Java 9 is a major feature release. The following summarizes features and enhancements in Java  language version 9.

• Allow @SafeVargs annotation on private instance methods
• Allow final variables to be used as resources in the try-with-resources statement
• Allow the use of diamond with anonymous classes if the type of the inferred type is denotable.
• Complete the removal of underscore from the set of legal identifier names. This was started in java version 8.
• Add support for private interface methods.

Reference : https://docs.oracle.com/javase/9/language/toc.htm#JSLAN-GUID-16A5183A-DC0D-4A96-B9D8-AAC9671222DD