1)
oh! I never noticed that the "MIN_VALUE" for Double and Integer are so
different!
Yes, for Double, MIN_VALUE is the smallest value >0,
while for Integer, MIN_VALUE is the smallest value.
I just wonder why they make it this way? So confusing
....................
Thanks Patricia to point it out!
And, yes, if I use NEGATIVE_INFINITY, it will work out as what we
expect.

2)
as to Roedy's suggestion, add Double to the arraylist will be faster
(the above code is just a hello world).
But do you have any idea of this situation (which is more close to my
real situation):
the Strings in ArrayList come from some records (sorry, I can not
change the input strings), like: "xyz_3.4_zex", "xex_6.5_seze", etc. in
which, '_' is a divider and the middle item is a double type value. The
first and third item are some strings (can be arbitrary long having any
chars except the divider char '_'). It is required to sort these
strings based on the middle double value, and output the strings (so we
can not just take out the double value and replace the string with a
Double).
Any idea how to sort these kind of thing?
I am not expert, and the only way I can think out (if not using the new
Comparator() as in above code) is to map each string back to a class
similar to ("xyz_3.4_zex" will become):
class X
{
String S1 = "xyz";
Double D = new Double(3.4);
String S2 = "zex";
}
and add it to a arraylist, sort them base on Double D, and convert the
class back to the string. But I am thinking that is slow too.

Thank you guys a lot. :-)