* @return the escaped form of {@code string}
   * @throws NullPointerException if {@code string} is null
   * @throws IllegalArgumentException if {@code string} contains badly formed UTF-16 or cannot be
   *     escaped for any other reason
   */
  public abstract String escape(String string);

  private final Function<String, String> asFunction = this::escape;

   * @param cp the Unicode code point to escape
   */
  public static void assertEscaping(UnicodeEscaper escaper, String expected, int cp) {

    String escaped = computeReplacement(escaper, cp);
    Assert.assertNotNull(escaped);
    Assert.assertEquals(expected, escaped);
  }

  /**
   * Asserts that an escaper does not escape the given character.
   *
   * @param escaper the non-null escaper to test

   * @param cp the Unicode code point to escape
   */
  public static void assertEscaping(UnicodeEscaper escaper, String expected, int cp) {

    String escaped = computeReplacement(escaper, cp);
    Assert.assertNotNull(escaped);
    Assert.assertEquals(expected, escaped);
  }

  /**
   * Asserts that an escaper does not escape the given character.
   *
   * @param escaper the non-null escaper to test

  public static Escaper xmlAttributeEscaper() {
    return XML_ATTRIBUTE_ESCAPER;
  }

  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

  /**
   * Returns an {@link Escaper} instance that escapes strings so they can be safely included in <a
   * href="https://url.spec.whatwg.org/#syntax-url-path-segment">URL path segments</a>. The returned
   * escaper escapes all non-ASCII characters, even though <a
   * href="https://url.spec.whatwg.org/#url-code-points">many of these are accepted in modern
   * URLs</a>. (<a href="https://url.spec.whatwg.org/#path-state">If the escaper were to leave these

 * UTF-8.
 *
 * <p><b>Note:</b> This escaper produces <a
 * href="https://url.spec.whatwg.org/#percent-encode">uppercase</a> hexadecimal sequences.
 *
 * @author David Beaumont
 * @since 15.0
 */
@GwtCompatible
public final class PercentEscaper extends UnicodeEscaper {

  // In some escapers spaces are escaped to '+'
  private static final char[] plusSign = {'+'};

 */
@GwtCompatible
public final class Escapers {
  private Escapers() {}

  /**
   * Returns an {@link Escaper} that does no escaping, passing all character data through unchanged.
   */
  public static Escaper nullEscaper() {
    return NULL_ESCAPER;
  }

  // An Escaper that efficiently performs no escaping.
  // Extending CharEscaper (instead of Escaper) makes Escapers.compose() easier.

  /**
   * Returns an {@link Escaper} instance that escapes strings so they can be safely included in <a
   * href="https://url.spec.whatwg.org/#syntax-url-path-segment">URL path segments</a>. The returned
   * escaper escapes all non-ASCII characters, even though <a
   * href="https://url.spec.whatwg.org/#url-code-points">many of these are accepted in modern
   * URLs</a>. (<a href="https://url.spec.whatwg.org/#path-state">If the escaper were to leave these

public abstract class CharEscaper extends Escaper {
  /** Constructor for use by subclasses. */
  protected CharEscaper() {}

  /**
   * Returns the escaped form of a given literal string.
   *
   * @param string the literal string to be escaped
   * @return the escaped form of {@code string}
   * @throws NullPointerException if {@code string} is null
   */
  @Override
  public String escape(String string) {

    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) {