{"id":5269,"date":"2021-08-27T09:43:41","date_gmt":"2021-08-27T01:43:41","guid":{"rendered":"https:\/\/www.xttblog.com\/?p=5269"},"modified":"2021-08-27T09:43:41","modified_gmt":"2021-08-27T01:43:41","slug":"%e4%bd%bf%e7%94%a8java8%e5%bf%ab%e9%80%9f%e5%ae%9e%e7%8e%b0list%e8%bd%acmap%e3%80%81%e5%88%86%e7%bb%84%e3%80%81%e8%bf%87%e6%bb%a4%e7%ad%89%e6%93%8d%e4%bd%9c","status":"publish","type":"post","link":"https:\/\/www.xttblog.com\/?p=5269","title":{"rendered":"\u4f7f\u7528Java8\u5feb\u901f\u5b9e\u73b0List\u8f6cmap\u3001\u5206\u7ec4\u3001\u8fc7\u6ee4\u7b49\u64cd\u4f5c"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">\u5229\u7528 java8 \u65b0\u7279\u6027\uff0c\u53ef\u4ee5\u7528\u7b80\u6d01\u9ad8\u6548\u7684\u4ee3\u7801\u6765\u5b9e\u73b0\u4e00\u4e9b\u6570\u636e\u5904\u7406\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u5148\u5b9a\u4e49 1 \u4e2a Apple \u5bf9\u8c61\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class Apple {\n    private Integer id;\n    private String name;\n    private BigDecimal money;\n    private Integer num;\n    public Apple(Integer id, String name, BigDecimal money, Integer num) {\n        this.id = id;\n        this.name = name;\n        this.money = money;\n        this.num = num;\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">\u6dfb\u52a0\u4e00\u4e9b\u6d4b\u8bd5\u6570\u636e\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>List&lt;Apple&gt; appleList = new ArrayList&lt;&gt;();\/\/\u5b58\u653eapple\u5bf9\u8c61\u96c6\u5408\n\nApple apple1 =  new Apple(1,\"\u82f9\u679c1\",new BigDecimal(\"3.25\"),10);\nApple apple12 = new Apple(1,\"\u82f9\u679c2\",new BigDecimal(\"1.35\"),20);\nApple apple2 =  new Apple(2,\"\u9999\u8549\",new BigDecimal(\"2.89\"),30);\nApple apple3 =  new Apple(3,\"\u8354\u679d\",new BigDecimal(\"9.99\"),40);\n\nappleList.add(apple1);\nappleList.add(apple12);\nappleList.add(apple2);\nappleList.add(apple3);<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">\u5206\u7ec4<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">List \u91cc\u9762\u7684\u5bf9\u8c61\u5143\u7d20\uff0c\u4ee5\u67d0\u4e2a\u5c5e\u6027\u6765\u5206\u7ec4\uff0c\u4f8b\u5982\uff0c\u4ee5 id \u5206\u7ec4\uff0c\u5c06 id \u76f8\u540c\u7684\u653e\u5728\u4e00\u8d77\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/\/List \u4ee5ID\u5206\u7ec4 Map&lt;Integer,List&lt;Apple&gt;&gt;\nMap&lt;Integer, List&lt;Apple&gt;&gt; groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));\n\nSystem.err.println(\"groupBy:\"+groupBy);\n{1=[Apple{id=1, name='\u82f9\u679c1', money=3.25, num=10}, Apple{id=1, name='\u82f9\u679c2', money=1.35, num=20}], 2=[Apple{id=2, name='\u9999\u8549', money=2.89, num=30}], 3=[Apple{id=3, name='\u8354\u679d', money=9.99, num=40}]}<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">List \u8f6c Map<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">id \u4e3a key\uff0capple \u5bf9\u8c61\u4e3a value\uff0c\u53ef\u4ee5\u8fd9\u4e48\u505a\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/**\n * List -&gt; Map\n * \u9700\u8981\u6ce8\u610f\u7684\u662f\uff1a\n * toMap \u5982\u679c\u96c6\u5408\u5bf9\u8c61\u6709\u91cd\u590d\u7684key\uff0c\u4f1a\u62a5\u9519Duplicate key ....\n *  apple1,apple12\u7684id\u90fd\u4e3a1\u3002\n *  \u53ef\u4ee5\u7528 (k1,k2)-&gt;k1 \u6765\u8bbe\u7f6e\uff0c\u5982\u679c\u6709\u91cd\u590d\u7684key,\u5219\u4fdd\u7559key1,\u820d\u5f03key2\n *\/\nMap&lt;Integer, Apple&gt; appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -&gt; a,(k1,k2)-&gt;k1));<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">\u6253\u5370 appleMap\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>{1=Apple{id=1, name='\u82f9\u679c1', money=3.25, num=10}, 2=Apple{id=2, name='\u9999\u8549', money=2.89, num=30}, 3=Apple{id=3, name='\u8354\u679d', money=9.99, num=40}}<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">\u8fc7\u6ee4 Filter<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">\u4ece\u96c6\u5408\u4e2d\u8fc7\u6ee4\u51fa\u6765\u7b26\u5408\u6761\u4ef6\u7684\u5143\u7d20\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/\/\u8fc7\u6ee4\u51fa\u7b26\u5408\u6761\u4ef6\u7684\u6570\u636e\nList&lt;Apple&gt; filterList = appleList.stream().filter(a -&gt; a.getName().equals(\"\u9999\u8549\")).collect(Collectors.toList());\n\nSystem.err.println(\"filterList:\"+filterList);\n[Apple{id=2, name='\u9999\u8549', money=2.89, num=30}]<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">\u6c42\u548c<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">\u5c06\u96c6\u5408\u4e2d\u7684\u6570\u636e\u6309\u7167\u67d0\u4e2a\u5c5e\u6027\u6c42\u548c:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/\/\u8ba1\u7b97 \u603b\u91d1\u989d\nBigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);\nSystem.err.println(\"totalMoney:\"+totalMoney);  \/\/totalMoney:17.48<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">\u67e5\u627e\u6d41\u4e2d\u6700\u5927\u3001\u6700\u5c0f\u503c<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.maxBy \u548c Collectors.minBy \u6765\u8ba1\u7b97\u6d41\u4e2d\u7684\u6700\u5927\u6216\u6700\u5c0f\u503c\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Optional&lt;Dish&gt; maxDish = Dish.menu.stream().\n      collect(Collectors.maxBy(Comparator.comparing(Dish::getCalories)));\nmaxDish.ifPresent(System.out::println);\n\nOptional&lt;Dish&gt; minDish = Dish.menu.stream().\n      collect(Collectors.minBy(Comparator.comparing(Dish::getCalories)));\nminDish.ifPresent(System.out::println);<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">\u53bb\u91cd<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>import static java.util.Comparator.comparingLong;\nimport static java.util.stream.Collectors.collectingAndThen;\nimport static java.util.stream.Collectors.toCollection;\n\n\/\/ \u6839\u636eid\u53bb\u91cd\nList&lt;Person&gt; unique = appleList.stream().collect(\n     ollectingAndThen(\n         toCollection(() -&gt; new TreeSet&lt;&gt;(comparingLong(Apple::getId))), ArrayList::new)\n);<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">\u4e0b\u8868\u5c55\u793a Collectors \u7c7b\u7684\u9759\u6001\u5de5\u5382\u65b9\u6cd5\u3002<\/p>\n\n\n\n<table class=\"wp-block-table\"><thead><tr><th>\u5de5\u5382\u65b9\u6cd5<\/th><th>\u8fd4\u56de\u7c7b\u578b<\/th><th>\u4f5c\u7528<\/th><\/tr><\/thead><tbody><tr><td>toList<\/td><td><code>List&lt;T&gt;<\/code><\/td><td>\u628a\u6d41\u4e2d\u6240\u6709\u9879\u76ee\u6536\u96c6\u5230\u4e00\u4e2a List<\/td><\/tr><tr><td>toSet<\/td><td><code>Set&lt;T&gt;<\/code><\/td><td>\u628a\u6d41\u4e2d\u6240\u6709\u9879\u76ee\u6536\u96c6\u5230\u4e00\u4e2a Set\uff0c\u5220\u9664\u91cd\u590d\u9879<\/td><\/tr><tr><td>toCollection<\/td><td><code>Collection&lt;T&gt;<\/code><\/td><td>\u628a\u6d41\u4e2d\u6240\u6709\u9879\u76ee\u6536\u96c6\u5230\u7ed9\u5b9a\u7684\u4f9b\u5e94\u6e90\u521b\u5efa\u7684\u96c6\u5408<code>menuStream.collect(toCollection(), ArrayList::new)<\/code><\/td><\/tr><tr><td>counting<\/td><td>Long<\/td><td>\u8ba1\u7b97\u6d41\u4e2d\u5143\u7d20\u7684\u4e2a\u6570<\/td><\/tr><tr><td>sumInt<\/td><td>Integer<\/td><td>\u5bf9\u6d41\u4e2d\u9879\u76ee\u7684\u4e00\u4e2a\u6574\u6570\u5c5e\u6027\u6c42\u548c<\/td><\/tr><tr><td>averagingInt<\/td><td>Double<\/td><td>\u8ba1\u7b97\u6d41\u4e2d\u9879\u76ee Integer \u5c5e\u6027\u7684\u5e73\u5747\u503c<\/td><\/tr><tr><td>summarizingInt<\/td><td>IntSummaryStatistics<\/td><td>\u6536\u96c6\u5173\u4e8e\u6d41\u4e2d\u9879\u76ee Integer \u5c5e\u6027\u7684\u7edf\u8ba1\u503c\uff0c\u4f8b\u5982\u6700\u5927\u3001\u6700\u5c0f\u3001 \u603b\u548c\u4e0e\u5e73\u5747\u503c<\/td><\/tr><tr><td>joining<\/td><td>String<\/td><td>\u8fde\u63a5\u5bf9\u6d41\u4e2d\u6bcf\u4e2a\u9879\u76ee\u8c03\u7528 toString \u65b9\u6cd5\u6240\u751f\u6210\u7684\u5b57\u7b26\u4e32<code>collect(joining(\", \"))<\/code><\/td><\/tr><tr><td>maxBy<\/td><td><code>Optional&lt;T&gt;<\/code><\/td><td>\u4e00\u4e2a\u5305\u88f9\u4e86\u6d41\u4e2d\u6309\u7167\u7ed9\u5b9a\u6bd4\u8f83\u5668\u9009\u51fa\u7684\u6700\u5927\u5143\u7d20\u7684 Optional\uff0c \u6216\u5982\u679c\u6d41\u4e3a\u7a7a\u5219\u4e3a Optional.empty()<\/td><\/tr><tr><td>minBy<\/td><td><code>Optional&lt;T&gt;<\/code><\/td><td>\u4e00\u4e2a\u5305\u88f9\u4e86\u6d41\u4e2d\u6309\u7167\u7ed9\u5b9a\u6bd4\u8f83\u5668\u9009\u51fa\u7684\u6700\u5c0f\u5143\u7d20\u7684 Optional\uff0c \u6216\u5982\u679c\u6d41\u4e3a\u7a7a\u5219\u4e3a Optional.empty()<\/td><\/tr><tr><td>reducing<\/td><td>\u5f52\u7ea6\u64cd\u4f5c\u4ea7\u751f\u7684\u7c7b\u578b<\/td><td>\u4ece\u4e00\u4e2a\u4f5c\u4e3a\u7d2f\u52a0\u5668\u7684\u521d\u59cb\u503c\u5f00\u59cb\uff0c\u5229\u7528 BinaryOperator \u4e0e\u6d41 \u4e2d\u7684\u5143\u7d20\u9010\u4e2a\u7ed3\u5408\uff0c\u4ece\u800c\u5c06\u6d41\u5f52\u7ea6\u4e3a\u5355\u4e2a\u503c<code>\u7d2f\u52a0int totalCalories = menuStream.collect(reducing(0, Dish::getCalories, Integer::sum));<\/code><\/td><\/tr><tr><td>collectingAndThen<\/td><td>\u8f6c\u6362\u51fd\u6570\u8fd4\u56de\u7684\u7c7b\u578b<\/td><td>\u5305\u88f9\u53e6\u4e00\u4e2a\u6536\u96c6\u5668\uff0c\u5bf9\u5176\u7ed3\u679c\u5e94\u7528\u8f6c\u6362\u51fd\u6570<code>int howManyDishes = menuStream.collect(collectingAndThen(toList(), List::size))<\/code><\/td><\/tr><tr><td>groupingBy<\/td><td><code>Map&lt;K, List&lt;T&gt;&gt;<\/code><\/td><td>\u6839\u636e\u9879\u76ee\u7684\u4e00\u4e2a\u5c5e\u6027\u7684\u503c\u5bf9\u6d41\u4e2d\u7684\u9879\u76ee\u4f5c\u95ee\u7ec4\uff0c\u5e76\u5c06\u5c5e\u6027\u503c\u4f5c \u4e3a\u7ed3\u679c Map \u7684\u952e<\/td><\/tr><tr><td>partitioningBy<\/td><td><code>Map&lt;Boolean,List&lt;T&gt;&gt;<\/code><\/td><td>\u6839\u636e\u5bf9\u6d41\u4e2d\u6bcf\u4e2a\u9879\u76ee\u5e94\u7528\u8c13\u8bcd\u7684\u7ed3\u679c\u6765\u5bf9\u9879\u76ee\u8fdb\u884c\u5206\u533a<\/td><\/tr><\/tbody><\/table>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.averagingDouble()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.averagingDouble() \u65b9\u6cd5\u5c06\u6d41\u4e2d\u7684\u6240\u6709\u5143\u7d20\u89c6\u4e3a double \u7c7b\u578b\u5e76\u8ba1\u7b97\u4ed6\u4eec\u7684\u5e73\u5747\u503c\u3002\u8be5\u65b9\u6cd5\u8fd4\u56de\u7684\u662f\u540c\u4e00\u4e2a Collectors \u5b9e\u4f8b\uff0c\u56e0\u6b64\u53ef\u4ee5\u8fdb\u884c\u94fe\u5f0f\u64cd\u4f5c\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.averagingDouble() \u63a5\u53d7\u4e00\u4e2a\u53c2\u6570\uff0c\u8fd9\u4e2a\u53c2\u6570\u662f\u4e00\u4e2a lambda \u8868\u8fbe\u5f0f\uff0c\u7528\u4e8e\u5bf9\u6240\u6709\u7684\u5143\u7d20\u6267\u884c\u4e00\u4e2a map \u64cd\u4f5c\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Java \u6240\u6709\u96c6\u5408\u7684 stream().collect() \u53ef\u4ee5\u63a5\u53d7\u4e00\u4e2a\u6536\u96c6\u5668\u5b9e\u4f8b\u4f5c\u4e3a\u5176\u53c2\u6570\u5e76\u8fd4\u56de\u8be5\u6536\u96c6\u5668\u7684\u8ba1\u7b97\u7ed3\u679c<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u53c2\u8003\u4e0b\u9762\u7684\u4ee3\u7801\uff0ccollect() \u65b9\u6cd5\u4f1a\u628a\u6240\u6709\u7684\u5143\u7d20\u6536\u96c6\u8d77\u6765\u7136\u540e\u4f20\u9012\u7ed9 Collectors.averagingDouble(d-&gt;d*2) \u6536\u96c6\u5668\uff0c\u5bf9\u6bcf\u4e2a\u5143\u7d20\u6267\u884c *2 \u64cd\u4f5c\u540e\u8ba1\u7b97\u5e73\u5747\u503c<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class AveragingDoubleTest {\n\n    public static void main(String[] args) {\n        List&lt;Integer&gt; list = Arrays.asList(1,2,3,4);\n        Double result = list.stream().collect(Collectors.averagingDouble(d-&gt;d*2));\n        System.out.println(result);\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.AveragingDoubleTest\n5.0\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.averagingInt()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">\u8be5\u65b9\u6cd5\u548c Collectors.averagingDouble() \u4e00\u6837\uff0c\u4e0d\u540c\u7684\u662f\u5b83\u628a\u6d41\u4e2d\u7684\u6240\u6709\u5143\u7d20\u770b\u6210\u662f int \u7c7b\u578b\uff0c\u5e76\u8fd4\u56de\u4e00\u4e2a\u6d6e\u70b9\u7c7b\u578b\u7684\u5e73\u5747\u503c<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class AveragingIntTest {\n\n    public static void main(String[] args) {\n        List&lt;Integer&gt; list = Arrays.asList(1,2,3,4);\n        Double result = list.stream().collect(Collectors.averagingInt(v-&gt;v*2));\n        System.out.println(result);\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.AveragingIntTest\n5.0\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.averagingLong()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">\u8be5\u65b9\u6cd5\u4e5f\u548c Collectors.averagingDouble() \u7c7b\u4f3c\uff0c\u4e0d\u540c\u7684\u662f\u5b83\u628a\u6d41\u4e2d\u7684\u6240\u6709\u5143\u7d20\u770b\u6210\u662f long \u7c7b\u578b\uff0c\u5e76\u8fd4\u56de\u4e00\u4e2a double \u7c7b\u578b\u7684\u5e73\u5747\u503c<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class AveragingLongTest {\n\n    public static void main(String[] args) {\n        List&lt;Integer&gt; list = Arrays.asList(1,2,3,4);\n        Double result = list.stream().collect(Collectors.averagingLong(v-&gt;v*2));\n        System.out.println(result);\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.AveragingLongTest\n5.0\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.averagingDouble()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.collectingAndThen()\u51fd\u6570\u5e94\u8be5\u6700\u50cf map and reduce \u4e86\uff0c\u5b83\u53ef\u63a5\u53d7\u4e24\u4e2a\u53c2\u6570\uff0c\u7b2c\u4e00\u4e2a\u53c2\u6570\u7528\u4e8e reduce \u64cd\u4f5c\uff0c\u800c\u7b2c\u4e8c\u53c2\u6570\u7528\u4e8e map \u64cd\u4f5c\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u4e5f\u5c31\u662f\uff0c\u5148\u628a\u6d41\u4e2d\u7684\u6240\u6709\u5143\u7d20\u4f20\u9012\u7ed9\u7b2c\u4e8c\u4e2a\u53c2\u6570\uff0c\u7136\u540e\u628a\u751f\u6210\u7684\u96c6\u5408\u4f20\u9012\u7ed9\u7b2c\u4e00\u4e2a\u53c2\u6570\u6765\u5904\u7406\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u4f8b\u5982\u4e0b\u9762\u7684\u4ee3\u7801\uff0c\u5148\u628a [1,2,3,4] \u8fd9\u4e2a\u96c6\u5408\u4f20\u9012\u7ed9 s-&gt; s<em>s lambda \u8868\u8fbe\u5f0f\uff0c\u8ba1\u7b97\u5f97\u51fa\u7ed3\u679c\u4e3a [1,4,9,16] \uff0c\u7136\u540e\u518d\u628a [1,4,9,16] \u4f20\u9012\u7ed9 v-&gt;v<\/em>2 \u8868\u8fbe\u5f0f\uff0c\u8ba1\u7b97\u5f97\u51fa [2,8,18,32] \uff0c\u7136\u540e\u4f20\u9012\u7ed9 Collectors.averagingLong() \u8ba1\u7b97\u5f97\u5230\u7ed3\u679c\u4e3a 25.0<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class CollectingAndThenTest {\n\n    public static void main(String[] args) {\n        List&lt;Integer&gt; list = Arrays.asList(1,2,3,4);\n        Double result = list.stream().collect(Collectors.collectingAndThen(Collectors.averagingLong(v-&gt;v*2),\n                s-&gt; s*s));\n        System.out.println(result);\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.CollectingAndThenTest\n25.0\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.counting()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.counting() \u7528\u4e8e\u7edf\u8ba1\u6d41\u4e2d\u5143\u7d20\u7684\u4e2a\u6570\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class CountingTest {\n\n    public static void main(String[] args) {\n        List&lt;Integer&gt; list = Arrays.asList(1,2,3,4);\n        long result= list.stream().collect(Collectors.counting());\n        System.out.println(result);\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.CountingTest\n4\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.joining()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.joining() \u65b9\u6cd5\u7528\u67d0\u4e2a\u6307\u5b9a\u7684\u62fc\u63a5\u5b57\u7b26\u4e32\u628a\u6240\u6709\u5143\u7d20\u62fc\u63a5\u6210\u4e00\u4e2a\u5b57\u7b26\u4e32\uff0c\u5e76\u6dfb\u52a0\u53ef\u9009\u7684\u524d\u7f00\u548c\u540e\u7f00<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class JoiningTest {\n\n    public static void main(String[] args) {\n        List&lt;String&gt; list = Arrays.asList(\"A\",\"B\",\"C\",\"D\");\n        String result= list.stream().collect(Collectors.joining(\",\",\"(\",\")\"));\n        System.out.println(result);\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c:\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.JoiningTest\n(A,B,C,D)\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.maxBy() \u548c Collectors.minBy()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><code>Collectors.maxBy()<\/code>\u548c<code>Collectors.minBy()<\/code>\u4e24\u4e2a\u65b9\u6cd5\u5206\u522b\u7528\u4e8e\u8ba1\u7b97\u6d41\u4e2d\u6240\u6709\u5143\u7d20\u7684\u6700\u5927\u503c\u548c\u6700\u5c0f\u503c\u3002<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u4e24\u4e2a\u65b9\u6cd5\u90fd\u53ef\u4ee5\u63a5\u53d7\u4e00\u4e2a\u6bd4\u8f83\u5668\u4f5c\u4e3a\u53c2\u6570\uff0c\u7528\u4e8e\u5982\u4f55\u8ba1\u7b97\u6700\u5927\u503c\u6216\u6700\u5c0f\u503c<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class MaxByMinByTest {\n\n    public static void main(String[] args) {\n        List&lt;Integer&gt; list = Arrays.asList(30,10,20,35);\n        \/\/Get Max\n        list.stream().collect(Collectors.maxBy(new MaxByMinByTest().new IntegerComp()))\n                .ifPresent(i-&gt;System.out.println(i));\n        \/\/Get Min\n        list.stream().collect(Collectors.minBy(new MaxByMinByTest().new IntegerComp()))\n                .ifPresent(i-&gt;System.out.println(i));\n    }\n    class IntegerComp implements Comparator&lt;Integer&gt; {\n        @Override\n        public int compare(Integer i1, Integer i2) {\n            if(i1 &gt;=i2 ){\n                return 1;\n            }else{\n                return -1;\n            }\n        }\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.MaxByMinByTest\n35\n10\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.summingInt()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.summingInt() \u65b9\u6cd5\u5c06\u6d41\u4e2d\u7684\u6240\u6709\u5143\u7d20\u89c6\u4e3a int \u7c7b\u578b\uff0c\u5e76\u8ba1\u7b97\u6240\u6709\u5143\u7d20\u7684\u603b\u548c ( sum )<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class SummingIntTest {\n\n    public static void main(String[] args) {\n        List&lt;Integer&gt; list = Arrays.asList(30,10,20,35);\n        int result = list.stream().collect(Collectors.summingInt(i-&gt;i));\n        System.out.println(result);\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.SummingIntTest\n95\n\nProcess finished with exit code 0 <\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.summingLong()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.summingLong() \u5c06\u6d41\u4e2d\u7684\u6240\u6709\u5143\u7d20\u89c6\u4e3a long \u7c7b\u578b\uff0c\u5e76\u8ba1\u7b97\u6240\u6709\u5143\u7d20\u7684\u603b\u548c<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class SummingLongTest {\n\n    public static void main(String[] args) {\n        List&lt;Long&gt; list = new ArrayList&lt;&gt;();\n        list.add((long)340);\n        list.add((long)240);\n        list.add((long)360);\n        long result = list.stream().collect(Collectors.summingLong(l-&gt;l));\n        System.out.println(result);\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.SummingLongTest\n940\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.summingDouble()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.summingDouble() \u5c06\u6d41\u4e2d\u7684\u6240\u6709\u5143\u7d20\u89c6\u4e3a double \u7c7b\u578b\uff0c\u5e76\u8ba1\u7b97\u6240\u6709\u5143\u7d20\u7684\u603b\u548c<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class SummingDoubleTest {\n\n    public static void main(String[] args) {\n        List&lt;Double&gt; list = Arrays.asList(340.5,234.56,672.76);\n        Double result = list.stream().collect(Collectors.summingDouble(d-&gt;d));\n        System.out.println(result);\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.SummingDoubleTest\n1247.82\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300csummingInt()\u3001summingLong()\u3001summingDouble()\u8fd9\u4e09\u4e2a\u51fd\u6570\u7684\u7ed3\u679c\u7684\u7c7b\u578b\uff0c\u5c31\u662f\u5b83\u4eec\u5982\u4f55\u770b\u5f85\u5143\u7d20\u7684\u7c7b\u578b\u3002\u300d<\/strong><\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.toList()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.toList() \u5c06\u6d41\u4e2d\u7684\u6240\u6709\u5143\u7d20\u5bfc\u51fa\u5230\u4e00\u4e2a\u5217\u8868 ( List ) \u4e2d<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class ToListTest {\n\n    public static void main(String[] args) {\n        List&lt;String&gt; list = Stream.of(\"AA\",\"BB\",\"CC\").collect(Collectors.toList());\n        list.forEach(s-&gt;System.out.println(s));\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.ToListTest\nAA\nBB\nCC\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.toSet()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.toSet() \u628a\u6d41\u4e2d\u7684\u6240\u6709\u5143\u7d20\u5bfc\u51fa\u5230\u4e00\u4e2a\u96c6\u5408 ( Set ) \u4e2d\uff0c\u5e76\u6392\u9664\u91cd\u590d\u7684\u5143\u7d20 ( Set \u7684\u7279\u6027 )<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class ToSetTest {\n\n    public static void main(String[] args) {\n        Set&lt;String&gt; set = Stream.of(\"AA\",\"AA\",\"BB\").collect(Collectors.toSet());\n        set.forEach(s-&gt;System.out.println(s));\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.ToSetTest\nAA\nBB\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.toMap()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.toMap() \u5c06\u6d41\u4e2d\u7684\u6240\u6709\u5143\u7d20\u5bfc\u51fa\u5230\u4e00\u4e2a\u54c8\u5e0c\u8868 ( Map ) \u4e2d\u3002\u8be5\u65b9\u6cd5\u63a5\u53d7\u4e24\u4e2a\u53c2\u6570\uff0c\u7b2c\u4e00\u4e2a\u53c2\u6570\u7528\u4e8e\u751f\u6210\u952e ( key ) \uff0c\u7b2c\u4e8c\u4e2a\u53c2\u6570\u7528\u4e8e\u751f\u6210\u503c ( value )\u3002\u4e24\u4e2a\u53c2\u6570\u90fd\u662f Lambda \u8868\u8fbe\u5f0f\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class ToMapTest {\n\n    public static void main(String[] args) {\n        Map&lt;String,String&gt; map = Stream.of(\"AA\",\"BB\",\"CC\").collect(Collectors.toMap(k-&gt;k, v-&gt;v+v));\n        map.forEach((k,v)-&gt;System.out.println(\"key:\"+k +\" value:\"+v));\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.ToMapTest\nkey:CC value:CCCC\nkey:BB value:BBBB\nkey:AA value:AAAA\n\nProcess finished with exit code 0<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">Collectors.mapping()<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Collectors.mapping() \u4e00\u822c\u7528\u4e8e\u591a\u91cd map and reduce \u4e2d\u3002Java \u6587\u6863\u4e2d\u63cf\u8ff0\u7684\u539f\u578b\u5982\u4e0b\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>mapping(Function&lt;? super T,? extends U&gt; mapper, Collector&lt;? super U,A,R&gt; downstream)<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">\u7b2c\u4e00\u4e2a\u53c2\u6570\u7528\u4e8e map \uff0c\u7b2c\u4e8c\u4e2a\u53c2\u6570\u7528\u4e8e reduce<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public class MappingTest {\n\n    public static void main(String[] args) {\n        List&lt;Person&gt; list = Person.getList();\n        Map&lt;Integer, String&gt; nameByAge\n                = list.stream().collect(Collectors.groupingBy(Person::getAge,\n                Collectors.mapping(Person::getName, Collectors.joining(\",\"))));\n        nameByAge.forEach((k,v)-&gt;System.out.println(\"Age:\"+k +\" Persons: \"+v));\n    }\n}\nclass Person {\n    private String name;\n    private int age;\n    public Person(String name, int age) {\n        this.name = name;\n        this.age = age;\n    }\n    public String getName() {\n        return name;\n    }\n    public int getAge() {\n        return age;\n    }\n    public static List&lt;Person&gt; getList() {\n        List&lt;Person&gt; list = new ArrayList&lt;&gt;();\n        list.add(new Person(\"JayLa\", 30));\n        list.add(new Person(\"LayMan\", 20));\n        list.add(new Person(\"Tom\", 20));\n        list.add(new Person(\"XinYi\", 30));\n        return list;\n    }\n}<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>\u300c\u8fd0\u884c\u7ed3\u679c\uff1a\u300d<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\/Library\/Java\/JavaVirtualMachines\/jdk1.8.0_171.jdk\/...com.sjh.test.java8.collectors.MappingTest\nAge:20  Persons: LayMan,Tom\nAge:30  Persons: JayLa,XinYi\n\nProcess finished with exit code 0<\/code><\/pre>\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 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-->","protected":false},"excerpt":{"rendered":"<p>\u5229\u7528 java8 \u65b0\u7279\u6027\uff0c\u53ef\u4ee5\u7528\u7b80\u6d01\u9ad8\u6548\u7684\u4ee3\u7801\u6765\u5b9e\u73b0\u4e00\u4e9b\u6570\u636e\u5904\u7406\u3002 \u5148\u5b9a\u4e49 1 \u4e2a Apple \u5bf9\u8c61\uff1a \u6dfb\u52a0\u4e00 [&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":[1741,1700,1727,618,1743],"class_list":["post-5269","post","type-post","status-publish","format-standard","hentry","category-java","tag-arraylist","tag-java-11","tag-java8","tag-linkedlist"],"_links":{"self":[{"href":"https:\/\/www.xttblog.com\/index.php?rest_route=\/wp\/v2\/posts\/5269","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=5269"}],"version-history":[{"count":1,"href":"https:\/\/www.xttblog.com\/index.php?rest_route=\/wp\/v2\/posts\/5269\/revisions"}],"predecessor-version":[{"id":5270,"href":"https:\/\/www.xttblog.com\/index.php?rest_route=\/wp\/v2\/posts\/5269\/revisions\/5270"}],"wp:attachment":[{"href":"https:\/\/www.xttblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5269"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.xttblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5269"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.xttblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5269"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}