OpenJDK 18 GA is scheduled for 2022/03/22.
So we should add support for that with the next PMD version.

The following JEPs are important for us:

• JEP 413: Code Snippets in Java API Documentation
• JEP 420: Pattern Matching for switch (Second Preview)

In summary: https://javaalmanac.io/jdk/18/

This issue is similar to the previous: #3366

EA Release of JDK18 is available here: http://jdk.java.net/18/

Tasks:

• Add new java language versions 18 and 18-preview.
  • 18 is the new default
  • update also java.md and ant.md
  • and add in KotlinTestingDsl.kt.
• Update asm dependency:
  • https://search.maven.org/artifact/org.ow2.asm/asm
  • https://asm.ow2.io/versions.html
  • Update net.sourceforge.pmd.lang.java.typeresolution.visitors.PMDASMVisitor.ASM_API if necessary
  • ✔️ we already use the latest version 9.2
• Update implementation for "Pattern Matching for switch" (406, 420) as a preview feature, mark AST classes as experimental, if needed.
  • https://openjdk.java.net/jeps/406 (preview 1)
  • https://openjdk.java.net/jeps/420 (preview 2)
  • http://cr.openjdk.java.net/~gbierman/jep420/jep420-20211208/specs/patterns-switch-jls.html
• Remove java language version 16-preview
  • update also java.md and ant.md
  • update KotlinTestingDsl.kt
• Write release notes
• Prepare merge for PMD 7 -> [java] Merge "Support JDK 18 #3837" into pmd/7.0.x #3838