{"id":5066,"date":"2020-08-09T15:20:29","date_gmt":"2020-08-09T07:20:29","guid":{"rendered":"https:\/\/www.xttblog.com\/?p=5066"},"modified":"2020-08-09T15:20:29","modified_gmt":"2020-08-09T07:20:29","slug":"java8%e4%b8%admap%e6%96%b0%e5%a2%9e%e7%9a%84getordefault%ef%bc%8cputifabsent%e5%92%8ccomputeifabsent%e4%b8%89%e4%b8%aa%e6%96%b9%e6%b3%95","status":"publish","type":"post","link":"https:\/\/www.xttblog.com\/?p=5066","title":{"rendered":"Java8\u4e2dMap\u65b0\u589e\u7684getOrDefault()\uff0cputIfAbsent()\u548ccomputeIfAbsent()\u4e09\u4e2a\u65b9\u6cd5"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">\u5148\u8bf4\u4e00\u4e0b\uff0c\u5728 Java8 \u4e2d computeIfAbsent \u65b9\u6cd5\u6709\u4e00\u4e2a\u4e25\u91cd\u7684\u6b7b\u5faa\u73af bug\uff0cJava9 \u4e2d\u5df2\u7ecf\u4fee\u590d\u4e86\uff0c\u8fd9\u4e2a\u540e\u9762\u6211\u4eec\u5355\u72ec\u5217\u4e3e\u6587\u7ae0\u6765\u8be6\u7ec6\u8bf4\u5b83\u662f\u5982\u4f55\u4ea7\u751f\u4ee5\u53ca\u5982\u4f55\u88ab\u4fee\u590d\u7684\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u5b9e\u9645\u4e0a\u4ece Java 8 \u5f00\u59cb\uff0cMap \u63d0\u4f9b\u4e86 computeIfAbsent() \u65b9\u6cd5\u3002\u4e3a\u4e86\u8bb2\u660e\u767d\u5b83\u7684\u7528\u6cd5\uff0c\u6211\u4eec\u5148\u6765\u770b\u4e00\u4e2a\u4ee3\u7801\u573a\u666f\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Map&lt;String, List&lt;String&gt;&gt; map = new HashMap&lt;&gt;();\nList&lt;String&gt; list = map.get(\"codedq\");\nif (list == null) {\n  list = new ArrayList&lt;&gt;();\n  map.put(\"list1\", list);\n}\nlist.add(\"\u4e1a\u4f59\u8349\");<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">\u8fd9\u6bb5\u4ee3\u7801\u770b\u8d77\u6765\u5f88\u591a\u4eba\u90fd\u5199\u8fc7\uff0c\u6240\u4ee5 Java8 \u63a8\u51fa\u4e86 computeIfAbsent \u65b9\u6cd5\uff0c\u7528\u6765\u7b80\u5316\u6574\u4e2a\u4ee3\u7801\u5757\uff0c\u4f7f\u5176\u53ea\u9700\u8981\u4e00\u884c\u4ee3\u7801\u5c31\u53ef\u4ee5\u641e\u5b9a\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>map.computeIfAbsent(\"codedq\", k -&gt; new ArrayList&lt;&gt;()).add(\"\u4e1a\u4f59\u8349\");<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">\u5176\u4e2d\u53d8\u91cf k \u662f Map \u7684 key\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u662f\u4e0d\u662f\u5f88\u65b9\u4fbf\uff1f\u4f46\u662f\u9664\u6b64\u4e4b\u5916\uff0cMap \u8fd8\u6709\u4e24\u4e2a\u65b9\u6cd5\uff1agetOrDefault() \u548c putIfAbsent()\uff0c\u8fd9\u4e09\u4e2a\u65b9\u6cd5\u90fd\u63a5\u53d7 Key \u548c\u4e00\u4e2a\u201c\u9ed8\u8ba4\u503c\u201d\u4f5c\u4e3a\u53c2\u6570\uff0c\u4e14\u8fd4\u56de\u4e00\u4e2a Value\u3002\u5982\u679c\u4e0d\u5c0f\u5fc3\u628a\u5b83\u4eec\u641e\u6df7\u7528\u9519\u4e86\uff0c\u53ef\u80fd\u4f1a\u5e26\u6765\u5927\u95ee\u9898\u3002\u4e0b\u9762\u5206\u522b\u4ecb\u7ecd\u4e0b\u3002<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">computeIfAbsent<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>V computeIfAbsent(K, Function&lt;? super K, ? extends V&gt;)<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">\u8fd9\u4e2a\u65b9\u6cd5\u6709\u4e24\u4e2a\u53c2\u6570\uff0cKey \u548c\u4e00\u4e2a\u6839\u636e Key \u6765\u4ea7\u751f Value \u7684 Function\uff1b\u7136\u540e\u8fd4\u56de\u4e00\u4e2a Value\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u8fd9\u4e2a\u65b9\u6cd5\u4f1a\u68c0\u67e5 Map \u4e2d\u7684 Key\uff0c\u5982\u679c\u53d1\u73b0 Key \u4e0d\u5b58\u5728\u6216\u8005\u5bf9\u5e94\u7684\u503c\u662f null\uff0c\u5219\u8c03\u7528 Function \u6765\u4ea7\u751f\u4e00\u4e2a\u503c\uff0c\u7136\u540e\u5c06\u5176\u653e\u5165 Map\uff0c\u6700\u540e\u8fd4\u56de\u8fd9\u4e2a\u503c\uff1b\u5426\u5219\u7684\u8bdd\u8fd4\u56de Map \u5df2\u7ecf\u5b58\u5728\u7684\u503c\u3002<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\"><p>\u8865\u5145\uff1a\u5728 ConcurrentHashMap \u4e2d\uff0c\u8fd9\u4e2a\u65b9\u6cd5\u4fdd\u8bc1\u7ebf\u7a0b\u5b89\u5168\uff0c\u4e14\u591a\u7ebf\u7a0b\u5e76\u53d1\u6267\u884c\u7684\u60c5\u51b5\u4e0b\uff0cFunction \u53c2\u6570\u53ea\u4f1a\u88ab\u8c03\u7528\u4e00\u6b21\u3002\u6240\u4ee5\u8fd9\u4e2a\u65b9\u6cd5\u5b8c\u5168\u53ef\u4ee5\u4ee3\u66ff DCL\uff08Double Check Lock\uff09\u5199\u6cd5\u3002<\/p><\/blockquote>\n\n\n\n<h2 class=\"wp-block-heading\">getOrDefault<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>V getOrDefault(Object, V)<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">\u8fd9\u4e2a\u65b9\u6cd5\u540c\u6837\u68c0\u67e5 Map \u4e2d\u7684 Key\uff0c\u5982\u679c\u53d1\u73b0 Key \u4e0d\u5b58\u5728\u6216\u8005\u5bf9\u5e94\u7684\u503c\u662f null\uff0c\u5219\u8fd4\u56de\u7b2c\u4e8c\u4e2a\u53c2\u6570\u5373\u9ed8\u8ba4\u503c\u3002\u8981\u6ce8\u610f\uff0c\u8fd9\u4e2a\u9ed8\u8ba4\u503c\u4e0d\u4f1a\u653e\u5165 Map\u3002\u6240\u4ee5\u5982\u679c\u4f60\u8fd9\u6837\u5199\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Map&lt;String, List&lt;String&gt;&gt; map = new HashMap&lt;&gt;();\nmap.getOrDefault(\"codedq\", new ArrayList&lt;&gt;()).add(\"\u4e1a\u4f59\u8349\");<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">\u6267\u884c\u5b8c\u4e4b\u540e map \u4ecd\u7136\u662f\u7a7a\u7684\uff01<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">putIfAbsent<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>V putIfAbsent(K, V)<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">\u8fd9\u4e2a\u65b9\u6cd5\u7684\u903b\u8f91\u5b8c\u5168\u4e0d\u540c\uff0c\u6ce8\u610f\u5b83\u4e0d\u662f\u4e00\u4e2a get() \u65b9\u6cd5\uff0c\u800c\u662f put() \u65b9\u6cd5\u7684\u53d8\u79cd\uff01\u8fd9\u4e2a\u65b9\u6cd5\u7684\u903b\u8f91\u662f\uff0c\u5982\u679c Key \u4e0d\u5b58\u5728\u6216\u8005\u5bf9\u5e94\u7684\u503c\u662f null\uff0c\u5219\u5c06 Value \u8bbe\u7f6e\u8fdb\u53bb\uff0c\u7136\u540e\u8fd4\u56de null\uff1b\u5426\u5219\u53ea\u8fd4\u56de Map \u5f53\u4e2d\u5bf9\u5e94\u7684\u503c\uff0c\u800c\u4e0d\u505a\u5176\u4ed6\u64cd\u4f5c\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u6240\u4ee5\u663e\u800c\u6613\u89c1\uff0c\u5728\u6700\u5f00\u59cb\u7684\u4f8b\u5b50\u4e2d\uff0c\u5982\u679c\u5c06 computeIfAbsent() \u66ff\u6362\u6210\u5176\u4ed6\u4e24\u4e2a\u65b9\u6cd5\u90fd\u662f\u9519\u7684\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u76ee\u524d\u6839\u636e\u6211\u5728 codereview \u4ee5\u53ca\u9762\u8bd5\u8001\u7a0b\u5e8f\u5458\u7684\u8fc7\u7a0b\u4e2d\uff0c\u53d1\u73b0\u5f88\u591a\u4eba\u4e0d\u77e5\u9053 Map \u4e2d\u65b0\u589e\u4e86\u65b9\u6cd5\uff0c\u4e5f\u4ece\u6765\u6ca1\u7528\u8fc7\u3002\u8fd8\u662f\u5efa\u8bae\u5927\u5bb6\u5728\u7a7a\u95f2\u65f6\u95f4\u591a\u5173\u6ce8\u4e00\u4e9b Java \u4e0d\u540c\u7248\u672c\u7684\u65b0\u7279\u6027\uff0c\u4e0d\u7136\u5728\u7f16\u7a0b\u7684\u9053\u8def\u4e0a\uff0c\u4f60\u4f1a\u5403\u4e8f\u7684\uff01<\/p>\n<!-- <audio autoplay=\"autoplay\" height=\"100\" width=\"100\" hidden=\"true\"><source src=\"https:\/\/c.coka.la\/orAvPR.mp3\" type=\"audio\/mp3\"\/><\/audio><img decoding=\"async\" src=\"https:\/\/images.weserv.nl\/?url=https:\/\/ws3.sinaimg.cn\/large\/8fa5dcfcgy1g4em3rzar5j20ib089ae0.jpg\" alt=\"\u4e1a\u4f59\u8349\u516c\u4f17\u53f7\"\/> --><p><!-- <img decoding=\"async\" src=\"https:\/\/raw.githubusercontent.com\/xmt1139057136\/xttblog\/master\/8fa5dcfcgy1g4em3rzar5j20ib089ae0.jpg\" alt=\"\u4e1a\u4f59\u8349\u516c\u4f17\u53f7\"\/>--><img decoding=\"async\" src=\"https:\/\/saas.51autoparts.cn\/api\/admin\/sys-file\/oss\/file?fileName=84db51f8561e448f9a4fda2eff0811f9.jpg\" alt=\"\u4e1a\u4f59\u8349\u516c\u4f17\u53f7\"\/><\/p><p>\u6700\u540e\uff0c\u6b22\u8fce\u5173\u6ce8\u6211\u7684\u4e2a\u4eba\u5fae\u4fe1\u516c\u4f17\u53f7\uff1a\u4e1a\u4f59\u8349\uff08yyucao\uff09\uff01<b>\u53ef\u52a0\u4f5c\u8005\u5fae\u4fe1\u53f7\uff1a<font color=\"red\">xttblog2<\/font>\u3002\u5907\u6ce8\uff1a\u201c1\u201d\uff0c\u6dfb\u52a0\u535a\u4e3b\u5fae\u4fe1\u62c9\u4f60\u8fdb\u5fae\u4fe1\u7fa4\u3002\u5907\u6ce8\u9519\u8bef\u4e0d\u4f1a\u540c\u610f\u597d\u53cb\u7533\u8bf7\u3002<\/b>\u518d\u6b21\u611f\u8c22\u60a8\u7684\u5173\u6ce8\uff01\u540e\u7eed\u6709\u7cbe\u5f69\u5185\u5bb9\u4f1a\u7b2c\u4e00\u65f6\u95f4\u53d1\u7ed9\u60a8\uff01\u539f\u521b\u6587\u7ae0\u6295\u7a3f\u8bf7\u53d1\u9001\u81f3532009913@qq.com\u90ae\u7bb1\u3002\u5546\u52a1\u5408\u4f5c\u4e5f\u53ef\u6dfb\u52a0\u4f5c\u8005\u5fae\u4fe1\u8fdb\u884c\u8054\u7cfb\uff01<\/p><p>\u672c\u6587\u539f\u6587\u51fa\u5904\uff1a<a href=\"https:\/\/www.xttblog.com\">\u4e1a\u4f59\u8349<\/a>\uff1a &raquo; <a href=\"https:\/\/www.xttblog.com\/?p=5066\">Java8\u4e2dMap\u65b0\u589e\u7684getOrDefault()\uff0cputIfAbsent()\u548ccomputeIfAbsent()\u4e09\u4e2a\u65b9\u6cd5<\/a><\/p><!-- <div class=\"bd-reward-stl\"><button id=\"bdRewardBtn\"><span><\/span><\/button><\/div> -->","protected":false},"excerpt":{"rendered":"<p>\u5148\u8bf4\u4e00\u4e0b\uff0c\u5728 Java8 \u4e2d computeIfAbsent \u65b9\u6cd5\u6709\u4e00\u4e2a\u4e25\u91cd\u7684\u6b7b\u5faa\u73af bug\uff0cJava9 \u4e2d\u5df2 [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[1164,1161,618,1846,2069],"class_list":["post-5066","post","type-post","status-publish","format-standard","hentry","category-java","tag-concurrenthashmap","tag-hashmap","tag-java8","tag-jmap","tag-linkedmultivaluemap"],"_links":{"self":[{"href":"https:\/\/www.xttblog.com\/index.php?rest_route=\/wp\/v2\/posts\/5066","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.xttblog.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.xttblog.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.xttblog.com\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.xttblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=5066"}],"version-history":[{"count":1,"href":"https:\/\/www.xttblog.com\/index.php?rest_route=\/wp\/v2\/posts\/5066\/revisions"}],"predecessor-version":[{"id":5067,"href":"https:\/\/www.xttblog.com\/index.php?rest_route=\/wp\/v2\/posts\/5066\/revisions\/5067"}],"wp:attachment":[{"href":"https:\/\/www.xttblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5066"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.xttblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5066"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.xttblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5066"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}