{"id":1612,"date":"2021-12-26T19:05:42","date_gmt":"2021-12-26T11:05:42","guid":{"rendered":"https:\/\/www.zeroheart.xyz\/wordpress\/?p=1612"},"modified":"2021-12-30T11:43:31","modified_gmt":"2021-12-30T03:43:31","slug":"java%e5%b0%8f%e7%bb%86%e8%8a%82%ef%bc%88%e6%8c%81%e7%bb%ad%e6%9b%b4%e6%96%b0%ef%bc%89","status":"publish","type":"post","link":"https:\/\/www.zeroheart.xyz\/wordpress\/?p=1612","title":{"rendered":"java\u5c0f\u7ec6\u8282\uff5c\u963f\u91ccjava\u5f00\u53d1\u624b\u518c\uff5c\uff08\u6301\u7eed\u66f4\u65b0\uff09"},"content":{"rendered":"\n
1.ConcurrentHashMap \u7684key\u548cvalue\u90fd\u4e0d\u80fd\u4e3anull<\/pre>\n\n\n\n
\"\"<\/figure>\n\n\n\n
2.jdk8\u4ee5\u4e0b map \u5faa\u73af\u4f7f\u7528entrySet,jdk8\u4ee5\u540e\u4f7f\u7528Map.forEach<\/pre>\n\n\n\n
1.7\nSet<Map.Entry<String, String>> entryseSet=map.entrySet(); \nfor (Map.Entry<String, String> entry:entryseSet) {\n \n    System.out.println(entry.getKey()+\",\"+entry.getValue());\n \n}\n\n1.8\nmap.forEach((k,v) -> {\n            System.out.println(k);\n            System.out.println(v);\n        });\n        <\/code><\/pre>\n\n\n\n
3.\u5982\u679c\u6bcf\u6b21\u8bbf\u95ee\u51b2\u7a81\u6982\u7387\u5c0f\u4e8e 20%\uff0c\u63a8\u8350\u4f7f\u7528\u4e50\u89c2\u9501\uff0c\u5426\u5219\u4f7f\u7528\u60b2\u89c2\u9501\u3002\u4e50\u89c2\u9501\u7684\u91cd\u8bd5\u6b21\u6570\u4e0d\u5f97\u5c0f\u4e8e 3 \u6b21\u3002<\/pre>\n\n\n\n
4.\u65e5\u671f\u683c\u5f0f\u5316\u65f6\uff0c\u4f20\u5165pattern\u4e2d\u8868\u793a\u5e74\u4efd\u7edf\u4e00\u4f7f\u7528\u5c0f\u5199\u7684y\u3002\n\nyyyy-MM-dd HH:mm:ss\n\n\u5927\u5199\u7684 YYYY \u4ee3\u8868\u662f week in which year (JDK7 \u4e4b\u540e\u5f15\u5165\u7684\u6982\u5ff5)\uff0c\u610f\u601d\u662f\u5f53\u5929\u6240\u5728\u7684\u5468\u5c5e\u4e8e\u7684\u5e74\u4efd\uff0c\u4e00\u5468\u4ece\u5468\u65e5\u5f00\u59cb\uff0c\u5468\u516d\u7ed3\u675f\uff0c \u53ea\u8981\u672c\u5468\u8de8\u5e74\uff0c\u8fd4\u56de\u7684 YYYY \u5c31\u662f\u4e0b\u4e00\u5e74<\/pre>\n\n\n\n
5.\u4e0d\u8981\u4f7f\u7528count(\u5217\u540d)\u6216count(\u5e38\u91cf)\u6765\u66ff\u4ee3count(*)\uff0ccount(*)\u662fSQL92\u5b9a\u4e49\u7684 \u6807\u51c6\u7edf\u8ba1\u884c\u6570\u7684\u8bed\u6cd5\uff0c\u8ddf\u6570\u636e\u5e93\u65e0\u5173\uff0c\u8ddf NULL \u548c\u975e NULL \u65e0\u5173\u3002\n<\/pre>\n\n\n\n
6.\u4efb\u4f55\u8d27\u5e01\u91d1\u989d\uff0c\u5747\u4ee5\u6700\u5c0f\u8d27\u5e01\u5355\u4f4d\u4e14\u6574\u578b\u7c7b\u578b\u6765\u8fdb\u884c\u5b58\u50a8\u3002<\/pre>\n\n\n\n
7.\u6d6e\u70b9\u6570\u4e4b\u95f4\u7684\u7b49\u503c\u5224\u65ad\uff0c\u57fa\u672c\u6570\u636e\u7c7b\u578b\u4e0d\u80fd\u7528==\u6765\u6bd4\u8f83\uff0c\u5305\u88c5\u6570\u636e\u7c7b\u578b\u4e0d\u80fd\u7528\nequals\u6765\u5224\u65ad\u3002\n(1)\u6307\u5b9a\u4e00\u4e2a\u8bef\u5dee\u8303\u56f4\uff0c\u4e24\u4e2a\u6d6e\u70b9\u6570\u7684\u5dee\u503c\u5728\u6b64\u8303\u56f4\u4e4b\u5185\uff0c\u5219\u8ba4\u4e3a\u662f\u76f8\u7b49\u7684\u3002\nfloat a = 1.0F - 0.9F;\nfloat b = 0.9F - 0.8F;\nfloat diff = 1e-6F;\nif (Math.abs(a - b) < diff)\n{ \nSystem.out.println(\"true\");\n}\n(2)\n\u4f7f\u7528BigDecimal\u6765\u5b9a\u4e49\u503c\uff0c\u518d\u8fdb\u884c\u6d6e\u70b9\u6570\u7684\u8fd0\u7b97\u64cd\u4f5c\u3002\nBigDecimal a = new BigDecimal(\"1.0\");\nBigDecimal b = new BigDecimal(\"0.9\");\nBigDecimal c = new BigDecimal(\"0.8\");\nBigDecimal x = a.subtract(b);\nBigDecimal y = b.subtract(c);\nif (x.compareTo(y) == 0)\n{ \nSystem.out.println(\"true\");\n}<\/pre>\n\n\n\n
8.BigDecimal\u7684\u7b49\u503c\u6bd4\u8f83\u5e94\u4f7f\u7528compareTo()\u65b9\u6cd5\uff0c\u800c\u4e0d\u662fequals()\u65b9\u6cd5\u3002\nequals()\u65b9\u6cd5\u4f1a\u6bd4\u8f83\u503c\u548c\u7cbe\u5ea6\uff081.0\u4e0e1.00\u8fd4\u56de\u7ed3\u679c\u4e3afalse\uff09\uff0c\u800ccompareTo()\u5219\u4f1a\u5ffd\u7565\u7cbe\u5ea6\u3002<\/pre>\n\n\n\n
9.\u5904\u7406double\u8f6cBigDecimal\uff0c\u9700\u8981\u6ce8\u610f<\/pre>\n\n\n\n
\"\"<\/figure>\n\n\n\n
10.\u5173\u4e8e\u57fa\u672c\u6570\u636e\u7c7b\u578b\u4e0e\u5305\u88c5\u6570\u636e\u7c7b\u578b\u7684\u4f7f\u7528\u6807\u51c6\u5982\u4e0b\uff1a<\/pre>\n\n\n\n
\"\"<\/figure>\n\n\n\n
11.\u5728\u4f7f\u7528\u963b\u585e\u7b49\u5f85\u83b7\u53d6\u9501\u7684\u65b9\u5f0f\u4e2d\uff0c\u5fc5\u987b\u5728try\u4ee3\u7801\u5757\u4e4b\u5916\uff0c\u5e76\u4e14\u5728\u52a0\u9501\u65b9\u6cd5\u4e0etry\u4ee3\u7801\u5757\u4e4b\u95f4\u6ca1\u6709\u4efb\u4f55\u53ef\u80fd\u629b\u51fa\u5f02\u5e38\u7684\u65b9\u6cd5\u8c03\u7528\uff0c\u907f\u514d\u52a0\u9501\u6210\u529f\u540e\uff0c\u5728finally\u4e2d\u65e0\u6cd5\u89e3\u9501\u3002<\/pre>\n\n\n\n
\"\"<\/figure>\n\n\n\n
12.\u5982\u679c\u4f7f\u7528\u5c1d\u8bd5\u673a\u5236\u6765\u83b7\u53d6\u9501\uff0c\u9700\u8981\u5148\u5224\u65ad\u662f\u5426\u6301\u6709\u9501<\/pre>\n\n\n\n
\"\"<\/figure>\n\n\n\n
13.\u591a\u7ebf\u7a0b\u5e76\u884c\u5904\u7406\u5b9a\u65f6\u4efb\u52a1\u65f6\uff0cTimer\u8fd0\u884c\u591a\u4e2aTimeTask\u65f6\uff0c\u53ea\u8981\u5176\u4e2d\u4e4b\u4e00\u6ca1\u6709\u6355\u83b7\u629b\u51fa\u7684\u5f02\u5e38\uff0c\u5176\u5b83\u4efb\u52a1\u4fbf\u4f1a\u81ea\u52a8\u7ec8\u6b62\u8fd0\u884c\uff0c\u4f7f\u7528ScheduledExecutorService\n\u5219\u6ca1\u6709\u8fd9\u4e2a\u95ee\u9898\u3002<\/pre>\n\n\n\n
14.\u5f53switch\u62ec\u53f7\u5185\u7684\u53d8\u91cf\u7c7b\u578b\u4e3aString\u5e76\u4e14\u6b64\u53d8\u91cf\u4e3a\u5916\u90e8\u53c2\u6570\u65f6\uff0c\u5fc5\u987b\u5148\u8fdb\u884cnull\u5224\u65ad\u3002<\/pre>\n\n\n\n
15.\u4e09\u5143\u8fd0\u7b97\u7b26\u8bf7\u6ce8\u610f\u7c7b\u578b<\/pre>\n\n\n\n
\"\"<\/figure>\n\n\n\n
16.\u6570\u636e\u5e93\u5b9a\u4e49\uff0c\u5c0f\u6570\u7c7b\u578b\u4e3adecimal\uff0c\u7981\u6b62\u4f7f\u7528float\u548cdouble\u3002<\/pre>\n\n\n\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
1.ConcurrentHashMap \u7684key\u548cvalue\u90fd\u4e0d\u80fd\u4e3anull 2.jdk8\u4ee5\u4e0b map \u5faa\u73af\u4f7f […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6],"tags":[129,130],"_links":{"self":[{"href":"https:\/\/www.zeroheart.xyz\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/1612"}],"collection":[{"href":"https:\/\/www.zeroheart.xyz\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.zeroheart.xyz\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.zeroheart.xyz\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.zeroheart.xyz\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1612"}],"version-history":[{"count":41,"href":"https:\/\/www.zeroheart.xyz\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/1612\/revisions"}],"predecessor-version":[{"id":1699,"href":"https:\/\/www.zeroheart.xyz\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/1612\/revisions\/1699"}],"wp:attachment":[{"href":"https:\/\/www.zeroheart.xyz\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1612"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.zeroheart.xyz\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1612"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.zeroheart.xyz\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1612"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}