https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * Copyright (C) 2008 The Guava Authors
 *
 * 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.google.common.escape;

import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;


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.

For example, an XML escaper would convert the literal string "Foo<Bar>" into 
"Foo&lt;Bar&gt;" to prevent "<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 "Foo<Bar>". 
An Escaper instance is required to be stateless, and safe when used concurrently by multiple threads. 
Because, in general, escaping operates on the code points of a string and not on its individual char values, it is not safe to assume that escape(s) is equivalent to escape(s.substring(0, n)) + escape(s.substring(n)) for arbitrary n. This is because of the possibility of splitting a surrogate pair. The only case in which it is safe to escape strings and concatenate the results is if you can rule out this possibility, either by splitting an existing long string into short strings adaptively around surrogate pairs, or by starting with short strings already known to be free of unpaired surrogates. 
The two primary implementations of this interface are CharEscaper and 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. 
Popular escapers are defined as constants in classes like HtmlEscapers and XmlEscapers. To create your own escapers, use CharEscaperBuilder, or extend CharEscaper or 
UnicodeEscaper. 
Author:David Beaumont
Since:15.0
/**
 * 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>Because, in general, escaping operates on the code points of a string and not on its
 * individual {@code char} values, it is not safe to assume that {@code escape(s)} is equivalent to
 * {@code escape(s.substring(0, n)) + escape(s.substring(n))} for arbitrary {@code n}. This is
 * because of the possibility of splitting a surrogate pair. The only case in which it is safe to
 * escape strings and concatenate the results is if you can rule out this possibility, either by
 * splitting an existing long string into short strings adaptively around {@linkplain
 * Character#isHighSurrogate surrogate} {@linkplain Character#isLowSurrogate pairs}, or by starting
 * with short strings already known to be free of unpaired surrogates.
 *
 * <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>Popular escapers are defined as constants in classes like {@link
 * com.google.common.html.HtmlEscapers} and {@link com.google.common.xml.XmlEscapers}. To create
 * your own escapers, use {@link CharEscaperBuilder}, or extend {@code CharEscaper} or {@code
 * UnicodeEscaper}.
 *
 * @author David Beaumont
 * @since 15.0
 */
@GwtCompatible
public abstract class Escaper {
  // TODO(user): evaluate custom implementations, considering package private constructor.
  
Constructor for use by subclasses. 
/** Constructor for use by subclasses. */
  protected Escaper() {}

  
Returns the escaped form of a given literal string.

Note that this method may treat input characters differently depending on the specific
escaper implementation.

    
  
  • UnicodeEscaper handles UTF-16 correctly, including surrogate character pairs. If the input is badly formed the escaper should throw IllegalArgumentException. 
  • 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). 


Params:
  • string – the literal string to be escaped
Throws:
Returns:the escaped form of string
/**
   * 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 {@code 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);

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

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