xref: /packages/apps/UnifiedEmail/src/com/android/mail/lib/base/Escaper.java

/*
 * Copyright (C) 2008 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.android.mail.lib.base;

/**
 * An object that converts literal text into a format safe for inclusion in a particular context
 * (such as an XML document). Typically (but not always), the inverse process of "unescaping" the
 * text is performed automatically by the relevant parser.
 *
 * <p>For example, an XML escaper would convert the literal string {@code "Foo<Bar>"} into {@code
 * "Foo&lt;Bar&gt;"} to prevent {@code "<Bar>"} from being confused with an XML tag. When the
 * resulting XML document is parsed, the parser API will return this text as the original literal
 * string {@code "Foo<Bar>"}.
 *
 * <p>An {@code Escaper} instance is required to be stateless, and safe when used concurrently by
 * multiple threads.
 *
 * <p>The two primary implementations of this interface are {@link CharEscaper} and {@link
 * UnicodeEscaper}. They are heavily optimized for performance and greatly simplify the task of
 * implementing new escapers. It is strongly recommended that when implementing a new escaper you
 * extend one of these classes. If you find that you are unable to achieve the desired behavior
 * using either of these classes, please contact the Java libraries team for advice.
 *
 * <p>Several popular escapers are defined as constants in the class {@link CharEscapers}. To create
 * your own escapers, use {@link CharEscaperBuilder}, or extend {@link CharEscaper} or {@code
 * UnicodeEscaper}.
 *
 * @author dbeaumont@google.com (David Beaumont)
 */
public abstract class Escaper {
  /**
   * Returns the escaped form of a given literal string.
   *
   * <p>Note that this method may treat input characters differently depending on the specific
   * escaper implementation.
   *
   * <ul>
   * <li>{@link UnicodeEscaper} handles <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a>
   *     correctly, including surrogate character pairs. If the input is badly formed the escaper
   *     should throw {@link IllegalArgumentException}.
   * <li>{@link CharEscaper} handles Java characters independently and does not verify the input
   *     for well formed characters. A CharEscaper should not be used in situations where input is
   *     not guaranteed to be restricted to the Basic Multilingual Plane (BMP).
   * </ul>
   *
   * @param string the literal string to be escaped
   * @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);

  /**
   * Returns an {@code Appendable} instance which automatically escapes all text appended to it
   * before passing the resulting text to an underlying {@code Appendable}.
   *
   * <p>Note that the Appendable returned by this method may treat input characters differently
   * depending on the specific escaper implementation.
   *
   * <ul>
   * <li>{@link UnicodeEscaper} handles <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a>
   *     correctly, including surrogate character pairs. If the input is badly formed the escaper
   *     should throw {@link IllegalArgumentException}.
   * <li>{@link CharEscaper} handles Java characters independently and does not verify the input
   *     for well formed characters. A CharEscaper should not be used in situations where input is
   *     not guaranteed to be restricted to the Basic Multilingual Plane (BMP).
   * </ul>
   *
   * <p>In all implementations the escaped Appendable should throw {@code NullPointerException} if
   * given a {@code null} {@link CharSequence}.
   *
   * @param out the underlying {@code Appendable} to append escaped output to
   * @return an {@code Appendable} which passes text to {@code out} after escaping it
   */
  public abstract Appendable escape(Appendable out);

  private final Function<String, String> asFunction =
      new Function<String, String>() {
        public String apply(String from) {
          return escape(from);
        }
      };

  /**
   * Returns a {@link Function} that invokes {@link #escape(String)} on this escaper.
   */
  public Function<String, String> asFunction() {
    return asFunction;
  }
}