{"id":366,"date":"2013-02-16T16:20:28","date_gmt":"2013-02-16T16:20:28","guid":{"rendered":"http:\/\/lg.yusemo.com\/?p=366"},"modified":"2016-08-26T02:08:13","modified_gmt":"2016-08-26T02:08:13","slug":"two-ways-to-traverse-the-hashmap","status":"publish","type":"post","link":"https:\/\/lg.yusemo.com\/?p=366","title":{"rendered":"HashMap\u4e2d\u904d\u5386\u7684\u4e24\u79cd\u65b9\u5f0f"},"content":{"rendered":"

\u6839\u636ejdk\u7684api\u4e2d\u7684\u4ecb\u7ecd\uff0chashMap\u6709\u4ee5\u4e0b\u7279\u70b9\uff1a<\/p>\n

1\uff09\u5141\u8bb8\u4f7f\u7528 null<\/tt> \u503c\u548c null<\/tt> \u952e<\/p>\n

2\uff09\u975e\u540c\u6b65\uff08\u540c\u6b65\u9700\u8981\u5c01\u88c5\uff1a<\/p>\n


<span style=\"font-family: Consolas, Monaco, monospace; font-size: 12px; line-height: 18px;\">Map m = Collections.synchronizedMap(new HashMap(...));)<\/span>\n\n<\/code><\/pre>\n
3)HashMap<\/tt> \u7684\u5b9e\u4f8b\u6709\u4e24\u4e2a\u53c2\u6570\u5f71\u54cd\u5176\u6027\u80fd\uff1a\u521d\u59cb\u5bb9\u91cf \u548c\u52a0\u8f7d\u56e0\u5b50\u3002\u5982\u679c\u8981\u4fdd\u8bc1\u904d\u5386\u7684\u6548\u7387\uff0c\u5219\u4e0d\u8981\u5c06\u521d\u59cb\u5bb9\u91cf\u8bbe\u7f6e\u5f97\u592a\u9ad8\uff08\u6216\u5c06\u52a0\u8f7d\u56e0\u5b50\u8bbe\u7f6e\u5f97\u592a\u4f4e\uff09\u3002\u9ed8\u8ba4\u7684\u52a0\u8f7d\u5f15\u5b50\u662f0.75f\u3002\u5982\u679c\u5b58\u50a8\u7684entries\u4e0d\u8d85\u8fc7\u5f53\u524d\u5bb9\u91cf\u548c\u52a0\u8f7d\u56e0\u5b50\u7684\u7d2f\u79ef\u65f6\uff0c\u5219\u4e0d\u4f1a\u53d1\u751f rehash \u64cd\u4f5c\uff0c\u8fd9\u6837\u6548\u7387\u67e5\u8be2\u548c\u5b58\u50a8\u7684\u6548\u7387\u90fd\u4f1a\u8f83\u9ad8\uff08ps:\u3002hashmap\u6700\u5927\u5b58\u50a8\u5bb9\u91cf1<<30(\u5373\uff1a2^30\uff09,\u521d\u59cb\u5bb9\u91cf\u662f16\uff081<<4\uff09<\/p>\n
\u5927\u90e8\u5206\u60c5\u51b5\u4e0b\uff0c\u90fd\u4e0d\u4f1a\u53bb\u66f4\u6539\u521d\u59cb\u5bb9\u91cf\u548c\u52a0\u8f7d\u56e0\u5b50\u7684\uff0c\u7cfb\u7edf\u9ed8\u8ba4\u76840.75\u5bf9\u65f6\u95f4\u548c\u7a7a\u95f4\u53d6\u4e86\u4e00\u79cd\u6298\u4e2d\u7684\u65b9\u5f0f\u3002<\/p>\n
\u5728\u65e5\u5e38\u5e94\u7528\u4e2d\u5e38\u5e38\u4f1a\u904d\u5386hashmap\uff0c\u6839\u636e\u4e00\u4e2a\u6d4b\u8bd5\u4ee3\u7801\uff0c\u53d1\u73b0\u4e0b\u9762\u4e24\u79cd\u904d\u5386\u65b9\u5f0f\u7684\u6548\u7387\u622a\u7136\u4e0d\u540c\u3002
\n\u4e00\u3001<\/p>\n
Map map = new HashMap();\nIterator iter = map.entrySet().iterator();\nwhile (iter.hasNext()) {\nMap.Entry entry = (Map.Entry) iter.next();\nObject key = entry.getKey();\nObject val = entry.getValue();\n}\n<\/code><\/pre>\n
\u6548\u7387\u9ad8,\u4ee5\u540e\u5efa\u8bae\u91c7\u7528\u4f7f\u7528\u6b64\u79cd\u65b9\u5f0f
\n\u4e8c\u3001<\/p>\n
Map map = new HashMap();\nIterator iter = map.keySet().iterator();\nwhile (iter.hasNext()) {\nObject key = iter.next();\nObject val = map.get(key);\n}\n<\/code><\/pre>\n
\u6548\u7387\u4f4e,\u4ee5\u540e\u5c3d\u91cf\u5c11\u4f7f\u7528
\n\u4e24\u79cd\u65b9\u5f0f\u7684\u6d4b\u8bd5\u4ee3\u7801\uff1a<\/p>\n
public class HashMapTest {\npublic static void main(String[] args) {\nHashMap<Integer, String> hashmap = new HashMap<Integer, String>();\nfor (int i = 0; i < 20000; i++) {\nhashmap.put(i, \"keyset\");\n}\nlong bs = Calendar.getInstance().getTimeInMillis();\nIterator<Integer> iterator = hashmap.keySet().iterator();\nwhile (iterator.hasNext()) {\nSystem.out.print(hashmap.get(iterator.next()));\n}\nSystem.out.println(\"\\r\\n---keyset\u904d\u5386------\" + bs);\nSystem.out.println(Calendar.getInstance().getTimeInMillis() - bs);\nlistHashMap();\n}\n\/**\n*\n* listHashMap \u65b9\u6cd5\n* <p>\u65b9\u6cd5\u8bf4\u660e:entry iterator<\/p>\n*\/\npublic static void listHashMap() {\nHashMap<Integer, String> hashmap = new HashMap<Integer, String>();\nfor (int i = 0; i < 20000; i++) {\nhashmap.put(i, \"entry\");\n}\nlong bs = Calendar.getInstance().getTimeInMillis();\nIterator<Entry<Integer, String>> it = hashmap.entrySet().iterator();\nwhile (it.hasNext()) {\nEntry<Integer, String> entry = it.next();\n\/\/ entry.getKey() \u8fd4\u56de\u4e0e\u6b64\u9879\u5bf9\u5e94\u7684\u952e\n\/\/ entry.getValue() \u8fd4\u56de\u4e0e\u6b64\u9879\u5bf9\u5e94\u7684\u503c\nSystem.out.print(entry.getValue());\n}\nSystem.out.println(\"\\r\\n---keyEntry\u904d\u5386------\");\nSystem.out.println(Calendar.getInstance().getTimeInMillis() - bs);\n}\n}\n<\/code><\/pre>\n
\u5bf9\u4e8ekeySet\u5176\u5b9e\u662f\u904d\u5386\u4e862\u6b21\uff0c\u4e00\u6b21\u662f\u8f6c\u4e3aiterator\uff0c\u4e00\u6b21\u5c31\u4ecehashmap\u4e2d\u53d6\u51fakey\u6240\u5bf9\u4e8e\u7684value\u3002\u800centryset\u53ea\u662f\u904d\u5386\u4e86\u7b2c\u4e00\u6b21\uff0c\u4ed6\u628akey\u548cvalue\u90fd\u653e\u5230\u4e86entry\u4e2d\uff0c\u6240\u4ee5\u5c31\u5feb\u4e86<\/p>\n","protected":false},"excerpt":{"rendered":"
\u6839\u636ejdk\u7684api\u4e2d\u7684\u4ecb\u7ecd\uff0chashMap\u6709\u4ee5\u4e0b\u7279\u70b9\uff1a 1\uff09\u5141\u8bb8\u4f7f\u7528 null \u503c\u548c null \u952e 2\uff09\u975e\u540c\u6b65\uff08\u540c\u6b65\u9700\u8981\u5c01\u88c5\uff1a <span style=”font-family: Consolas, Monaco, monospace; font-size: 12px; line-height…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[22],"tags":[3],"_links":{"self":[{"href":"https:\/\/lg.yusemo.com\/index.php?rest_route=\/wp\/v2\/posts\/366"}],"collection":[{"href":"https:\/\/lg.yusemo.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/lg.yusemo.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/lg.yusemo.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/lg.yusemo.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=366"}],"version-history":[{"count":15,"href":"https:\/\/lg.yusemo.com\/index.php?rest_route=\/wp\/v2\/posts\/366\/revisions"}],"predecessor-version":[{"id":446,"href":"https:\/\/lg.yusemo.com\/index.php?rest_route=\/wp\/v2\/posts\/366\/revisions\/446"}],"wp:attachment":[{"href":"https:\/\/lg.yusemo.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=366"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/lg.yusemo.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=366"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/lg.yusemo.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=366"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}