Java 15 introduced several new features, enhancements, and improvements to the language, libraries, and tools. Some of the notable features and changes in Java 15 include:

1. Sealed Classes: Java 15 introduced the “sealed classes” feature, which allows developers to restrict which other classes or interfaces can extend or implement a class. This helps in creating more maintainable and predictable class hierarchies.

2. Pattern Matching (Preview): The “pattern matching” feature was introduced as a preview feature in Java 15. It includes enhancements to the instanceof operator and introduces the switch expression, which simplifies code by eliminating boilerplate code related to type casting.

3. Text Blocks: Java 15 improved text blocks by allowing you to remove trailing white spaces. Text blocks provide a cleaner way to write multi-line string literals in code.

4. Hidden Classes: Hidden classes were introduced as an incubator feature. They provide a way to define classes that are not discoverable by reflection, making it more challenging for attackers to exploit vulnerabilities.

5. Unix Domain Socket Support: Java 15 added support for Unix domain sockets, allowing Java applications to communicate with other processes using Unix domain sockets on Unix-based systems.

6. Deprecation of RMI Activation System: The Remote Method Invocation (RMI) Activation System was deprecated in Java 15, marking it as a candidate for removal in future Java releases. It’s recommended to use alternative technologies for remote object activation.

7. Deprecation of Applet API: The Applet API was deprecated in Java 15, signaling that it may be removed in future releases. Applets have become obsolete due to security concerns and the availability of more modern web technologies.

8. Records and Sealed Classes Enhancements: Java 15 introduced various enhancements to records (introduced in Java 14) and sealed classes. These enhancements improve the usability and functionality of these language features.

9. Library and Tool Updates: Java 15 includes updates to various libraries and tools, such as updated versions of the GraalVM compiler and improvements to the jpackage tool for packaging Java applications.