i updated a project to the latest forge version,
when i try to start the dev minecraft environment, the game crashes (doesn't even start)

where exactly is the error?

THIS is the full log:

Working Directory: C:\Users\luiss\Documents\GitHub\Forge-1.16.5-Nero
Gradle user home: C:\Users\luiss\.gradle
Gradle Distribution: Gradle wrapper from target build
Gradle Version: 6.9
Java Home: C:\Users\luiss\.p2\pool\plugins\org.eclipse.justj.openjdk.hotspot.jre.full.win32.x86_64_15.0.1.v20201027-0507\jre
JVM Arguments: None
Program Arguments: None
Build Scans Enabled: false
Offline Mode Enabled: false
Gradle Tasks: runClient


> Configure project :
Java: 15.0.1 JVM: 15.0.1+9-18(Oracle Corporation) Arch: amd64
WARNING: This project is configured to use the official obfuscation mappings provided by Mojang. These mapping fall under their associated license, you should be fully aware of this license. For the latest license text, refer below, or the reference copy here: https://github.com/MinecraftForge/MCPConfig/blob/master/Mojang.md, You can hide this warning by running the `hideOfficialWarningUntilChanged` task
WARNING: (c) 2020 Microsoft Corporation. These mappings are provided "as-is" and you bear the risk of using them. You may copy and use the mappings for development purposes, but you may not redistribute the mappings complete and unmodified. Microsoft makes no warranties, express or implied, with respect to the mappings provided here.  Use and modification of this document or the source code (in any form) of Minecraft: Java Edition is governed by the Minecraft End User License Agreement available at https://account.mojang.com/documents/minecraft_eula.

> Task :compileJava
C:\Users\luiss\Documents\GitHub\Forge-1.16.5-Nero\src\main\java\net\luis\nero\common\world\feature\ModDungeonsFeature.java:30: error: cannot find symbol
	private final Set<Block> replaceBlocks = Set.of(ModBlocks.DEEPSLATE.get(), ModBlocks.TUFF.get());
	                                            ^
  symbol:   method of(net.minecraft.block.Block,net.minecraft.block.Block)
  location: interface java.util.Set
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
1 error

> Task :compileJava FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':compileJava'.
> Compilation failed; see the compiler error output for details.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.9/userguide/command_line_interface.html#sec:command_line_warnings

BUILD FAILED in 3s
1 actionable task: 1 executed

 

46 minutes ago, diesieben07 said:

Set.of is a Java 9 feature, do not use it.

okay make sense,
and why?

14 minutes ago, diesieben07 said:

Because the Minecraft launcher ships with Java 8, so your mod will crash.

then why does forge support java 8-15?