The API documentation for CharsetEncoder.replaceWith() gives the following
information for the "newReplacement" parameter:

   newReplacement - The new replacement; must not be null, must have
   non-zero length, must not be longer than the value returned by the
   maxBytesPerChar method, and must be legal

   
http://docs.oracle.com/javase/7/docs/api/java/nio/charset/CharsetEncoder.html#replaceWith(byte[])

The equivalent section of documentation for CharsetDecoder.replaceWith()
looks like this:

   newReplacement - The new replacement; must not be null and must have
   non-zero length

   
http://docs.oracle.com/javase/7/docs/api/java/nio/charset/CharsetDecoder.html#replaceWith(java.lang.String)

Note that the part about the maximum length of the parameter is missing.
The implementation reveals that the length of newReplacement is checked
against the value returned by CharsetDecoder.maxCharsPerByte(). Therefore
we would expect the documentation to read as follows:

   newReplacement - The new replacement; must not be null, must have
   non-zero length, must not be longer than the value returned by the
   maxCharsPerByte method, and must be legal