Problem with executing Postgres function from javacode

boolean retval = false;
StringBuffer sql = new StringBuffer();
ResultSet rst = null;
conn.setAutoCommit(false);
sql.append("select assign_customers() ");
Statement st = conn.createStatement();
retval = st.execute(sql.toString());

When I execute assign_customers through pgadmin , it works. But within javacode
I do not get any error nor the result gets executed
Assign_customers doesnt return any values.

Can somebody help me to call this function from javacode