Java 9 introduced several significant features and changes to the Java programming language. Some of the key features and improvements in Java 9 include:

1. Module System (Project Jigsaw): Java 9 introduced the module system, which allows developers to create modular and maintainable code by encapsulating classes and their dependencies into modules. This helps in better organization and separation of concerns in large-scale applications.

2. jshell (Interactive REPL): Java 9 introduced the jshell tool, which provides an interactive Read-Eval-Print Loop (REPL) environment for experimenting with Java code and testing snippets without the need to write a complete program.

3. Improved Stream API: Java 9 introduced several enhancements to the Stream API, including additional methods for working with streams more efficiently and effectively.

4. Enhanced Process API: Java 9 introduced the java.lang.ProcessHandle class, which allows you to interact with and manage native processes more effectively.

5. Compact Strings: Java 9 introduced the concept of compact strings to improve the memory efficiency of strings. This feature optimizes the storage of strings with Latin-1 characters.

6. New Factory Methods for Collections: Java 9 introduced new factory methods for creating immutable and mutable collections more easily.

7. HTTP/2 Support: Java 9 added support for the HTTP/2 protocol in the java.net.http package, making it easier to work with HTTP/2 in Java applications.

8. New Version-String Scheme: Java 9 introduced a new version-string scheme to simplify versioning and avoid common versioning-related issues.

9. Improved Garbage Collection: Java 9 introduced improvements to the garbage collection algorithms, including the Garbage-First (G1) collector.

10. Deprecation and Removal of Old Features: Java 9 deprecated and removed some older and less-used features and APIs to maintain a more streamlined and modern Java platform.