{"id":10008293,"date":"2023-09-19T20:42:55","date_gmt":"2023-09-19T12:42:55","guid":{"rendered":"https:\/\/blog.itmax.cn\/?p=10008293"},"modified":"2023-09-19T20:42:55","modified_gmt":"2023-09-19T12:42:55","slug":"java-%e4%b8%adfor%e5%be%aa%e7%8e%af%e5%92%8cforeach%e5%be%aa%e7%8e%af%e5%93%aa%e4%b8%aa%e6%9b%b4%e5%bf%ab%ef%bc%9f","status":"publish","type":"post","link":"https:\/\/itmax.cn\/?p=10008293","title":{"rendered":"Java \u4e2dfor\u5faa\u73af\u548cforeach\u5faa\u73af\u54ea\u4e2a\u66f4\u5feb\uff1f"},"content":{"rendered":"<p><strong>\u524d\u8a00<\/strong><\/p>\n<p>\u5728Java\u7f16\u7a0b\u4e2d\uff0c\u5faa\u73af\u7ed3\u6784\u662f\u7a0b\u5e8f\u5458\u5e38\u7528\u7684\u63a7\u5236\u6d41\u7a0b\uff0c\u800cfor\u5faa\u73af\u548cforeach\u5faa\u73af\u662f\u5176\u4e2d\u6bd4\u8f83\u5e38\u89c1\u7684\u4e24\u79cd\u5f62\u5f0f\u3002\u5173\u4e8e\u5b83\u4eec\u54ea\u4e00\u4e2a\u66f4\u5feb\u7684\u8ba8\u8bba\u4e00\u76f4\u5b58\u5728\u3002\u672c\u6587\u65e8\u5728\u63a2\u7a76Java\u4e2d\u7684for\u5faa\u73af\u548cforeach\u5faa\u73af\u7684\u6027\u80fd\u5dee\u5f02\uff0c\u5e76\u5e2e\u52a9\u8bfb\u8005\u66f4\u597d\u5730\u9009\u62e9\u9002\u5408\u81ea\u8eab\u9700\u6c42\u7684\u5faa\u73af\u65b9\u5f0f\u3002\u901a\u8fc7\u8be6\u7ec6\u6bd4\u8f83\u5b83\u4eec\u7684\u904d\u5386\u6548\u7387\u3001\u6570\u636e\u7ed3\u6784\u9002\u7528\u6027\u548c\u7f16\u8bd1\u5668\u4f18\u5316\u7b49\u56e0\u7d20\uff0c\u6211\u4eec\u5c06\u4e3a\u5927\u5bb6\u63ed\u793a\u5b83\u4eec\u7684\u5dee\u5f02\u548c\u9002\u7528\u573a\u666f\uff0c\u4ee5\u4fbf\u60a8\u80fd\u591f\u505a\u51fa\u66f4\u660e\u667a\u7684\u7f16\u7a0b\u51b3\u7b56\u3002<\/p>\n<p><img decoding=\"async\"   referrerpolicy=\"no-referrer\" src=\"https:\/\/segmentfault.com\/img\/remote\/1460000044230402\" alt=\"\" title=\"\"><\/p>\n<p><strong>for\u5faa\u73af\u4e0eforeach\u5faa\u73af\u7684\u6bd4\u8f83<\/strong><\/p>\n<p>\u5c0f\u7f16\u8ba4\u4e3afor\u548cforeach \u4e4b\u95f4\u552f\u4e00\u7684\u5b9e\u9645\u533a\u522b\u662f\uff0c\u5bf9\u4e8e\u53ef\u7d22\u5f15\u5bf9\u8c61\uff0c\u6211\u4eec\u65e0\u6743\u8bbf\u95ee\u7d22\u5f15\u3002<\/p>\n<pre><code class=\"Java\">for(int i = 0; i &lt; mylist.length; i++) {\n if(i &lt; 5) {\n \/\/do something\n } else {\n \/\/do other stuff\n }\n}<\/code><\/pre>\n<p>\u4f46\u662f\uff0c\u6211\u4eec\u53ef\u4ee5\u4f7f\u7528 foreach \u521b\u5efa\u4e00\u4e2a\u5355\u72ec\u7684\u7d22\u5f15 int \u53d8\u91cf\u3002\u4f8b\u5982\uff1a<\/p>\n<pre><code class=\"Java\">int index = -1;\nfor(int myint : mylist) {\n index++;\n if(index &lt; 5) {\n \/\/do something\n } else {\n \/\/do other stuff\n }\n}<\/code><\/pre>\n<p>\u73b0\u5728\u5199\u4e00\u4e2a\u7b80\u5355\u7684\u7c7b\uff0c\u5176\u4e2d\u6709 foreachTest() \u65b9\u6cd5\uff0c\u8be5\u65b9\u6cd5\u4f7f\u7528 forEach \u8fed\u4ee3\u5217\u8868\u3002<\/p>\n<pre><code class=\"Java\">import java.util.List;\n\npublic class ForEachTest {\n        List&lt;Integer&gt; intList;\n        \n    public void foreachTest(){\n        for(Integer i : intList){\n\n        }\n    }\n}<\/code><\/pre>\n<p>\u7f16\u8bd1\u8fd9\u4e2a\u7c7b\u65f6\uff0c\u7f16\u8bd1\u5668\u4f1a\u5728\u5185\u90e8\u5c06\u8fd9\u6bb5\u4ee3\u7801\u8f6c\u6362\u4e3a\u8fed\u4ee3\u5668\u5b9e\u73b0\u3002\u5c0f\u7f16\u901a\u8fc7\u6267\u884c javap -verbose IterateListTest \u53cd\u7f16\u8bd1\u4ee3\u7801\u3002<\/p>\n<pre><code class=\"Java\">public void foreachTest();\n    descriptor: ()V\n    flags: ACC_PUBLIC\n    Code:\n      stack=1, locals=3, args_size=1\n         0: aload_0\n         1: getfield      #19                 \/\/ Field intList:Ljava\/util\/List;\n         4: invokeinterface #21,  1           \/\/ InterfaceMethod java\/util\/List.iterator:()Ljava\/util\/Iterator;\n         9: astore_2\n        10: goto          23\n        13: aload_2\n        14: invokeinterface #27,  1           \/\/ InterfaceMethod java\/util\/Iterator.next:()Ljava\/lang\/Object;\n        19: checkcast     #33                 \/\/ class java\/lang\/Integer\n        22: astore_1\n        23: aload_2\n        24: invokeinterface #35,  1           \/\/ InterfaceMethod java\/util\/Iterator.hasNext:()Z\n        29: ifne          13\n        32: return\n      LineNumberTable:\n        line 9: 0\n        line 12: 32\n      LocalVariableTable:\n        Start  Length  Slot  Name   Signature\n            0      33     0  this   Lcom\/greekykhs\/springboot\/ForEachTest;\n      StackMapTable: number_of_entries = 2\n        frame_type = 255 \/* full_frame *\/\n          offset_delta = 13\n          locals = [ class com\/greekykhs\/springboot\/ForEachTest, top, class java\/util\/Iterator ]\n          stack = []\n        frame_type = 9 \/* same *\/<\/code><\/pre>\n<p>\u4ece\u4e0a\u9762\u7684\u5b57\u8282\u7801\u6211\u4eec\u53ef\u4ee5\u770b\u5230:<\/p>\n<p>a). getfield\u547d\u4ee4\u7528\u4e8e\u83b7\u53d6\u53d8\u91cf\u6574\u6570\u3002<\/p>\n<p>b).\u8c03\u7528List.iterator\u83b7\u53d6\u8fed\u4ee3\u5668\u5b9e\u4f8b<\/p>\n<p>c).\u8c03\u7528iterator.hasNext\uff0c\u5982\u679c\u8fd4\u56detrue\uff0c\u5219\u8c03\u7528iterator.next\u65b9\u6cd5\u3002<\/p>\n<p>\u4e0b\u8fb9\u6765\u505a\u4e00\u4e0b\u6027\u80fd\u6d4b\u8bd5\u3002\u5728 IterateListTest \u7684\u4e3b\u8981\u65b9\u6cd5\u4e2d\uff0c\u521b\u5efa\u4e86\u4e00\u4e2a\u5217\u8868\u5e76\u4f7f\u7528 for \u548c forEach \u5faa\u73af\u5bf9\u5176\u8fdb\u884c\u8fed\u4ee3\u3002<\/p>\n<pre><code class=\"Java\">import java.util.ArrayList;\nimport java.util.List;\n\npublic class IterateListTest {\n        public static void main(String[] args) {\n                List&lt;Integer&gt; mylist = new ArrayList&lt;&gt;();\n        for (int i = 0; i &lt; 1000000; i++) {\n            mylist.add(i);\n        }\n\n        long forLoopStartTime = System.currentTimeMillis();\n        for (int i = 0; i &lt; mylist.size(); i++) {mylist.get(i);}\n\n        long forLoopTraversalCost =System.currentTimeMillis()-forLoopStartTime;\n        System.out.println(&quot;for loop traversal cost for ArrayList= &quot;+ forLoopTraversalCost);\n\n        long forEachStartTime = System.currentTimeMillis();\n        for (Integer integer : mylist) {}\n\n        long forEachTraversalCost =System.currentTimeMillis()-forEachStartTime;\n        System.out.println(&quot;foreach traversal cost for ArrayList= &quot;+ forEachTraversalCost);\n        }\n}<\/code><\/pre>\n<p>\u7ed3\u679c\u5982\u4e0b\uff1a<br \/><img decoding=\"async\"   referrerpolicy=\"no-referrer\" src=\"https:\/\/segmentfault.com\/img\/remote\/1460000044230403\" alt=\"\" title=\"\" loading=\"lazy\"><\/p>\n<p><strong>\u603b\u7ed3<\/strong><\/p>\n<p>\u89c2\u5bdf\u7ed3\u679c\u663e\u793a\uff0cfor\u5faa\u73af\u7684\u6027\u80fd\u4f18\u4e8efor-each\u5faa\u73af\u3002\u7136\u540e\u518d\u4f7f\u7528LinkedList\u6bd4\u8f83\u5b83\u4eec\u7684\u6027\u80fd\u5dee\u5f02\u3002\u5bf9\u4e8e LinkedList \u6765\u8bf4\uff0cfor-each\u5faa\u73af\u5c55\u73b0\u51fa\u66f4\u597d\u7684\u6027\u80fd\u3002ArrayList\u5185\u90e8\u4f7f\u7528\u8fde\u7eed\u5b58\u50a8\u7684\u6570\u7ec4\uff0c\u56e0\u6b64\u6570\u636e\u7684\u68c0\u7d22\u65f6\u95f4\u590d\u6742\u5ea6\u4e3a O(1)\uff0c\u901a\u8fc7\u7d22\u5f15\u53ef\u4ee5\u76f4\u63a5\u8bbf\u95ee\u6570\u636e\u3002\u800c LinkedList \u4f7f\u7528\u53cc\u5411\u94fe\u8868\u7ed3\u6784\uff0c\u5f53\u6211\u4eec\u4f7f\u7528 for \u5faa\u73af\u8fdb\u884c\u904d\u5386\u65f6\uff0c\u6bcf\u6b21\u90fd\u9700\u8981\u4ece\u94fe\u8868\u5934\u8282\u70b9\u5f00\u59cb\uff0c\u5bfc\u81f4\u65f6\u95f4\u590d\u6742\u5ea6\u8fbe\u5230\u4e86 O(n*n)\uff0c\u56e0\u6b64\u5728\u8fd9\u79cd\u60c5\u51b5\u4e0b\uff0cfor-each \u5faa\u73af\u66f4\u9002\u5408\u64cd\u4f5c LinkedList\u3002<\/p>\n<p><a href=\"https:\/\/segmentfault.com\/a\/1190000044230400\">https:\/\/segmentfault.com\/a\/1190000044230400<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u524d\u8a00 \u5728Java\u7f16\u7a0b\u4e2d\uff0c\u5faa\u73af\u7ed3\u6784\u662f\u7a0b\u5e8f\u5458\u5e38\u7528\u7684\u63a7\u5236\u6d41\u7a0b\uff0c\u800cfor\u5faa\u73af\u548cforeach\u5faa\u73af\u662f\u5176\u4e2d\u6bd4\u8f83\u5e38\u89c1\u7684\u4e24\u79cd\u5f62 [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[4],"tags":[],"class_list":["post-10008293","post","type-post","status-publish","format-standard","hentry","category-share"],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/itmax.cn\/index.php?rest_route=\/wp\/v2\/posts\/10008293","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/itmax.cn\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/itmax.cn\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/itmax.cn\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/itmax.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=10008293"}],"version-history":[{"count":0,"href":"https:\/\/itmax.cn\/index.php?rest_route=\/wp\/v2\/posts\/10008293\/revisions"}],"wp:attachment":[{"href":"https:\/\/itmax.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10008293"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/itmax.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10008293"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/itmax.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10008293"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}