001// ***************************************************************************************************************************
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006// *                                                                                                                         *
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012// ***************************************************************************************************************************
013package org.apache.juneau.http;
014
015import org.apache.juneau.http.annotation.*;
016
017/**
018 * Represents a parsed <l>ETag</l> HTTP response header.
019 *
020 * <p>
021 * An identifier for a specific version of a resource, often a message digest.
022 *
023 * <h5 class='figure'>Example</h5>
024 * <p class='bcode w800'>
025 *    ETag: "737060cd8c284d8af7ad3082f209582d"
026 * </p>
027 *
028 * <h5 class='topic'>RFC2616 Specification</h5>
029 *
030 * The ETag response-header field provides the current value of the entity tag for the requested variant.
031 * The headers used with entity tags are described in sections 14.24, 14.26 and 14.44.
032 * The entity tag MAY be used for comparison with other entities from the same resource (see section 13.3.3).
033 *
034 * <p class='bcode w800'>
035 *    ETag = "ETag" ":" entity-tag
036 * </p>
037 *
038 * <p>
039 * Examples:
040 * <p class='bcode w800'>
041 *    ETag: "xyzzy"
042 *    ETag: W/"xyzzy"
043 *    ETag: ""
044 * </p>
045 *
046 * <ul class='seealso'>
047 *    <li class='extlink'>{@doc RFC2616}
048 * </ul>
049 */
050@Header("ETag")
051public final class ETag extends HeaderString {
052
053   /**
054    * Returns a parsed <c>ETag</c> header.
055    *
056    * @param value The <c>ETag</c> header string.
057    * @return The parsed <c>ETag</c> header, or <jk>null</jk> if the string was null.
058    */
059   public static ETag forString(String value) {
060      if (value == null)
061         return null;
062      return new ETag(value);
063   }
064
065   private ETag(String value) {
066      super(value);
067   }
068}