*
   * @param escaper the instance to be wrapped
   * @return a UnicodeEscaper with the same behavior as the given instance
   * @throws NullPointerException if escaper is null
   * @throws IllegalArgumentException if escaper is not a UnicodeEscaper or a CharEscaper
   */
  static UnicodeEscaper asUnicodeEscaper(Escaper escaper) {
    checkNotNull(escaper);
    if (escaper instanceof UnicodeEscaper) {

   *
   * @param escaper the instance to be wrapped
   * @return a UnicodeEscaper with the same behavior as the given instance
   * @throws NullPointerException if escaper is null
   * @throws IllegalArgumentException if escaper is not a UnicodeEscaper or a CharEscaper
   */
  static UnicodeEscaper asUnicodeEscaper(Escaper escaper) {
    checkNotNull(escaper);
    if (escaper instanceof UnicodeEscaper) {

  }

  private static final Escaper XML_ESCAPER;
  private static final Escaper XML_CONTENT_ESCAPER;
  private static final Escaper XML_ATTRIBUTE_ESCAPER;

  static {
    Escapers.Builder builder = Escapers.builder();
    // The char values \uFFFE and \uFFFF are explicitly not allowed in XML
    // (Unicode code points above \uFFFF are represented via surrogate pairs
    // which means they are treated as pairs of safe characters).

 * your own escapers, use {@link CharEscaperBuilder}, or extend {@code CharEscaper} or {@code
 * UnicodeEscaper}.
 *
 * @author David Beaumont
 * @since 15.0
 */
@DoNotMock("Use Escapers.nullEscaper() or another methods from the *Escapers classes")
@GwtCompatible
@ElementTypesAreNonnullByDefault
public abstract class Escaper {

   * characters, even though <a href="https://goo.gl/e7E0In">many of these are accepted in modern
   * URLs</a>. (<a href="https://goo.gl/jfVxXW">If the escaper were to leave these characters
   * unescaped, they would be escaped by the consumer at parse time, anyway.</a>) Additionally, the
   * escaper escapes the slash character ("/"). While slashes are acceptable in URL paths, they are

 * your own escapers, use {@link CharEscaperBuilder}, or extend {@code CharEscaper} or {@code
 * UnicodeEscaper}.
 *
 * @author David Beaumont
 * @since 15.0
 */
@DoNotMock("Use Escapers.nullEscaper() or another methods from the *Escapers classes")
@GwtCompatible
@ElementTypesAreNonnullByDefault
public abstract class Escaper {

   * characters, even though <a href="https://goo.gl/e7E0In">many of these are accepted in modern
   * URLs</a>. (<a href="https://goo.gl/jfVxXW">If the escaper were to leave these characters
   * unescaped, they would be escaped by the consumer at parse time, anyway.</a>) Additionally, the
   * escaper escapes the slash character ("/"). While slashes are acceptable in URL paths, they are

  }

  private static final Escaper XML_ESCAPER;
  private static final Escaper XML_CONTENT_ESCAPER;
  private static final Escaper XML_ATTRIBUTE_ESCAPER;

  static {
    Escapers.Builder builder = Escapers.builder();
    // The char values \uFFFE and \uFFFF are explicitly not allowed in XML
    // (Unicode code points above \uFFFF are represented via surrogate pairs
    // which means they are treated as pairs of safe characters).

    assertUnicodeEscaping(e, "%F4%8F%BF%BF", '\uDBFF', '\uDFFF');

    assertEquals("", e.escape(""));
    assertEquals("safestring", e.escape("safestring"));
    assertEquals("embedded%00null", e.escape("embedded\0null"));
    assertEquals("max%EF%BF%BFchar", e.escape("max\uffffchar"));
  }

  // Helper to assert common expected behaviour of uri escapers.
  static void assertBasicUrlEscaper(UnicodeEscaper e) {

    assertUnicodeEscaping(e, "%F4%8F%BF%BF", '\uDBFF', '\uDFFF');

    assertEquals("", e.escape(""));
    assertEquals("safestring", e.escape("safestring"));
    assertEquals("embedded%00null", e.escape("embedded\0null"));
    assertEquals("max%EF%BF%BFchar", e.escape("max\uffffchar"));
  }

  // Helper to assert common expected behaviour of uri escapers.
  static void assertBasicUrlEscaper(UnicodeEscaper e) {