Pressing and releasing the numpad-5 key produces X-events when the numlock 
is off, but does not produce keypressed/keyreleased Java events.  We ought to 
give some indication that a key has been pressed/released.  However, we should 
not produce a keytyped event unless a unicode character is generated.  

Here is the result of running xev on a sparc (Solaris 9 update 2) 
KeyPress event, serial 23, synthetic NO, window 0x1680001,
    root 0x31, subw 0x0, time 2576694049, (163,460), root:(475,532),
    state 0x0, keycode 99 (keysym 0xffdc, F31), same_screen YES,
    XLookupString gives 0 characters:  ""

KeyRelease event, serial 23, synthetic NO, window 0x1680001,
    root 0x31, subw 0x0, time 2576694147, (163,460), root:(475,532),
    state 0x0, keycode 99 (keysym 0xffdc, F31), same_screen YES,
    XLookupString gives 0 characters:  ""

A synonym for XK_F31 is XK_R11 which doesn't help much.   
keysymdef.h says: 
#define XK_F31                  0xFFDC
#define XK_R11                  0xFFDC

Here is a section of my xmodmap: 
keycode  74 = Multi_key
keycode  75 = F27 F27 KP_7 Home
keycode  76 = Up F28 KP_8
keycode  77 = F29 F29 KP_9 Prior
keycode  78 = F24 F24 KP_Subtract
keycode  79 = F17 F17 SunOpen
keycode  80 = F18 F18 SunPaste
keycode  81 = End
keycode  82 =
...
keycode  96 = Return
keycode  97 = KP_Enter
keycode  98 = Left F30 KP_4
keycode  99 = F31 F31 KP_5
keycode 100 = Right F32 KP_6
keycode 101 = KP_Insert KP_Insert KP_0
keycode 102 = F19 F19 Find
keycode 103 = Prior

Interestingly, my Linux xmodmap says: 
keycode  83 = KP_Left KP_4
keycode  84 = KP_Begin KP_5
keycode  85 = KP_Right KP_6

keysymdef.h 
#define XK_KP_Page_Down         0xFF9B
#define XK_KP_End               0xFF9C
#define XK_KP_Begin             0xFF9D
#define XK_KP_Insert            0xFF9E
#define XK_KP_Delete            0xFF9F 

#define XK_Prior                0xFF55  /* Prior, previous */
#define XK_Page_Up              0xFF55
#define XK_Next                 0xFF56  /* Next */
#define XK_Page_Down            0xFF56
#define XK_End                  0xFF57  /* EOL */
#define XK_Begin                0xFF58  /* BOL */

Note that on Solaris, the numpad /*- keys produce F24, F25, F26, but in 
Java we map them to /*- anyway.  dtterm acts similar to Java.  dtterm also 
produces a 5 when the numpad 5 key is pressed, but numlock is off (F31).  
However, some applications, e.g. bugtraq, do not produce /*-5 when numlock 
is off.  StarOffice accepts - but not /* .