RFR: JDK-8013900: More warnings compiling jaxp.

[Daniel Fuchs]

This is a fix for JDK-8013900: More warnings compiling jaxp.

<http://cr.openjdk.java.net/~dfuchs/JDK-8013900/webrev.00/>

Although the title might suggest a trivial fix, it goes a bit
beyond a simple warning fix because the root cause of those warnings
is that some classes redefine equals without redefining
hashCode() - and devising a hashCode() method that respects
its contract with equals() is not always trivial.

The proposed fix adds hashCode() methods where necessary, ensuring
that:

    if (o1.equals(o2) == true), then (o1.hashCode() == o2.hashCode())

The fix also contains some cosmetic/sanity changes - like:

1. adding @Override wherever NetBeans complained they were
missing - and
2. replacing StringBuffer with StringBuilder when
possible.
3. In one instance, AbstractDateTimeDV.java I also had
to reformat the whole file (due to its weird original formatting)
- apologies for the noise it causes in the webrev.
4. I also removed a couple of private isEqual(obj1, obj2) methods
replacing them by calls to Objects.equals(obj1, obj2) which did
the same thing.
5. finally I refactored some of the existing equals (e.g. replacing
    try { (Sometype) other.... } catch (ClassCastException x) {
    return false; } with use of 'other instanceof Sometype'...)

There are however a couple of more serious changes that could
deserve to be reviewed in more details:

a. The new implementation of hashCode() in
    AbstractDateTimeDV.DateTimeData, where I had to figure
    out a way to convert the date to UTC so that the hashCode() would
    match equals:

    AbstractDateTimeDV.java: lines 972-992

and b. in PrecisionDecimalDV.XPrecisionDecimal - where I had to
invent a canonical string representation to devise a hashCode that
would match equals:

    PrecisionDecimalDV.java: lines 147-237

For this last one - I have added a new test in jdk/test to check
the implementation of the new canonical String representation
for hashCode() - since the code of that is not trivial.


best regards,

-- daniel