Have you already looked at this website http://iotdk.intel.com/docs/master/mraa/java/classmraa_1_1_uart.html . This site has the Public Member Functions for the UART class. Have you already considered using the setMode ()? Is this what you need?
That function is not directly implemented in the UART class.
As a workaround, you can try to edit the uart functions for read and write string. You can try to edit these files so that they write and read bytes based on the string functions.
https://github.com/intel-iot-devkit/mraa/blob/master/api/mraa/uart.hpp for the writeStr function
for the mraa_uart_write function.