In Java, according to the JSSE Reference Guide, there is no default for the keystore, the default for the truststore is “jssecacerts, if it exists. Otherwise, cacerts“.
A few applications use ~/.keystore as a default keystore, but this is not without problems (mainly because you might not want all the application run by the user to use that trust store).
I’d suggest using application-specific values that you bundle with your application instead, it would tend to be more applicable in general.