Connection to HID USB device (keyboard and mouse) in android

Connection to HID USB device (keyboard and mouse) in android

I am trying to develop an android app for managing HID devices. Using
UsbManager and getDeviceList() provided from google sdk and following the
android sdk official documentation, I am in condition to set up a
connection with various devices (as pen drives, external HD, usb "phone"
etc.), but I am not in condition to connect the android smartphone to usb
mouse and keyboard: they are not listed by getDeviceList() method at all,
even if android OS is able to recognize and use it! In fact, I am using
the mouse and the keyboard with the mobile without any problem. I am using
for my test a samsung galaxy S4. Any one can give me some explanation
about this, please? Thank you!
p.s.: I also tried to use app like "USB Host Diagnostic" and the result is
the same: the app can not recognize connected mouse and keyboard...but
mouse and keyboard are still working perfectly...