guava/src/com/google/common/io/Files.java
@Beta
  @Deprecated
  public
  static void append(CharSequence from, File to, Charset charset) throws IOException {
    asCharSink(to, charset, FileWriteMode.APPEND).write(from);
  }
@Beta
  @Deprecated
  public
  static void append(CharSequence from, File to, Charset charset) throws IOException {
    asCharSink(to, charset, FileWriteMode.APPEND).write(from);
  }
Appends a character sequence (such as a string) to a file using the given character set.

@param from the character sequence to append
@param to the destination file
@param charset the charset used to encode the output stream; see {@link StandardCharsets} for
helpful predefined constants
@throws IOException if an I/O error occurs
@deprecated Prefer {@code asCharSink(to, charset, FileWriteMode.APPEND).write(from)}. This
method is scheduled to be removed in January 2019.