1) Don't write code that depends on the evaluation being done in a
specific order. It's too confusing. Break it up into several statements, so
that the order is clear.
   2) They are not undefined. The Java Language Specification is pretty
good with respect to defining the order of evaluation of everything.
http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.7
   3) You can probably find out the answers by writing short programs and
testing for yourself what the results are.

   - Oliver

my answer is, you should not write code that depends on quirky things
like that. They are very likely to be wrong in a given implementation
since they are so rarely tested. Further they baffle people coming
after you.

Write what you mean clearly.

i += 2;
i *= 2;

p = a[i];

Man goes into a doctor's office and says, "Doctor, when I swing my arm
wildly above my head like this [man swings arm wildly] it hurts. Can you
help me with that?" The doctor responds with, "Don't swing your arm like
that."

In other words, don't do it. If I have something like:

    int i = 3;
    // some code here
    foo(++i, i++);

and I want to pass the values 4, 4 then leave i with the value 5 I'd just
change it to:

    int i = 3;
    // some code here
    ++i;
    foo(i, i);
    i++;

It is then obvious that I want to pass i in with the same value, twice.

In other words, use what you know will work. This has a few advantages: 1)
it is more readable, 2) a junior programmer will not get confused, 3) you
have one less thing you need to memorize about the language.