{"id":123943,"date":"2026-11-08T14:32:45","date_gmt":"2026-11-08T06:32:45","guid":{"rendered":"https:\/\/www.wsisp.com\/helps\/123943.html"},"modified":"2026-11-08T14:32:45","modified_gmt":"2026-11-08T06:32:45","slug":"%e4%b8%80%e9%81%93%e8%bf%87%e6%97%b6%e7%9a%84%e9%9d%a2%e8%af%95%e9%a2%98%ef%bc%9ahashmap-%e5%92%8c-hashtable-%e5%88%b0%e5%ba%95%e5%b7%ae%e5%9c%a8%e5%93%aa%ef%bc%9f","status":"publish","type":"post","link":"https:\/\/www.wsisp.com\/helps\/123943.html","title":{"rendered":"\u4e00\u9053\u201c\u8fc7\u65f6\u201d\u7684\u9762\u8bd5\u9898\uff1aHashMap \u548c Hashtable \u5230\u5e95\u5dee\u5728\u54ea\uff1f"},"content":{"rendered":"

\/* MD \/ \u5bcc\u6587\u672c\u4e2d\u7684 .toc\uff08\u542b\u535a\u5ba2\u56ed\u642c\u5bb6\u7b49\u5d4c\u5957\u7ed3\u6784\uff09\uff1b.toc-box \u5728\u4fa7\u680f\uff0c\u4e0d\u53d7\u5f71\u54cd *\/
\n #content_views .toc,
\n \/* \u7f16\u8f91\u5668\u5e38\u5728\u76ee\u5f55\u524d\u540e\u63d2\u5165\u7a7a p\uff08:empty \u4ecd\u5360 20px\uff09\uff0c\u4e00\u5e76\u53bb\u6389\u907f\u514d\u9876\u7a7a\u9699 *\/
\n #content_views.markdown_views > p:empty:has(+ .toc),
\n #content_views.markdown_views > .toc + p:empty,
\n \/* \u5bcc\u6587\u672c\u65e7\u7248\u76ee\u5f55\u6807\u8bb0 *\/
\n #content_views.htmledit_views #main-toc,
\n #content_views.htmledit_views #hr-toc,
\n #content_views.htmledit_views p[id*=\\”-toc\\”] {
\n display: none !important;
\n }
\n \/* \u76ee\u5f55\u53bb\u6389\u540e\uff0c\u7d27\u8ddf\u7684\u9996\u4e2a\u6807\u9898\u4e0d\u518d\u591a\u51fa\u4e00\u5757\u4e0a\u8fb9\u8ddd *\/
\n #content_views.markdown_views > .toc + h1,
\n #content_views.markdown_views > .toc + h2,
\n #content_views.markdown_views > .toc + h3,
\n #content_views.markdown_views > .toc + h4,
\n #content_views.markdown_views > .toc + p:empty + h1,
\n #content_views.markdown_views > .toc + p:empty + h2,
\n #content_views.markdown_views > .toc + p:empty + h3,
\n #content_views.markdown_views > .toc + p:empty + h4 {
\n margin-top: 0 !important;
\n }<\/p>\n

\u4e00\u9053\u201c\u8fc7\u65f6\u201d\u7684\u9762\u8bd5\u9898：HashMap \u548c Hashtable \u5230\u5e95\u5dee\u5728\u54ea？<\/h2>\n

Hashtable \u662f 1996 \u5e74 JDK 1.0 \u7684\u4ea7\u7269\u30021998 \u5e74 JDK 1.2 \u5f15\u5165\u96c6\u5408\u6846\u67b6\u540e，\u5b83\u5c31\u88ab\u5b98\u65b9\u6807\u6ce8\u4e3a\u8fc7\u65f6\u7684\u9057\u7559\u7c7b，\u4f46\u9762\u8bd5\u5b98\u51e0\u4e4e\u6bcf\u6b21\u90fd\u95ee\u3002<\/p>\n

\u56e0\u4e3a\u5b83\u80fd\u4e00\u53e3\u6c14\u4e32\u8d77\u56db\u6761\u4e3b\u7ebf：\u7ebf\u7a0b\u5b89\u5168\u7684\u5b9e\u73b0\u65b9\u5f0f\u3001null \u8bed\u4e49\u7684\u53d6\u820d\u3001\u5bb9\u91cf\u4e0e\u7d22\u5f15\u7b97\u6cd5\u3001\u8fed\u4ee3\u5668\u7684 fail-fast \u673a\u5236\u3002\u7b54\u5f97\u51fa\u6765\u662f\u53ca\u683c，\u7b54\u5f97\u51fa\u201c\u4e3a\u4ec0\u4e48\u8fd9\u4e48\u8bbe\u8ba1\u201d\u624d\u662f\u52a0\u5206\u3002<\/p>\n

0. \u5148\u7ed9\u9762\u8bd5\u7b54\u6848（30 \u79d2\u7248）<\/h3>\n

HashMap \u548c Hashtable \u6700\u6838\u5fc3\u7684\u533a\u522b\u5c31\u662f\u7ebf\u7a0b\u5b89\u5168\u6027\u3002 Hashtable \u7684\u65b9\u6cd5\u57fa\u672c\u90fd\u52a0\u4e86 synchronized，\u662f\u7ebf\u7a0b\u5b89\u5168\u7684，\u4f46\u4ee3\u4ef7\u662f\u6027\u80fd\u5dee；HashMap \u6ca1\u52a0\u9501，\u5355\u7ebf\u7a0b\u4e0b\u8dd1\u5f97\u5feb，\u4f46\u591a\u7ebf\u7a0b\u73af\u5883\u5f97\u81ea\u5df1\u4fdd\u8bc1\u540c\u6b65\u3002<\/p>\n

\u5176\u4ed6\u51e0\u4e2a\u5173\u952e\u533a\u522b：<\/p>\n

  • \u5bf9 null \u7684\u5904\u7406\u4e0d\u4e00\u6837\u3002HashMap \u5141\u8bb8\u4e00\u4e2a null \u952e\u548c\u591a\u4e2a null \u503c，Hashtable \u4e0d\u8ba9\u5b58 null，\u5f80\u91cc\u585e\u5c31\u629b NullPointerException；<\/li>\n
  • \u7ee7\u627f\u4f53\u7cfb\u4e0d\u540c\u3002HashMap \u7ee7\u627f AbstractMap，\u662f\u6b63\u7ecf\u7684 Map \u5b9e\u73b0；Hashtable \u7ee7\u627f\u7684\u662f\u53e4\u65e9\u7684 Dictionary \u7c7b，\u867d\u7136\u540e\u6765\u4e5f\u5b9e\u73b0\u4e86 Map \u63a5\u53e3，\u4f46 Dictionary \u65e9\u5c31\u88ab\u5b98\u65b9\u6807\u6ce8\u4e3a\u8fc7\u65f6（obsolete）；<\/li>\n
  • \u8fed\u4ee3\u5668\u673a\u5236\u6709\u5dee\u522b\u3002HashMap \u7528\u7684\u662f fail-fast \u7684 Iterator，\u8fed\u4ee3\u65f6\u6539\u7ed3\u6784\u76f4\u63a5\u629b ConcurrentModificationException；Hashtable \u7684 keys()\/elements() \u8fd4\u56de\u7684\u662f\u8001\u5f0f Enumerator，\u4e0d\u505a\u6821\u9a8c，\u6539\u4e86\u7ed3\u6784\u4e5f\u4e0d\u4e00\u5b9a\u62a5\u9519\u2014\u2014\u4e0d\u8fc7\u8981\u6ce8\u610f，\u5b83\u7684 keySet().iterator() \u662f fail-fast \u7684（\u7b2c 5 \u8282\u6709\u5b9e\u6d4b）\u3002<\/li>\n

    \u5bb9\u91cf\u7b97\u6cd5（16 vs 11\u3001\u4f4d\u4e0e vs \u53d6\u6a21）\u3001hash \u6270\u52a8\u8fd9\u4e9b\u4e5f\u5e38\u88ab\u8ffd\u95ee，\u626b\u4e00\u773c\u4e0b\u9762\u7684\u901f\u67e5\u8868\u5373\u53ef：<\/p>\n\n\u7ef4\u5ea6HashMapHashtable<\/tr>\n\n\n\n\n\n\n\n\n\n\n
    \u5f15\u5165\u7248\u672c<\/td>\nJDK 1.2<\/td>\nJDK 1.0<\/td>\n<\/tr>\n
    \u7236\u7c7b<\/td>\nAbstractMap<\/td>\nDictionary（\u5b98\u65b9\u6807\u6ce8\u5df2\u5e9f\u5f03）<\/td>\n<\/tr>\n
    \u7ebf\u7a0b\u5b89\u5168<\/td>\n\u274c \u4e0d\u5b89\u5168<\/td>\n\u2705 \u5b89\u5168（\u65b9\u6cd5\u7ea7 synchronized，\u9501 this）<\/td>\n<\/tr>\n
    null \u652f\u6301<\/td>\n1 \u4e2a null key + \u4efb\u610f null value<\/td>\nkey \u548c value \u90fd\u4e0d\u5141\u8bb8<\/td>\n<\/tr>\n
    \u9ed8\u8ba4\u5bb9\u91cf<\/td>\n16<\/td>\n11<\/td>\n<\/tr>\n
    \u6269\u5bb9\u7b56\u7565<\/td>\n2 \u500d（\u4fdd\u6301 2 \u7684\u5e42）<\/td>\n2n + 1（\u4fdd\u6301\u5947\u6570）<\/td>\n<\/tr>\n
    \u7d22\u5f15\u8ba1\u7b97<\/td>\n(n – 1) & hash<\/td>\n(hash & 0x7FFFFFFF) % length<\/td>\n<\/tr>\n
    hash \u6270\u52a8<\/td>\n\u6709（h ^ (h >>> 16)）<\/td>\n\u65e0，\u76f4\u63a5\u7528 key.hashCode()<\/td>\n<\/tr>\n
    \u8fed\u4ee3\u5668<\/td>\nfail-fast<\/td>\nkeySet() \u89c6\u56fe fail-fast，keys()\/elements() \u4e0d\u662f<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    \u4e00\u53e5\u8bdd\u8bb0\u4f4f：Hashtable \u7528\u4e00\u628a\u9501\u628a\u6574\u4e2a\u8868\u9501\u4f4f，\u5b89\u5168\u4f46\u6162；HashMap \u4e0d\u9501\u6240\u4ee5\u5feb，\u4f46\u5e76\u53d1\u8981\u81ea\u5df1\u7ba1\u3002<\/p>\n

    \u4e0b\u9762\u628a\u6bcf\u4e00\u6761\u62c6\u5f00，\u6700\u540e\u7ed9\u51e0\u4e2a\u5b9e\u6d4b\u6570\u636e\u3002<\/p>\n

    \n

    1. \u51fa\u8eab\u4e0d\u540c：\u4e00\u4e2a 1998 \u5e74\u7684\u9057\u7559\u7c7b<\/h3>\n

    Hashtable \u662f JDK 1.0 \u5c31\u6709\u7684\u7c7b，\u6bd4 Collection \u6846\u67b6\u8fd8\u65e9\u4e24\u5e74\u3002\u7b49\u5230 JDK 1.2 \u5f15\u5165\u96c6\u5408\u6846\u67b6\u65f6，\u5b98\u65b9\u624d\u8865\u4e86 HashMap，\u5e76\u987a\u624b\u628a Hashtable \u5f52\u5165\u201c\u4e0d\u63a8\u8350\u4f7f\u7528\u201d\u7684\u5386\u53f2\u9057\u7559\u3002<\/p>\n

    \u8bc1\u636e\u5728 Dictionary \u7684 javadoc \u91cc：<\/p>\n

    NOTE: This class is obsolete. New implementations should implement the Map interface, rather than extending this class.<\/p>\n

    Hashtable \u6070\u6070\u7ee7\u627f\u4e86 Dictionary，\u6240\u4ee5\u5b83\u7684\u4f4d\u7f6e\u5f88\u5c34\u5c2c：<\/p>\n

      \n
    • \u5b83\u6ca1\u6709\u88ab @Deprecated \u6807\u6ce8（\u5b9e\u6d4b Hashtable.class.isAnnotationPresent(Deprecated.class) \u8fd4\u56de false），\u6240\u4ee5\u7f16\u8bd1\u4e0d\u62a5\u8b66；<\/li>\n
    • \u4f46\u6587\u6863\u660e\u786e\u8bf4\u201c\u65b0\u5b9e\u73b0\u5e94\u8be5\u5b9e\u73b0 Map \u63a5\u53e3，\u800c\u4e0d\u662f\u7ee7\u627f\u8fd9\u4e2a\u7c7b\u201d\u3002<\/li>\n<\/ul>\n

      \u547d\u540d\u4e5f\u4e0d\u89c4\u8303\u3002 \u6309 Java \u547d\u540d\u7ea6\u5b9a\u5e94\u8be5\u662f HashTable，\u800c Hashtable \u628a t \u5199\u6210\u4e86\u5c0f\u5199\u2014\u2014\u8fd9\u662f JDK 1.0 \u65f6\u4ee3\u547d\u540d\u89c4\u8303\u5c1a\u672a\u5b9a\u578b\u7559\u4e0b\u7684\u5386\u53f2\u5305\u88b1\u3002<\/p>\n

      \u65b9\u6cd5\u540d\u66f4\u8ba9\u4eba\u56f0\u60d1：<\/p>\n

      Hashtable<\/span><<\/span>String<\/span>,<\/span> String<\/span>><\/span><\/span> ht =<\/span> new<\/span> Hashtable<\/span><<\/span>><\/span><\/span>(<\/span>)<\/span>;<\/span>
      \nht.<\/span>put<\/span>(<\/span>"a"<\/span>,<\/span> "v"<\/span>)<\/span>;<\/span>
      \nSystem<\/span>.<\/span>out.<\/span>println<\/span>(<\/span>ht.<\/span>contains<\/span>(<\/span>"v"<\/span>)<\/span>)<\/span>;<\/span> \/\/ true \u2014\u2014 \u5b83\u5176\u5b9e\u662f containsValue！<\/span><\/p>\n

      contains(Object) \u5224\u65ad\u7684\u662f\u503c\u662f\u5426\u5b58\u5728，\u548c\u76f4\u89c9\u5b8c\u5168\u76f8\u53cd\u3002HashMap \u5e72\u8106\u5220\u6389\u4e86\u8fd9\u4e2a\u65b9\u6cd5，\u53ea\u4fdd\u7559 containsKey \u548c containsValue，\u4ece API \u8bbe\u8ba1\u4e0a\u6d88\u706d\u4e86\u6b67\u4e49\u3002<\/p>\n

      \n

      2. \u7ebf\u7a0b\u5b89\u5168：\u4e00\u628a\u9501\u9501\u4f4f\u6574\u5f20\u8868<\/h3>\n

      \u5b9e\u8bc1\u6570\u636e： \u53cd\u5c04\u7edf\u8ba1 Hashtable \u7684 36 \u4e2a public \u65b9\u6cd5，\u5176\u4e2d 26 \u4e2a\u5e26 synchronized\u3002<\/p>\n

      public<\/span> synchronized<\/span> V<\/span> put<\/span>(<\/span>K<\/span> key,<\/span> V<\/span> value)<\/span> {<\/span> .<\/span>.<\/span>.<\/span> }<\/span>
      \npublic<\/span> synchronized<\/span> V<\/span> get<\/span>(<\/span>Object<\/span> key)<\/span> {<\/span> .<\/span>.<\/span>.<\/span> }<\/span>
      \npublic<\/span> synchronized<\/span> V<\/span> remove<\/span>(<\/span>Object<\/span> key)<\/span> {<\/span> .<\/span>.<\/span>.<\/span> }<\/span><\/p>\n

      \u9501\u5bf9\u8c61\u662f this，\u4e5f\u5c31\u662f\u8bf4\u6574\u5f20\u8868\u5171\u7528\u4e00\u628a\u9501\u3002\u5e26\u6765\u4e09\u4e2a\u540e\u679c：<\/p>\n

    • \u8bfb\u4e5f\u88ab\u963b\u585e：\u7ebf\u7a0b A \u5728 put，\u7ebf\u7a0b B \u7684 get \u5fc5\u987b\u7b49\u2014\u2014\u54ea\u6015\u5b83\u4eec\u64cd\u4f5c\u7684\u662f\u5b8c\u5168\u4e0d\u540c\u7684\u6876；<\/li>\n
    • \u5355\u7ebf\u7a0b\u4e5f\u8981\u4ed8\u9501\u5f00\u9500：synchronized \u5728\u65e0\u7ade\u4e89\u65f6\u867d\u7136\u5df2\u4f18\u5316\u5230\u504f\u5411\u9501\/\u8f7b\u91cf\u7ea7\u9501，\u4f46\u4ecd\u6709\u5bf9\u8c61\u5934\u64cd\u4f5c\u548c\u5185\u5b58\u5c4f\u969c\u7684\u6210\u672c；<\/li>\n
    • \u5e76\u53d1\u541e\u5410\u6781\u5dee：\u7ebf\u7a0b\u6570\u4e00\u4e0a\u6765，\u5168\u90e8\u9000\u5316\u6210\u4e32\u884c\u3002<\/li>\n

      \u4f46\u5b83\u662f\u201c\u7ebf\u7a0b\u5b89\u5168\u201d\u7684\u5417？\u6253\u5f15\u53f7<\/h4>\n

      if<\/span> (<\/span>!<\/span>ht.<\/span>containsKey<\/span>(<\/span>key)<\/span>)<\/span> {<\/span> \/\/ \u2460 \u68c0\u67e5<\/span>
      \n ht.<\/span>put<\/span>(<\/span>key,<\/span> value)<\/span>;<\/span> \/\/ \u2461 \u5199\u5165<\/span>
      \n}<\/span><\/p>\n

      \u2460\u548c\u2461\u662f\u4e24\u4e2a\u72ec\u7acb\u7684\u539f\u5b50\u64cd\u4f5c，\u4e2d\u95f4\u5b8c\u5168\u53ef\u80fd\u88ab\u5176\u4ed6\u7ebf\u7a0b\u63d2\u5165\u3002\u300c\u5148\u68c0\u67e5\u518d\u64cd\u4f5c\u300d\u7684\u590d\u5408\u903b\u8f91，Hashtable \u4fdd\u8bc1\u4e0d\u4e86\u3002<\/p>\n

      \u60f3\u8981\u771f\u6b63\u7684\u539f\u5b50\u6027，\u56e0\u4e3a\u9501\u5c31\u662f this，\u53ef\u4ee5\u81ea\u5df1\u518d\u5957\u4e00\u5c42：<\/p>\n

      synchronized<\/span> (<\/span>ht)<\/span> {<\/span>
      \n if<\/span> (<\/span>!<\/span>ht.<\/span>containsKey<\/span>(<\/span>key)<\/span>)<\/span> {<\/span>
      \n ht.<\/span>put<\/span>(<\/span>key,<\/span> value)<\/span>;<\/span>
      \n }<\/span>
      \n}<\/span><\/p>\n

      \u8fd9\u7b97 Hashtable \u7684\u4e00\u4e2a\u4fbf\u5229：\u5b83\u7684\u9501\u5bf9\u8c61\u5c31\u662f\u5bb9\u5668\u672c\u8eab（this），\u6240\u4ee5\u5916\u90e8\u53ef\u4ee5\u76f4\u63a5 synchronized (ht) \u628a\u591a\u4e2a\u64cd\u4f5c\u6253\u5305\u6210\u4e00\u4e2a\u539f\u5b50\u5757\u3002Collections.synchronizedMap \u540c\u7406\u2014\u2014\u9501\u5c31\u662f\u5305\u88c5\u540e\u8fd4\u56de\u7684\u90a3\u4e2a\u5bf9\u8c61，\u904d\u5386\u65f6\u540c\u6837\u8981 synchronized (syncMap)\u3002<\/p>\n

      \u4f46\u8fd9\u4e5f\u662f\u628a\u53cc\u5203\u5251：\u4efb\u4f55\u6301\u6709\u8be5\u5f15\u7528\u7684\u4ee3\u7801\u90fd\u80fd\u9501\u4f4f\u6574\u5f20\u8868，\u4e00\u65e6\u6709\u4eba\u5199\u4e86 synchronized (ht) { \u8017\u65f6\u64cd\u4f5c }，\u5168\u8868\u90fd\u4f1a\u88ab\u62d6\u4f4f\u3002\u8fd9\u662f\u201c\u9501\u5bf9\u8c61\u516c\u5f00\u201d\u7684\u4ee3\u4ef7\u3002<\/p>\n

      \n

      3. null \u8bed\u4e49：\u4e3a\u4ec0\u4e48 Hashtable \u4e00\u4e2a\u90fd\u4e0d\u8ba9\u653e<\/h3>\n

      Hashtable<\/span><<\/span>String<\/span>,<\/span> String<\/span>><\/span><\/span> ht =<\/span> new<\/span> Hashtable<\/span><<\/span>><\/span><\/span>(<\/span>)<\/span>;<\/span>
      \nht.<\/span>put<\/span>(<\/span>"k"<\/span>,<\/span> null<\/span>)<\/span>;<\/span> \/\/ NullPointerException<\/span>
      \nht.<\/span>put<\/span>(<\/span>null<\/span>,<\/span> "v"<\/span>)<\/span>;<\/span> \/\/ NullPointerException<\/span><\/p>\n

      \u5b9e\u6d4b\u4e24\u8005\u7684 NPE \u6765\u6e90\u8fd8\u4e0d\u4e00\u6837：<\/p>\n\n\u8c03\u7528\u5f02\u5e38\u4f4d\u7f6e\u539f\u56e0<\/tr>\n\n\n\n
      put("k", null)<\/td>\nHashtable.put:476<\/td>\n\u663e\u5f0f\u68c0\u67e5：if (value == null) throw new NullPointerException();<\/td>\n<\/tr>\n
      put(null, "v")<\/td>\nHashtable.put:481<\/td>\n\u9690\u5f0f：\u6267\u884c key.hashCode() \u65f6\u7a7a\u6307\u9488<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

      \u800c HashMap \u5141\u8bb8 1 \u4e2a null key + \u4efb\u610f\u591a\u4e2a null value：<\/p>\n

      HashMap<\/span><<\/span>String<\/span>,<\/span> String<\/span>><\/span><\/span> hm =<\/span> new<\/span> HashMap<\/span><<\/span>><\/span><\/span>(<\/span>)<\/span>;<\/span>
      \nhm.<\/span>put<\/span>(<\/span>null<\/span>,<\/span> null<\/span>)<\/span>;<\/span>
      \nhm.<\/span>put<\/span>(<\/span>"a"<\/span>,<\/span> null<\/span>)<\/span>;<\/span>
      \nSystem<\/span>.<\/span>out.<\/span>println<\/span>(<\/span>hm.<\/span>size<\/span>(<\/span>)<\/span>)<\/span>;<\/span> \/\/ 2<\/span><\/p>\n

      \u4e3a\u4ec0\u4e48 HashMap \u5141\u8bb8 null key？ \u56e0\u4e3a\u5b83\u7684 hash \u8ba1\u7b97\u5bf9 null \u505a\u4e86\u7279\u6b8a\u5904\u7406：<\/p>\n

      static<\/span> final<\/span> int<\/span> hash<\/span>(<\/span>Object<\/span> key)<\/span> {<\/span>
      \n int<\/span> h;<\/span>
      \n return<\/span> (<\/span>key ==<\/span> null<\/span>)<\/span> ?<\/span> 0<\/span> :<\/span> (<\/span>h =<\/span> key.<\/span>hashCode<\/span>(<\/span>)<\/span>)<\/span> ^<\/span> (<\/span>h >>><\/span> 16<\/span>)<\/span>;<\/span>
      \n}<\/span><\/p>\n

      null key \u7684 hash \u6052\u4e3a 0，\u56fa\u5b9a\u843d\u5728 0 \u53f7\u6876\u3002putVal \u91cc\u7684\u5224\u65ad\u4e5f\u968f\u4e4b\u517c\u5bb9：<\/p>\n

      if<\/span> (<\/span>p.<\/span>hash ==<\/span> hash &&<\/span>
      \n (<\/span>(<\/span>k =<\/span> p.<\/span>key)<\/span> ==<\/span> key ||<\/span> (<\/span>key !=<\/span> null<\/span> &&<\/span> key.<\/span>equals<\/span>(<\/span>k)<\/span>)<\/span>)<\/span>)<\/span><\/p>\n

      \u5f53 key == null \u65f6，p.key == key \u76f4\u63a5\u547d\u4e2d（null == null \u4e3a true），equals \u6839\u672c\u4e0d\u4f1a\u88ab\u8c03\u7528，\u4e0d\u4f1a NPE\u3002<\/p>\n

      \u90a3 ConcurrentHashMap \u4e3a\u4ec0\u4e48\u4e5f\u4e0d\u5141\u8bb8 null？<\/h4>\n

      \u8fd9\u662f\u8fd9\u9053\u9898\u6700\u80fd\u62c9\u5f00\u5dee\u8ddd\u7684\u8ffd\u95ee：<\/p>\n

      \u5e76\u53d1\u573a\u666f\u4e0b，get(key) \u8fd4\u56de null \u4f1a\u4ea7\u751f\u4e8c\u4e49\u6027\u2014\u2014\u65e0\u6cd5\u533a\u5206\u662f\u201c\u8fd9\u4e2a key \u4e0d\u5b58\u5728\u201d，\u8fd8\u662f\u201c\u8fd9\u4e2a key \u5b58\u5728\u4f46\u503c\u662f null\u201d\u3002<\/p>\n

      \u5355\u7ebf\u7a0b\u4e0b\u53ef\u4ee5\u7528 containsKey \u518d\u786e\u8ba4\u4e00\u6b21，\u4f46\u5728\u5e76\u53d1\u73af\u5883\u91cc，containsKey \u548c get \u4e4b\u95f4\u72b6\u6001\u53ef\u80fd\u5df2\u7ecf\u53d8\u4e86，\u4e24\u6b21\u8c03\u7528\u4e0d\u5177\u5907\u539f\u5b50\u6027，\u8fd9\u4e2a\u786e\u8ba4\u672c\u8eab\u4e0d\u53ef\u9760\u3002<\/p>\n

      ConcurrentHashMap \u7684\u8bbe\u8ba1\u4ece\u6e90\u5934\u4e0a\u62d2\u7edd null，\u5c31\u662f\u4e3a\u4e86\u907f\u514d\u8fd9\u79cd\u201c\u770b\u8d77\u6765\u80fd\u5224\u65ad\u3001\u5b9e\u9645\u5224\u65ad\u4e0d\u51c6\u201d\u7684\u9677\u9631\u2014\u2014\u5b83\u7684 javadoc \u4e5f\u660e\u786e\u5199\u7740：\u4e0e Hashtable \u4e00\u6837，\u4e0d\u5141\u8bb8 null key \u548c null value\u3002<\/p>\n

      \u5f15\u7533\u7684\u5b9e\u8df5\u5efa\u8bae： \u4e1a\u52a1\u4ee3\u7801\u91cc\u4e5f\u5c3d\u91cf\u522b\u7528 null \u5f53\u503c\u3002\u5982\u679c\u5fc5\u987b\u8868\u8fbe\u201c\u7a7a\u201d，\u7528 Optional \u6216\u7a7a\u5bf9\u8c61\u6a21\u5f0f，\u6bd4 map.get(k) == null \u5b89\u5168\u5f97\u591a\u3002<\/p>\n

      \n

      4. \u5bb9\u91cf\u4e0e\u6269\u5bb9：11 \u548c 16，\u5dee\u7684\u4e0d\u662f\u6570\u5b57\u800c\u662f\u7b97\u6cd5<\/h3>\n\nHashMapHashtable<\/tr>\n\n\n\n\n\n
      \u9ed8\u8ba4\u521d\u59cb\u5bb9\u91cf<\/td>\n16<\/td>\n11<\/td>\n<\/tr>\n
      \u8d1f\u8f7d\u56e0\u5b50<\/td>\n0.75<\/td>\n0.75<\/td>\n<\/tr>\n
      \u6269\u5bb9\u516c\u5f0f<\/td>\nnewCap = oldCap * 2<\/td>\nnewCap = oldCap * 2 + 1<\/td>\n<\/tr>\n
      \u6269\u5bb9\u540e\u5bb9\u91cf<\/td>\n16 \u2192 32 \u2192 64<\/td>\n11 \u2192 23 \u2192 47<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

      \u5b9e\u6d4b\u6269\u5bb9\u5e8f\u5217（\u53cd\u5c04\u8bfb\u53d6 table \u6570\u7ec4\u957f\u5ea6）：<\/p>\n

      HT default table : 11
      \nHT table after put : 11
      \nHT table grown : 47 (size=31)<\/p>\n

      \u4e3a\u4ec0\u4e48 HashMap \u575a\u6301 2 \u7684\u5e42<\/h4>\n

      \u56e0\u4e3a\u5b83\u7684\u7d22\u5f15\u662f (n – 1) & hash\u3002\u53ea\u6709 n \u662f 2 \u7684\u5e42\u65f6，n – 1 \u7684\u4e8c\u8fdb\u5236\u624d\u662f\u5168 1，\u4f4d\u4e0e\u8fd0\u7b97\u624d\u7b49\u4ef7\u4e8e\u53d6\u6a21，\u4e14\u80fd\u5b8c\u6574\u4fdd\u7559\u4f4e\u4f4d\u4fe1\u606f\u3002\u4f20\u975e 2 \u7684\u5e42\u8fdb\u53bb，\u6784\u9020\u5668\u4e5f\u4f1a\u7528 tableSizeFor \u5411\u4e0a\u53d6\u6574\u3002<\/p>\n

      \u4e3a\u4ec0\u4e48 Hashtable \u7528 11 \u548c 2n+1<\/h4>\n

      \u56e0\u4e3a\u5b83\u7528\u7684\u662f\u53d6\u6a21，\u4e0d\u9700\u8981 2 \u7684\u5e42：<\/p>\n

      int<\/span> index =<\/span> (<\/span>hash &<\/span> 0x7FFFFFFF<\/span>)<\/span> %<\/span> tab.<\/span>length;<\/span><\/p>\n

      （& 0x7FFFFFFF \u662f\u4e3a\u4e86\u628a\u8d1f\u7684 hash \u53d8\u6210\u6b63\u6570，\u5426\u5219 % \u4f1a\u5f97\u5230\u8d1f\u7d22\u5f15\u3002）<\/p>\n

        \n
      • \u521d\u59cb\u5bb9\u91cf\u9009 11：11 \u662f\u8d28\u6570，\u53d6\u6a21\u540e\u5206\u5e03\u66f4\u5747\u5300，\u51cf\u5c11\u51b2\u7a81；<\/li>\n
      • \u6269\u5bb9\u9009 2n+1：\u4fdd\u6301\u5bb9\u91cf\u59cb\u7ec8\u662f\u5947\u6570，\u907f\u514d hash % n \u5728 n \u4e3a\u5076\u6570\u65f6\u4ea7\u751f\u7684\u89c4\u5f8b\u6027\u805a\u96c6\u3002<\/li>\n<\/ul>\n

        \u6027\u80fd\u5bf9\u6bd4： \u4f4d\u4e0e（&）\u786e\u5b9e\u6bd4\u53d6\u6a21（%）\u5feb，\u4f46\u8fd9\u70b9\u5dee\u5f02\u5728\u73b0\u4ee3 CPU \u4e0a\u5fae\u4e4e\u5176\u5fae\u3002Hashtable \u771f\u6b63\u7684\u6027\u80fd\u95ee\u9898\u4e0d\u5728\u5bb9\u91cf\u7b97\u6cd5，\u800c\u5728\u9501\u2014\u2014\u5373\u4f7f\u7d22\u5f15\u7b97\u5f97\u518d\u5feb，26 \u4e2a synchronized \u65b9\u6cd5\u4e5f\u4f1a\u628a\u5e76\u53d1\u6027\u80fd\u5361\u6b7b\u3002<\/p>\n

        \n

        5. \u4e00\u4e2a\u5b9e\u6d4b\u51fa\u6765\u7684\u610f\u5916：\u4e24\u4e2a\u8fed\u4ee3\u5668\u884c\u4e3a\u4e0d\u540c<\/h3>\n

        \u8fd9\u662f\u5199\u8fd9\u7bc7\u6587\u7ae0\u65f6\u6700\u6709\u610f\u601d\u7684\u53d1\u73b0\u3002<\/p>\n

        Hashtable<\/span><<\/span>String<\/span>,<\/span>String<\/span>><\/span><\/span> ht =<\/span> new<\/span> Hashtable<\/span><<\/span>><\/span><\/span>(<\/span>)<\/span>;<\/span>
        \nfor<\/span> (<\/span>int<\/span> i =<\/span> 0<\/span>;<\/span> i <<\/span> 5<\/span>;<\/span> i++<\/span>)<\/span> ht.<\/span>put<\/span>(<\/span>"k"<\/span> +<\/span> i,<\/span> "v"<\/span> +<\/span> i)<\/span>;<\/span><\/p>\n

        \/\/ \u65b9\u5f0f\u4e00：keySet().iterator()<\/span>
        \nfor<\/span> (<\/span>String<\/span> k :<\/span> ht.<\/span>keySet<\/span>(<\/span>)<\/span>)<\/span> {<\/span> ht.<\/span>put<\/span>(<\/span>"x"<\/span> +<\/span> k,<\/span> "y"<\/span>)<\/span>;<\/span> }<\/span><\/p>\n

        \/\/ \u65b9\u5f0f\u4e8c：keys() \u8fd4\u56de\u7684 Enumeration<\/span>
        \nEnumeration<\/span><<\/span>String<\/span>><\/span><\/span> en =<\/span> ht.<\/span>keys<\/span>(<\/span>)<\/span>;<\/span>
        \nwhile<\/span> (<\/span>en.<\/span>hasMoreElements<\/span>(<\/span>)<\/span>)<\/span> {<\/span>
        \n String<\/span> k =<\/span> en.<\/span>nextElement<\/span>(<\/span>)<\/span>;<\/span>
        \n ht.<\/span>put<\/span>(<\/span>"x"<\/span> +<\/span> k,<\/span> "y"<\/span>)<\/span>;<\/span>
        \n}<\/span><\/p>\n

        JDK 21 \u5b9e\u6d4b\u8f93\u51fa：<\/p>\n

        keySet() + \u904d\u5386\u4e2d\u4fee\u6539 : java.util.ConcurrentModificationException
        \nkeys() + \u904d\u5386\u4e2d\u4fee\u6539 : \u65e0\u5f02\u5e38，size = 12<\/p>\n

        \u540c\u4e00\u4e2a\u7c7b，\u4e24\u79cd\u8fed\u4ee3\u65b9\u5f0f，\u884c\u4e3a\u5b8c\u5168\u76f8\u53cd\u3002<\/p>\n

        \u539f\u56e0\u5728 Hashtable.Enumerator \u7684\u6784\u9020\u53c2\u6570：<\/p>\n

        \/\/ \u7b80\u5316\u793a\u610f<\/span>
        \nEnumerator<\/span>(<\/span>Hashtable<\/span><<\/span>K<\/span>,<\/span>V<\/span>><\/span><\/span> table,<\/span> boolean<\/span> iterator)<\/span> {<\/span>
        \n .<\/span>.<\/span>.<\/span>
        \n this<\/span>.<\/span>expectedModCount =<\/span> iterator ?<\/span> table.<\/span>modCount :<\/span> 0<\/span>;<\/span> \/\/ \u53ea\u6709 iterator \u6a21\u5f0f\u624d\u8bb0\u5f55<\/span>
        \n}<\/span><\/p>\n

          \n
        • keySet() \/ entrySet() \u8d70\u7684\u662f Iterator \u6a21\u5f0f（iterator = true），\u6bcf\u6b21 next() \u6821\u9a8c modCount，\u6539\u4e86\u7ed3\u6784\u5c31\u629b CME \u2014\u2014 \u8fd9\u5c31\u662f fail-fast；<\/li>\n
        • keys() \/ elements() \u8fd4\u56de\u7684\u662f\u8001\u5f0f Enumeration（iterator = false），\u4e0d\u505a\u6821\u9a8c，\u904d\u5386\u671f\u95f4\u4fee\u6539\u4e0d\u62a5\u9519，\u4f46\u53ef\u80fd\u8bfb\u5230\u91cd\u590d\u6216\u5df2\u5220\u9664\u7684\u5143\u7d20，\u7ed3\u679c\u4e0d\u786e\u5b9a\u3002<\/li>\n<\/ul>\n

          \u8fd9\u4e2a\u5dee\u5f02\u7684\u5b9e\u9645\u5f71\u54cd：<\/p>\n

          \u8001\u9879\u76ee\u91cc\u7528 keys() \u904d\u5386 Hashtable \u4e00\u76f4\u76f8\u5b89\u65e0\u4e8b，\u67d0\u5929\u91cd\u6784\u628a\u5b83\u6539\u6210 keySet()，\u7a81\u7136\u5f00\u59cb\u5076\u53d1 ConcurrentModificationException\u2014\u2014\u8fd9\u4e0d\u662f bug，\u662f\u884c\u4e3a\u53d8\u4e86\u3002<\/p>\n

          \u6b63\u786e\u59ff\u52bf（\u4e24\u79cd\u90fd\u4e00\u6837）：\u904d\u5386 Hashtable \u5fc5\u987b\u81ea\u5df1\u52a0\u9501\u3002<\/p>\n

          synchronized<\/span> (<\/span>ht)<\/span> {<\/span>
          \n for<\/span> (<\/span>String<\/span> k :<\/span> ht.<\/span>keySet<\/span>(<\/span>)<\/span>)<\/span> {<\/span> .<\/span>.<\/span>.<\/span> }<\/span>
          \n}<\/span><\/p>\n

          \u56e0\u4e3a\u8fed\u4ee3\u5668\u672c\u8eab\u4e0d\u6301\u6709\u9501，\u5b83\u53ea\u662f\u5728\u8bfb table \u6570\u7ec4，\u5176\u4ed6\u7ebf\u7a0b\u968f\u65f6\u53ef\u80fd rehash \u628a\u6570\u7ec4\u6362\u6389\u3002<\/p>\n

          \u6700\u540e\u4e00\u53e5\u8bdd\u8bb0\u4f4f：fail-fast \u662f\u201c\u5c3d\u529b\u68c0\u6d4b\u201d，\u4e0d\u662f\u201c\u5e76\u53d1\u5b89\u5168\u4fdd\u969c\u201d\u3002 \u5b83\u53ea\u80fd\u5e2e\u4f60\u5c3d\u65e9\u53d1\u73b0\u8bef\u7528，\u4e0d\u80fd\u66ff\u4ee3\u540c\u6b65\u3002<\/p>\n

          \n

          6. \u4e09\u4e2a\u5bb9\u6613\u8e29\u7684 API \u9677\u9631<\/h3>\n

          6.1 contains() \u5224\u7684\u662f\u503c<\/h4>\n

          ht.<\/span>contains<\/span>(<\/span>"v"<\/span>)<\/span> \/\/ \u7b49\u4ef7\u4e8e containsValue("v")<\/span><\/p>\n

          \u60f3\u77e5\u9053 key \u5728\u4e0d\u5728，\u7528 containsKey\u3002\u8fd9\u4e2a\u547d\u540d\u5751\u5728\u7ef4\u62a4\u8001\u4ee3\u7801\u65f6\u5f88\u5e38\u89c1\u3002<\/p>\n

          6.2 putAll \u53ef\u80fd\u534a\u9014\u5931\u8d25<\/h4>\n

          Hashtable<\/span><<\/span>String<\/span>,<\/span>String<\/span>><\/span><\/span> ht =<\/span> new<\/span> Hashtable<\/span><<\/span>><\/span><\/span>(<\/span>)<\/span>;<\/span>
          \nMap<\/span><<\/span>String<\/span>,<\/span>String<\/span>><\/span><\/span> src =<\/span> new<\/span> HashMap<\/span><<\/span>><\/span><\/span>(<\/span>)<\/span>;<\/span>
          \nsrc.<\/span>put<\/span>(<\/span>"a"<\/span>,<\/span> "1"<\/span>)<\/span>;<\/span>
          \nsrc.<\/span>put<\/span>(<\/span>"b"<\/span>,<\/span> null<\/span>)<\/span>;<\/span> \/\/ \u8fd9\u4e00\u6761\u4f1a\u8ba9 putAll \u4e2d\u9014\u629b NPE<\/span>
          \nht.<\/span>putAll<\/span>(<\/span>src)<\/span>;<\/span> \/\/ \u7ed3\u679c：ht \u91cc\u6709 "a"，\u6ca1\u6709 "b"<\/span><\/p>\n

          \u90e8\u5206\u5199\u5165\u6210\u529f\u2014\u2014putAll \u6ca1\u6709\u4e8b\u52a1\u6027\u3002\u4ece HashMap \u8fc1\u79fb\u6570\u636e\u5230 Hashtable \u65f6，\u5982\u679c\u6e90\u6570\u636e\u91cc\u6709 null \u503c，\u5c31\u4f1a\u9047\u5230\u8fd9\u79cd\u201c\u534a\u4e2a\u8868\u201d\u7684\u72b6\u6001\u3002<\/p>\n

          6.3 Dictionary \u4e0d\u662f Map<\/h4>\n

          Hashtable \u5b9e\u73b0\u7684\u662f Map \u63a5\u53e3（JDK 1.2 \u540e\u8865\u7684），\u4f46\u5b83\u540c\u65f6\u7ee7\u627f\u4e86 Dictionary\u3002\u6240\u4ee5\u5b83\u80fd\u88ab\u5f53\u6210 Map \u7528，\u4f46 Dictionary \u7c7b\u578b\u672c\u8eab\u6ca1\u6709 entrySet()\u3001keySet() \u8fd9\u4e9b\u73b0\u4ee3 API\u3002\u8001\u4ee3\u7801\u91cc\u5982\u679c\u53d8\u91cf\u58f0\u660e\u6210 Dictionary，\u80fd\u8c03\u7684\u65b9\u6cd5\u975e\u5e38\u6709\u9650\u3002<\/p>\n

          \n

          7. \u73b0\u5728\u5230\u5e95\u8be5\u600e\u4e48\u9009<\/h3>\n\n\u573a\u666f\u9009\u62e9\u7406\u7531<\/tr>\n\n\n\n\n\n\n\n
          \u5355\u7ebf\u7a0b<\/td>\nHashMap<\/td>\n\u65e0\u9501\u5f00\u9500，\u6027\u80fd\u6700\u597d<\/td>\n<\/tr>\n
          \u591a\u7ebf\u7a0b\u8bfb\u5199<\/td>\nConcurrentHashMap<\/td>\n\u6876\u7ea7\u9501 + CAS，\u8bfb\u51e0\u4e4e\u65e0\u9501<\/td>\n<\/tr>\n
          \u9700\u8981\u5b58 null \u503c<\/td>\nHashMap<\/td>\nHashtable \u548c CHM \u90fd\u4e0d\u5141\u8bb8<\/td>\n<\/tr>\n
          \u5df2\u6709 Map \u8981\u52a0\u540c\u6b65<\/td>\nCollections.synchronizedMap<\/td>\n\u80fd\u5305\u88c5\u4efb\u610f Map，\u4f46\u540c\u6837\u662f\u5168\u8868\u9501<\/td>\n<\/tr>\n
          \u904d\u5386\u65f6\u8981\u5f3a\u4e00\u81f4<\/td>\n\u81ea\u5df1\u52a0\u9501\u6216\u7528\u5feb\u7167<\/td>\n\u4efb\u4f55\u5e76\u53d1 Map \u7684\u8fed\u4ee3\u5668\u90fd\u662f\u5f31\u4e00\u81f4<\/td>\n<\/tr>\n
          \u9057\u7559\u4ee3\u7801<\/td>\n\u4fdd\u6301，\u9010\u6b65\u66ff\u6362<\/td>\n\u76f4\u63a5\u6362\u4f1a\u8e29 null \u548c contains \u7684\u5751<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

          \u4e3a\u4ec0\u4e48 Collections.synchronizedMap \u4e5f\u4e0d\u63a8\u8350？ \u5b83\u548c Hashtable \u672c\u8d28\u76f8\u540c\u2014\u2014\u90fd\u662f\u5bf9\u6574\u5f20\u8868\u52a0\u540c\u4e00\u628a\u9501，\u5e76\u53d1\u6027\u80fd\u548c Hashtable \u4e00\u4e2a\u6c34\u5e73，\u53ea\u662f\u9501\u5bf9\u8c61\u53ef\u7531\u4f60\u6307\u5b9a\u3002<\/p>\n

          ConcurrentHashMap \u7684\u8fed\u4ee3\u5668\u662f\u5f31\u4e00\u81f4\u7684：\u904d\u5386\u65f6\u4e0d\u963b\u585e\u3001\u4e0d\u629b CME，\u4f46\u4e0d\u4fdd\u8bc1\u770b\u5230\u6700\u65b0\u7684\u5199\u5165\u3002\u8fd9\u662f\u5e76\u53d1\u5bb9\u5668\u201c\u7528\u4e00\u81f4\u6027\u6362\u541e\u5410\u201d\u7684\u5178\u578b\u8bbe\u8ba1\u3002<\/p>\n

          \n

          8. \u9ad8\u9891\u8ffd\u95ee 10 \u8fde<\/h3>\n

          Q1：Hashtable \u662f\u7ebf\u7a0b\u5b89\u5168\u7684\u5417？
          \n\u5355\u4e2a\u65b9\u6cd5\u662f（synchronized），\u4f46\u590d\u5408\u64cd\u4f5c\u4e0d\u662f\u3002\u201c\u5148\u5224\u65ad\u518d\u64cd\u4f5c\u201d\u5fc5\u987b\u81ea\u5df1 synchronized (ht) \u5305\u8d77\u6765\u3002<\/p>\n

          Q2：\u4e3a\u4ec0\u4e48\u521d\u59cb\u5bb9\u91cf\u662f 11？
          \n\u5386\u53f2\u9057\u7559 + \u53d6\u6a21\u7b97\u6cd5\u9700\u8981\u8d28\u6570\u300211 \u662f\u8d28\u6570，hash % 11 \u5206\u5e03\u5747\u5300\u3002\u6362\u6210 16 \u4e5f\u4e0d\u4f1a\u9519，\u53ea\u662f\u51b2\u7a81\u7387\u7565\u53d8\u3002<\/p>\n

          Q3：\u4e3a\u4ec0\u4e48\u6269\u5bb9\u662f 2n + 1？
          \n\u4fdd\u6301\u5bb9\u91cf\u59cb\u7ec8\u4e3a\u5947\u6570，\u907f\u514d\u5076\u6570\u5bb9\u91cf\u5728\u53d6\u6a21\u65f6\u4ea7\u751f\u7684\u89c4\u5f8b\u6027\u805a\u96c6\u3002<\/p>\n

          Q4：ConcurrentHashMap \u4e3a\u4ec0\u4e48\u6bd4 Hashtable \u5feb？
          \nHashtable \u662f\u6574\u8868\u4e00\u628a\u9501；CHM \u5728 JDK 7 \u662f\u5206\u6bb5\u9501，JDK 8 \u8d77\u662f CAS + synchronized \u9501\u5355\u4e2a\u6876\u7684\u5934\u8282\u70b9，\u9501\u7c92\u5ea6\u7ec6\u5316\u5230\u6876\u7ea7\u522b，\u8bfb\u64cd\u4f5c\u57fa\u672c\u65e0\u9501\u3002<\/p>\n

          Q5：HashMap \u7684 null key \u5b58\u5728\u54ea\u4e2a\u6876？
          \n0 \u53f7\u6876，\u56e0\u4e3a hash(null) \u8fd4\u56de 0\u3002<\/p>\n

          Q6：\u4e3a\u4ec0\u4e48 ConcurrentHashMap \u4e0d\u5141\u8bb8 null？
          \n\u5e76\u53d1\u4e0b get \u8fd4\u56de null \u65e0\u6cd5\u533a\u5206\u201c\u952e\u4e0d\u5b58\u5728\u201d\u548c\u201c\u503c\u4e3a null\u201d，\u800c containsKey \u4e0e get \u4e4b\u95f4\u6ca1\u6709\u539f\u5b50\u6027，\u8fd9\u4e2a\u5224\u65ad\u4e0d\u53ef\u9760，\u4f1a\u4ea7\u751f\u9690\u853d\u7684\u5e76\u53d1 bug\u3002<\/p>\n

          Q7：\u628a Hashtable \u6362\u6210 HashMap \u8981\u6ce8\u610f\u4ec0\u4e48？
          \n\u4e09\u4ef6\u4e8b：\u2460 null \u8bed\u4e49\u53d8\u4e86（Hashtable \u62d2\u7edd，HashMap \u63a5\u53d7，\u53ef\u80fd\u628a\u539f\u672c\u7684 NPE \u53d8\u6210\u201c\u6084\u6084\u5b58\u5165\u201d）；\u2461 contains(v) \u8981\u6539\u6210 containsValue(v)；\u2462 \u540c\u6b65\u8981\u9760\u8c03\u7528\u65b9\u81ea\u5df1\u4fdd\u8bc1\u3002<\/p>\n

          Q8：Dictionary \u548c Map \u4ec0\u4e48\u533a\u522b？
          \nDictionary \u662f\u62bd\u8c61\u7c7b，Map \u662f\u63a5\u53e3\u3002Dictionary \u5df2\u88ab\u5b98\u65b9\u6807\u6ce8 obsolete，\u65b0\u7684\u5b9e\u73b0\u90fd\u5e94\u8be5\u5b9e\u73b0 Map\u3002<\/p>\n

          Q9：\u4e3a\u4ec0\u4e48\u73b0\u5728\u8fd8\u4f1a\u88ab\u95ee\u5230 Hashtable？
          \n\u56e0\u4e3a\u5b83\u662f\u4e00\u9053\u201c\u6d3b\u5316\u77f3\u9898\u201d\u2014\u2014\u7531\u5b83\u5f15\u51fa\u7ebf\u7a0b\u5b89\u5168\u3001null \u8bed\u4e49\u3001\u6269\u5bb9\u7b97\u6cd5\u3001fail-fast \u56db\u4e2a\u77e5\u8bc6\u70b9，\u8003\u5bdf\u6210\u672c\u4f4e\u3001\u533a\u5206\u5ea6\u9ad8\u3002<\/p>\n

          Q10：HashMap \u7ebf\u7a0b\u4e0d\u5b89\u5168\u7684\u5177\u4f53\u8868\u73b0\u662f\u4ec0\u4e48？
          \n\u5e76\u53d1 put \u4e22\u6570\u636e\u3001JDK 7 \u6269\u5bb9\u5934\u63d2\u6cd5\u6210\u73af\u5bfc\u81f4 get \u6b7b\u5faa\u73af\u3001size \u8ba1\u6570\u4e0d\u51c6\u3002\u8fd9\u4e09\u70b9\u5c55\u5f00\u5c31\u662f\u4e0b\u4e00\u7bc7\u7684\u5185\u5bb9\u3002<\/p>\n

          \n

          9. 3 \u5206\u949f\u7b54\u9898\u6a21\u677f<\/h3>\n
        • \u7ed3\u8bba：Hashtable \u662f JDK 1.0 \u7684\u9057\u7559\u7c7b，\u65b9\u6cd5\u7ea7 synchronized，\u6574\u8868\u4e00\u628a\u9501；HashMap \u662f JDK 1.2 \u7684\u66ff\u4ee3\u54c1，\u4e0d\u4fdd\u8bc1\u7ebf\u7a0b\u5b89\u5168\u4f46\u6027\u80fd\u597d\u5f97\u591a；<\/li>\n
        • \u5dee\u5f02：\u5206\u56db\u6761\u8bb2\u2014\u2014\u7ebf\u7a0b\u5b89\u5168（\u5168\u8868\u9501 vs \u65e0\u9501）\u3001null（\u90fd\u4e0d\u5141\u8bb8 vs \u5141\u8bb8 1 \u4e2a null key）\u3001\u5bb9\u91cf（11 \u53d6\u6a21 vs 16 \u4f4d\u4e0e）\u3001\u8fed\u4ee3\u5668（Enumeration vs fail-fast Iterator）；<\/li>\n
        • \u4e3a\u4ec0\u4e48\u6dd8\u6c70：\u5168\u8868\u9501\u5bfc\u81f4\u5e76\u53d1\u9000\u5316\u6210\u4e32\u884c，\u73b0\u4ee3\u573a\u666f\u7528 ConcurrentHashMap；<\/li>\n
        • \u52a0\u5206：\u63d0\u4e00\u53e5\u201cHashtable \u7684 keys() \u4e0d\u662f fail-fast，keySet() \u624d\u662f\u201d，\u4ee5\u53ca\u201cCHM \u62d2\u7edd null \u662f\u4e3a\u4e86\u907f\u514d\u5e76\u53d1\u4e8c\u4e49\u6027\u201d\u3002<\/li>\n
          \n

          \u5c0f\u7ed3<\/h3>\n

          Hashtable \u2192 JDK 1.0，\u7ee7\u627f Dictionary，\u5168\u8868 synchronized
          \nHashMap \u2192 JDK 1.2，AbstractMap，\u65e0\u9501\u4f46\u9ad8\u6027\u80fd
          \nnull \u2192 HT \u5168\u62d2\u7edd；HM \u5141\u8bb8 1 \u4e2a null key（0 \u53f7\u6876）
          \n\u5bb9\u91cf \u2192 HT: 11 \u8d77\u6b65，2n+1，\u53d6\u6a21；HM: 16 \u8d77\u6b65，2 \u500d，\u4f4d\u4e0e
          \n\u8fed\u4ee3\u5668 \u2192 HT: keys() \u4e0d fail-fast，keySet() fail-fast
          \n\u73b0\u4ee3\u66ff\u4ee3 \u2192 ConcurrentHashMap<\/p>\n

          \u5b9e\u6d4b\u73af\u5883：JDK 21.0.8（Windows），\u53cd\u5c04\u90e8\u5206\u901a\u8fc7 –add-opens java.base\/java.util=ALL-UNNAMED \u8bfb\u53d6\u5185\u90e8 table \u5b57\u6bb5\u3002<\/p>\n

          \u4e0b\u4e00\u7bc7\u62c6 \u300csynchronized \u548c ReentrantLock \u7684\u533a\u522b\u300d\u2014\u2014\u4e00\u4e2a\u5173\u952e\u5b57\u4e00\u4e2a\u7c7b，\u4e3a\u4ec0\u4e48 ReentrantLock \u80fd\u4e2d\u65ad\u3001\u80fd\u8d85\u65f6\u3001\u80fd\u516c\u5e73，\u4ee5\u53ca\u4e3a\u4ec0\u4e48 JDK 6 \u4e4b\u540e\u300csynchronized \u6162\u300d\u8fd9\u4e2a\u7ed3\u8bba\u5df2\u7ecf\u8fc7\u671f\u4e86\u3002<\/p>\n

          \n","protected":false},"excerpt":{"rendered":"

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