I want to try BuildCraft 8. So I wanted to compile it. But an error occurs.

buildcraft/transport/pipe/behaviour/PipeBehaviourSeparate.java:21: error: cannot find symbol
return other.getClass() == getClass();

symbol: method getClass()
location: class PipeBehaviourSeparate
:compileJava FAILED

I'm on Linux/Debian,I use java8 and compile with Gradle.

That sounds like you haven't cloned the API submodule.

The commands are git submodule init and git submodule update