GitHub Copilot Instructions: Java 17 to Java 21 Upgrade Guide

These instructions help GitHub Copilot assist developers in upgrading Java projects from JDK 17 to JDK 21, focusing on new language features, API changes, and best practices.

Major Language Features in JDK 18-21

Pattern Matching for switch (JEP 441 - Standard in 21)

Enhanced switch Expressions and Statements