The default standard output device in the Oracle Java virtual machine (JVM) is the current trace file.
If you want to reroute all standard output from a program executing in the server--output from any System.out.println() calls, for example--to a user screen, you can execute the SET_OUTPUT() procedure of the DBMS_JAVA package as in the following example. Input the buffer size in bytes (10,000 bytes in this case).
sqlplus> execute dbms_java.set_output(10000);
Output exceeding the buffer size will be lost.
If you want your code executing in the server to expressly output to the user screen, you can also use the PL/SQL DBMS_OUTPUT.PUT_LINE() procedure instead of the Java System.out.println() method.