Compact Strings have introduced in Java 9 to replace Java 6's Compressed Strings. Its implementation uses byte array instead of char array and a new field coder has introduced to identify whether it is LATIN1 or UTF16 format while Compressed Strings have introduced in Java 6 that can be used byte array for one byte per character, and continued to use char array for two bytes per character, previously it can be turned on using -XX: + UseCompressedStrings.
Unlike Compressed Strings, Compact Strings do not require un-packing or re-packing. Hence Compact String gives better performance at runtime.
Compressed Strings are not enabled by default in Java 6, and need to be set explicitly using below command
Compact Strings are enabled by default in Java 9 by using the below command