pre-RFR (s): 8049847: Enhance PrintWriter line.separator handling

[Claes Redestad]

Hi,

  this is a proposal to resolve concerns that PrintWriter/BufferedWriter 
behave
inconsistently with j.u.Formatter when setting the line.separator 
property to override
system-appropriate line breaks.

  Instead of providing a set of new constructors, I propose to simply 
add a new default
method j.l.Appendable#lineSeparator, which by default delegates to 
System.lineSeparator().
This allows classes such as PrintWriter/BufferedWriter to provide 
overrides which
communicate to j.u.Formatter their intended behavior.

  This indirectly provides a way to control the lineSeparator by 
allowing users to
override PrintWriter or similar in custom classes which override 
lineSeparator, e.g.:

  PrintWriter unixPrintWriter = new PrintWriter(out) {
      @Override
      public String lineSeparator() { return "\n"; }
  };

  PrintWriter windowsPrintWriter = new PrintWriter(out) {
      @Override
      public String lineSeparator() { return "\r\n"; }
  };

  Bonus: This approach can be used to optimize j.l.Throwable to get rid 
of all static
inner classes to wrap PrintWriter/PrintStream and instead use 
j.l.Appendable, which
more than enough will mitigate the startup hit adding a method to 
Appendable will incur.

bug: https://bugs.openjdk.java.net/browse/JDK-8049847
webrev: http://cr.openjdk.java.net/~redestad/8049847/webrev.00/

  I've not filed requests to change the public API just yet, rather 
wanted to throw this out
for preview to see if there's some concern and feedback first.

/Claes