolved compilation problem: 
    Type mismatch: cannot convert from List<Object> to List<T>

[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.util.math.Cartesian$GetList.apply(Cartesian.java:57)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.util.math.Cartesian$GetList.apply(Cartesian.java:1)
[23:46:08] [Client thread/INFO]: [STDERR]:     at com.google.common.collect.Iterators$7.transform(Iterators.java:750)
[23:46:08] [Client thread/INFO]: [STDERR]:     at com.google.common.collect.TransformedIterator.next(TransformedIterator.java:47)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.state.BlockStateContainer.<init>(BlockStateContainer.java:73)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.Block.createBlockState(Block.java:1113)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.Block.<init>(Block.java:305)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.Block.<init>(Block.java:314)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.BlockAir.<init>(BlockAir.java:18)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.Block.registerBlocks(Block.java:1171)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.init.Bootstrap.register(Bootstrap.java:500)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.client.Minecraft.<init>(Minecraft.java:439)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.client.main.Main.main(Main.java:115)
[23:46:08] [Client thread/INFO]: [STDERR]:     at Start.main(Start.java:11)
Java HotSpot(TM) 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release

no i dont wanna make a mod i just want to mess with the games code to see what funny stuff i can do

10 hours ago, oke said:

no i dont wanna make a mod i just want to mess with the games code to see what funny stuff i can do

I think this is more data/resources packs. If you want replace all animals with others then you need was definitively a mod or at least knowledge in Json.

On 3/16/2020 at 3:46 PM, oke said:

First when i run it in debug mode nothing happens and when i press resume i get this

Exception in thread "Client thread" [23:46:08] [Client thread/INFO]: [STDERR]: java.lang.Error: Unresolved compilation problem: 
    Type mismatch: cannot convert from List<Object> to List<T>

[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.util.math.Cartesian$GetList.apply(Cartesian.java:57)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.util.math.Cartesian$GetList.apply(Cartesian.java:1)
[23:46:08] [Client thread/INFO]: [STDERR]:     at com.google.common.collect.Iterators$7.transform(Iterators.java:750)
[23:46:08] [Client thread/INFO]: [STDERR]:     at com.google.common.collect.TransformedIterator.next(TransformedIterator.java:47)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.state.BlockStateContainer.<init>(BlockStateContainer.java:73)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.Block.createBlockState(Block.java:1113)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.Block.<init>(Block.java:305)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.Block.<init>(Block.java:314)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.BlockAir.<init>(BlockAir.java:18)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.block.Block.registerBlocks(Block.java:1171)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.init.Bootstrap.register(Bootstrap.java:500)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.client.Minecraft.<init>(Minecraft.java:439)
[23:46:08] [Client thread/INFO]: [STDERR]:     at net.minecraft.client.main.Main.main(Main.java:115)
[23:46:08] [Client thread/INFO]: [STDERR]:     at Start.main(Start.java:11)
Java HotSpot(TM) 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release

im getting the same error and dont know how to fix it??