Prior to Java 9, the default KeyStore type was Java KeyStore (JKS). The current Java platform uses PKCS as the default KeyStore type, more specifically, PKCS12.
PKCS is an acronym for Public Key Cryptography Standards.
This change to PKCS provides stronger cryptographic algorithms as compared to JKS. As you would expect, JDK 9, 10, and 11 are still compatible with JKS to support previously developed systems.