A DESCRIPTION OF THE REQUEST :
There are several places in javax.swing.text where StringBuffer could be replaced with StringBuilder. Files that use StringBuffer and are candidates for change:
DefaultCaret
DefaultStyledDocument
InternationalFormatter
JTextComponent
MaskFormatter
NumberFormatter
PlainDocument
TabSet


JUSTIFICATION :
this will increase performance as acquiring the lock for the associated Object will no longer happen with each method invocation.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
useless locking removed.
ACTUAL -
useless locking happening

---------- BEGIN SOURCE ----------
Read the source of the noted files, see usage of StringBuffer within methods that could be replaced with StringBuilder.
---------- END SOURCE ----------

While Swing text stands to benefit the most, might as well do ALL of Swing.