AddPresets.java

package com.seedfinding.neil.mixin;

import net.minecraft.client.gui.screen.PresetsScreen;
import net.minecraft.text.Text;
import org.spongepowered.asm.mixin.Final;
import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.Shadow;
import org.spongepowered.asm.mixin.injection.At;
import org.spongepowered.asm.mixin.injection.Inject;
import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;

import java.util.List;

@Mixin(PresetsScreen.class)
public class AddPresets {
    @Mixin(targets = {"net.minecraft.client.gui.screen.PresetsScreen$SuperflatPreset" })
    static class SuperflatPreset {
        @Final
        @Shadow
        public Text name;
    }

    @Final
    @Shadow
    private static List<SuperflatPreset> PRESETS; // this is done because net.minecraft.client.gui.screen.PresetsScreen$SuperflatPreset ain't accessible

    @Inject(method = "<init>*", at = @At("RETURN"))
    public void sortList(CallbackInfo ci) {
        System.out.println(PRESETS.size());
        System.out.println(PRESETS.get(0).name);
    }
}

@Mixin(PresetsScreen.class)
public class AddPresets {
    @Mixin(targets = {"net.minecraft.client.gui.screen.PresetsScreen$SuperflatPreset"})
    public interface SuperflatPreset {
        @Accessor
        Text getName();
    }

    @Final
    @Shadow
    private static List<?> PRESETS; // this is done because net.minecraft.client.gui.screen.PresetsScreen$SuperflatPreset ain't accessible

    //    private static List<SuperflatPreset> PRESETS; // this is done because net.minecraft.client.gui.screen.PresetsScreen$SuperflatPreset ain't accessible
//
    @Inject(method = "<init>*", at = @At("RETURN"))
    public void sortList(CallbackInfo ci) {
        System.out.println(PRESETS.size());
        System.out.println(((SuperflatPreset) PRESETS.get(0)).getName());
    }

Caused by: java.lang.IllegalAccessError: class net.minecraft.client.gui.screen.PresetsScreen$SuperflatPreset cannot access its superinterface com.seedfinding.neil.mixin.AddPresets$SuperflatPreset (net.minecraft.client.gui.screen.PresetsScreen$SuperflatPreset and com.seedfinding.neil.mixin.AddPresets$SuperflatPreset are in unnamed module of loader net.fabricmc.loader.launch.knot.KnotClassLoader @67545b57)