I'm trying to get started writing mods, using this tutorial. I've done everything up through section 1.1. Gradle was successfully built, as far as I can tell. However, whenever I run minecraft as described in the tutorial, it first shows the following warnings in the Eclipse console:

Spoiler

2020-04-20 19:01:53,517 main WARN Advanced terminal features are not available in this environment
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by net.minecraftforge.userdev.LaunchTesting (file:/C:/Users/bildra01/.gradle/caches/forge_gradle/minecraft_user_repo/net/minecraftforge/forge/1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3/forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar) to method java.lang.ClassLoader.initializePath(java.lang.String)
WARNING: Please consider reporting this to the maintainers of net.minecraftforge.userdev.LaunchTesting
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

It then proceeds with launch, before ultimately crashing and printing the following crash report:

Spoiler

---- Minecraft Crash Report ----
// Uh... Did I do that?

Time: 4/20/20, 6:52 PM
Description: Initializing game

java.lang.ExceptionInInitializerError: null
    at net.minecraftforge.fml.ModLoader.<init>(ModLoader.java:121) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading}
    at net.minecraftforge.fml.ModLoader.get(ModLoader.java:146) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading}
    at net.minecraftforge.fml.client.ClientModLoader.lambda$begin$2(ClientModLoader.java:97) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at net.minecraftforge.fml.client.ClientModLoader.lambda$createRunnableWithCatch$5(ClientModLoader.java:113) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at net.minecraftforge.fml.client.ClientModLoader.begin(ClientModLoader.java:97) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at net.minecraft.client.Minecraft.<init>(Minecraft.java:393) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
    at net.minecraft.client.main.Main.main(Main.java:141) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?] {}
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:?] {}
    at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?] {}
    at java.lang.reflect.Method.invoke(Method.java:567) ~[?:?] {}
    at net.minecraftforge.userdev.FMLUserdevClientLaunchProvider.lambda$launchService$0(FMLUserdevClientLaunchProvider.java:55) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.Launcher.run(Launcher.java:81) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.Launcher.main(Launcher.java:65) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at net.minecraftforge.userdev.LaunchTesting.main(LaunchTesting.java:102) [forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {}
Caused by: java.lang.IllegalStateException: Failed to resolve consumer event type: net.minecraftforge.fml.network.simple.SimpleChannel$$Lambda$2200/0x00000008017f6440@64710a6
    at net.minecraftforge.eventbus.EventBus.addListener(EventBus.java:194) ~[eventbus-2.0.0-milestone.1-service.jar:?] {}
    at net.minecraftforge.eventbus.EventBus.addListener(EventBus.java:161) ~[eventbus-2.0.0-milestone.1-service.jar:?] {}
    at net.minecraftforge.eventbus.EventBus.addListener(EventBus.java:156) ~[eventbus-2.0.0-milestone.1-service.jar:?] {}
    at net.minecraftforge.eventbus.EventBus.addListener(EventBus.java:151) ~[eventbus-2.0.0-milestone.1-service.jar:?] {}
    at net.minecraftforge.fml.network.NetworkInstance.addListener(NetworkInstance.java:65) ~[?:?] {re:classloading}
    at net.minecraftforge.fml.network.simple.SimpleChannel.<init>(SimpleChannel.java:56) ~[?:?] {re:classloading}
    at net.minecraftforge.fml.network.simple.SimpleChannel.<init>(SimpleChannel.java:49) ~[?:?] {re:classloading}
    at net.minecraftforge.fml.network.NetworkRegistry$ChannelBuilder.simpleChannel(NetworkRegistry.java:409) ~[?:?] {re:classloading}
    at net.minecraftforge.fml.network.NetworkInitialization.getHandshakeChannel(NetworkInitialization.java:38) ~[?:?] {re:classloading}
    at net.minecraftforge.fml.network.FMLNetworkConstants.<clinit>(FMLNetworkConstants.java:48) ~[?:?] {re:classloading}
    ... 18 more

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Render thread
Stacktrace:
    at net.minecraftforge.fml.ModLoader.<init>(ModLoader.java:121)
    at net.minecraftforge.fml.ModLoader.get(ModLoader.java:146)
    at net.minecraftforge.fml.client.ClientModLoader.lambda$begin$2(ClientModLoader.java:97)
    at net.minecraftforge.fml.client.ClientModLoader.lambda$createRunnableWithCatch$5(ClientModLoader.java:113)
    at net.minecraftforge.fml.client.ClientModLoader.begin(ClientModLoader.java:97)
    at net.minecraft.client.Minecraft.<init>(Minecraft.java:393)

-- Initialization --
Details:
Stacktrace:
    at net.minecraft.client.main.Main.main(Main.java:141)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:567)
    at net.minecraftforge.userdev.FMLUserdevClientLaunchProvider.lambda$launchService$0(FMLUserdevClientLaunchProvider.java:55)
    at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37)
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54)
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72)
    at cpw.mods.modlauncher.Launcher.run(Launcher.java:81)
    at cpw.mods.modlauncher.Launcher.main(Launcher.java:65)
    at net.minecraftforge.userdev.LaunchTesting.main(LaunchTesting.java:102)

-- System Details --
Details:
    Minecraft Version: 1.15.2
    Minecraft Version ID: 1.15.2
    Operating System: Windows 10 (amd64) version 10.0
    Java Version: 12.0.2, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode, sharing), Oracle Corporation
    Memory: 310228872 bytes (295 MB) / 914358272 bytes (872 MB) up to 2122317824 bytes (2024 MB)
    CPUs: 4
    JVM Flags: 1 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump
    Launched Version: MOD_DEV
    Backend library: LWJGL version 3.2.2 build 10
    Backend API: Intel(R) HD Graphics 620 GL version 4.6.0 - Build 26.20.100.6859, Intel
    GL Caps: Using framebuffer using OpenGL 3.0
    Using VBOs: Yes
    Is Modded: Definitely; Client brand changed to 'forge'
    Type: Client (map_client.txt)
    CPU: 4x Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz
[m[32m[18:52:03] [Render thread/INFO] [STDOUT/]: [net.minecraft.util.registry.Bootstrap:printToSYSOUT:109]: #@!@# Game crashed! Crash report saved to: #@!@# C:\Users\bildra01\Desktop\Horrible-Blocks-Mod\run\.\crash-reports\crash-2020-04-20_18.52.03-client.txt
[m

What am I doing  wrong, and how can I fix it? I'm relatively familiar with Eclipse and Java (I'm just finishing my second semester of a Java course, so at least I hope so...).

Thanks in advance!

You need to use Java 8 for modding. If you get further errors after switching to Java 8, you may have to reset your Gradle cache.

I switched my JDK to Java 8. That got rid of the warnings at the beginning, but the game still crashes. However, it gives a different reason. Here is the crash log:

Spoiler

---- Minecraft Crash Report ----
// I let you down. Sorry

Time: 4/20/20 11:44 PM
Description: Initializing game

java.lang.NoSuchMethodError: java.nio.ByteBuffer.rewind()Ljava/nio/ByteBuffer;
    at net.minecraft.client.MainWindow.loadIcon(MainWindow.java:188) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at net.minecraft.client.MainWindow.setWindowIcon(MainWindow.java:152) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at net.minecraft.client.Minecraft.<init>(Minecraft.java:380) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
    at net.minecraft.client.main.Main.main(Main.java:141) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_251] {}
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_251] {}
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_251] {}
    at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_251] {}
    at net.minecraftforge.userdev.FMLUserdevClientLaunchProvider.lambda$launchService$0(FMLUserdevClientLaunchProvider.java:55) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.Launcher.run(Launcher.java:81) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.Launcher.main(Launcher.java:65) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at net.minecraftforge.userdev.LaunchTesting.main(LaunchTesting.java:102) [forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {}

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Render thread
Stacktrace:
    at net.minecraft.client.MainWindow.loadIcon(MainWindow.java:188)
    at net.minecraft.client.MainWindow.setWindowIcon(MainWindow.java:152)
    at net.minecraft.client.Minecraft.<init>(Minecraft.java:380)

-- Initialization --
Details:
Stacktrace:
    at net.minecraft.client.main.Main.main(Main.java:141)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at net.minecraftforge.userdev.FMLUserdevClientLaunchProvider.lambda$launchService$0(FMLUserdevClientLaunchProvider.java:55)
    at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37)
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54)
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72)
    at cpw.mods.modlauncher.Launcher.run(Launcher.java:81)
    at cpw.mods.modlauncher.Launcher.main(Launcher.java:65)
    at net.minecraftforge.userdev.LaunchTesting.main(LaunchTesting.java:102)

-- System Details --
Details:
    Minecraft Version: 1.15.2
    Minecraft Version ID: 1.15.2
    Operating System: Windows 10 (amd64) version 10.0
    Java Version: 1.8.0_251, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
    Memory: 542055496 bytes (516 MB) / 916455424 bytes (874 MB) up to 1886912512 bytes (1799 MB)
    CPUs: 4
    JVM Flags: 1 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump
    Launched Version: MOD_DEV
    Backend library: LWJGL version 3.2.2 build 10
    Backend API: Intel(R) HD Graphics 620 GL version 4.6.0 - Build 26.20.100.6859, Intel
    GL Caps: 
    Using VBOs: Yes
    Is Modded: Definitely; Client brand changed to 'forge'
    Type: Client (map_client.txt)
    CPU: <unknown>
[m[32m[23:44:18] [Render thread/INFO] [STDOUT/]: [net.minecraft.util.registry.Bootstrap:printToSYSOUT:109]: #@!@# Game crashed! Crash report saved to: #@!@# C:\Users\bildra01\Desktop\Horrible-Blocks-Mod\run\.\crash-reports\crash-2020-04-20_23.44.18-client.txt
[m

If I'm being a complete idiot, I apologize. I'm not yet familiar with Gradle (or, to be perfectly honest, the inner workings of Minecraft).

As I expected, you need to reset your Gradle cache. I'm honestly not super familiar with Gradle as a whole myself, but I believe the best way to do so is to navigate to the .gradle folder in your user directory and delete the caches folder. You will have to wait for Forge to redownload and reprocess the game, but it should work perfectly afterwards.

I deleted the caches and reran the process to build them. However, I'm getting this same problem still. Here's the new crash log, in case there's a difference I'm not noticing:

Spoiler

---- Minecraft Crash Report ----
// You're mean.

Time: 4/21/20 2:01 PM
Description: Initializing game

java.lang.NoSuchMethodError: java.nio.ByteBuffer.rewind()Ljava/nio/ByteBuffer;
    at net.minecraft.client.MainWindow.loadIcon(MainWindow.java:188) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at net.minecraft.client.MainWindow.setWindowIcon(MainWindow.java:152) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at net.minecraft.client.Minecraft.<init>(Minecraft.java:380) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
    at net.minecraft.client.main.Main.main(Main.java:141) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_251] {}
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_251] {}
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_251] {}
    at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_251] {}
    at net.minecraftforge.userdev.FMLUserdevClientLaunchProvider.lambda$launchService$0(FMLUserdevClientLaunchProvider.java:55) ~[forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.Launcher.run(Launcher.java:81) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at cpw.mods.modlauncher.Launcher.main(Launcher.java:65) [modlauncher-5.0.0-milestone.4.jar:?] {}
    at net.minecraftforge.userdev.LaunchTesting.main(LaunchTesting.java:102) [forge-1.15.2-31.1.0_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {}

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Render thread
Stacktrace:
    at net.minecraft.client.MainWindow.loadIcon(MainWindow.java:188)
    at net.minecraft.client.MainWindow.setWindowIcon(MainWindow.java:152)
    at net.minecraft.client.Minecraft.<init>(Minecraft.java:380)

-- Initialization --
Details:
Stacktrace:
    at net.minecraft.client.main.Main.main(Main.java:141)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at net.minecraftforge.userdev.FMLUserdevClientLaunchProvider.lambda$launchService$0(FMLUserdevClientLaunchProvider.java:55)
    at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37)
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54)
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72)
    at cpw.mods.modlauncher.Launcher.run(Launcher.java:81)
    at cpw.mods.modlauncher.Launcher.main(Launcher.java:65)
    at net.minecraftforge.userdev.LaunchTesting.main(LaunchTesting.java:102)

-- System Details --
Details:
    Minecraft Version: 1.15.2
    Minecraft Version ID: 1.15.2
    Operating System: Windows 10 (amd64) version 10.0
    Java Version: 1.8.0_251, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
    Memory: 236177984 bytes (225 MB) / 899153920 bytes (857 MB) up to 1886912512 bytes (1799 MB)
    CPUs: 4
    JVM Flags: 1 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump
    Launched Version: MOD_DEV
    Backend library: LWJGL version 3.2.2 build 10
    Backend API: Intel(R) HD Graphics 620 GL version 4.6.0 - Build 26.20.100.6859, Intel
    GL Caps: 
    Using VBOs: Yes
    Is Modded: Definitely; Client brand changed to 'forge'
    Type: Client (map_client.txt)
    CPU: <unknown>
[m[32m[14:01:11] [Render thread/INFO] [STDOUT/]: [net.minecraft.util.registry.Bootstrap:printToSYSOUT:109]: #@!@# Game crashed! Crash report saved to: #@!@# C:\Users\bildra01\Desktop\Horrible-Blocks-Mod\run\.\crash-reports\crash-2020-04-21_14.01.11-client.txt
[m

 

Huh, I was convinced that would fix it...

I did some more digging and found out that Lex made a workaround for the latest versions of Forge, so try updating to the latest version (31.1.47 at the time of writing) and see if that fixes it.

That finally worked! that's what I get for selecting the recommended version instead of the latest. Thank you so much for your help!