Compare a Path to a string

[Weijun Wang]

I'm profiling the new FilePermission implementation and found a lot of 
time spending on comparing a Path to a string (in fact, toString()), and 
in every case the path has only one element.

Here are them:

1. Comparing with "-":

    Path lastName = npath.getFileName();
    if (lastName != null && lastName.toString().equals("-")) { ... }

2. Comparing with "":

    p = p2.relativize(p1).normalize();
    if (p.getName(0).toString().isEmpty()) { ... }

3. Comparing with "..":

    for (Path item: p) {
       String s = item.toString();
       if (!s.equals("..")) { ... }
    }

Maybe it's better to compare them with static final Path fields? I 
create some benchmarks and they show comparing with objects is better 
than comparing with strings, the throughput is roughly 40% higher.

I am using linux-x64.

So I made one enhancement to UnixPath.<init>(fs, str) to store 
normalizeAndCheck(str) (it is calculated anyway) as stringValue (which 
will be returned by toString) in the constructor, and seems comparing 
with objects and comparing with strings have no significant difference now.

I admit I haven't analyzed if normalizeAndCheck(str) is always the 
correct toString() result.

Is there a better way?

Thanks
Max