Today, I found Oracle DB's username and password are case sensitive.

Please see the cases.

 

The account information are like these,

username     password

SYS                ADMIN

SRDEMO       ORACLE

 

The results to try,

command                                                   login result

sqlplus sys/admin@orcl                         success

sqlplus SYS/ADMIN@orcl                       success

sqlplus srdemo/oracle@orcl                 fail

sqlplus SRDEMO/ORACLE@ORCL    success

Please notice that it is the same case you are connecting with JDBC driver.