{"rsdb":{"rid":"239092","subhead":"","postdate":"0","aid":"164803","fid":"90","uid":"1","topic":"1","content":"<div id=\"cnblogs_post_body\">\n <p>\u3000\u3000http:\/\/docs.scala-lang.org\/tour\/lower-type-bounds.html\u4e2d\u6709\u4e00\u6bb5\u4ee3\u7801<\/p> \n <div class=\"cnblogs_code\"> \n  <pre><span style=\"color: #000000\">trait Node[+B] {\n  def prepend(elem: B): Unit\n}\n\ncase class ListNode[+B](h: B, t: Node[B]) extends Node[B] {\n  def prepend(elem: B) = ListNode[B](elem, this)\n  def head: B = h\n  def tail = t\n}\n\ncase class Nil[+B]() extends Node[B] {\n  def prepend(elem: B) = ListNode[B](elem, this)\n}<\/span><\/pre> \n <\/div> \n <p>\u3000\u3000\u6587\u4e2d\u8bf4\u8fd9\u6bb5\u4ee3\u7801\u4e0d\u4f1a\u901a\u8fc7\u7f16\u8bd1\uff0c\u56e0\u4e3aFunction1\u662f<em>contra<\/em>variant \u5728\u53c2\u6570\u7684\u4f4d\u7f6e\u4e0a\u3002\u770b\u5230\u8fd9\u91cc\u662f\u4e00\u4e2a\u5934\u5f88\u591a\u4e2a\u5927\u7684\u3002&nbsp;However, this program does&nbsp;<em>not<\/em>&nbsp;compile because the parameter&nbsp;<code class=\"highlighter-rouge\">elem<\/code>&nbsp;in&nbsp;<code class=\"highlighter-rouge\">prepend<\/code>&nbsp;is of type&nbsp;<code class=\"highlighter-rouge\">B<\/code>, which we declared&nbsp;<em>co<\/em>variant. This doesn\u2019t work because functions are&nbsp;<em>contra<\/em>variant in their parameter types and&nbsp;<em>co<\/em>variant in their result types.<\/p> \n <p>\u3000\u3000\u5148\u5047\u8bbe\u4e00\u4e0b\u5982\u679c\u80fd\u7f16\u8bd1\u901a\u8fc7\u7684\u8bdd\u3002<\/p> \n <p>\u3000\u3000\u5047\u8bbe\u6709\u8fd9\u6837\u5b50\u7684\u4e00\u6bb5\u4ee3\u7801<\/p> \n <div class=\"cnblogs_code\"> \n  <pre><span style=\"color: #000000\">trait Animal\ncase class Dog() extends Animal\ncase class Cat() extends Animal\ndef addDogToAnimal(animalNode : ListNode[Animal]) : Unit{\n<span style=\"color: #ff0000\"> animalNode.prepend(Dog()) <\/span>}<\/span><\/pre> \n <\/div> \n <pre>\u5982\u679cgeneric\u7684\u7c7b\u578b\u662fAnimal\u7684\u8bdd\uff0cListNode\u5c31\u53d8\u6210\u5982\u4e0b<br \/>case class ListNode[Animal](h: Animal, t: Node[Animal]) extends Node[Animal] {\n  def prepend(elem: Animal) = ListNode[Animal](elem, this)\n  def head: Animal = h\n  def tail = t\n}<\/pre> \n <pre><span>\u5982\u679cgeneric\u7684\u7c7b\u578b\u662fCat\u7684\u8bdd\uff0cListNode\u5c31\u53d8\u6210\u5982\u4e0b<br \/>case class ListNode[Cat](h:Cat, t: Node[Cat]) extends Node[Cat] {\n  <strong><span style=\"color: #ff0000\">def prepend(elem:Cat) = ListNode[Cat](elem, this)<\/span><\/strong>\n  def head: Cat= h\n  def tail = t\n}<\/span><\/pre> \n <p>\u3000addDogToAnimal\u65b9\u6cd5\u63a5\u53d7\u4e00\u4e2a<strong><span style=\"color: #ff0000\">ListNode[Animal]<\/span><\/strong>\uff0c\u56e0\u4e3a<span style=\"color: #ff0000\"><strong>ListNode[Cat] \u662f ListNode[Animal]\u7684\u5b50\u7c7b(\u56e0\u4e3a\u662fCovaraiance\u7684)<\/strong><\/span><\/p> \n <p><span style=\"color: #ff0000\">\u3000<span style=\"color: #000000\">\u6240\u4ee5\u6211\u4eec\u53ef\u4ee5addDogToAnimal(ListNode(Cat(), Nil()))\uff0c\u4f46\u662f<\/span><\/span>ListNode[Cat]\u53ea\u80fdprepend\u662fCat\u7c7b\u578b\u7684\u5bf9\u8c61\u3002\u6240\u4ee5\u4e00\u5b9a\u4f1a\u51fa\u95ee\u9898\u3002<\/p> \n <p>\u3000\u89e3\u51b3\u65b9\u6cd5\u5c31\u662f\u5728\u6240\u6709\u9700\u8981\u6d88\u8d39\u8005\u65b9\u6cd5\u4e2d&nbsp;introducing a new type parameter&nbsp;<code class=\"highlighter-rouge\">U<\/code>&nbsp;that has&nbsp;<code class=\"highlighter-rouge\">B<\/code>&nbsp;as a lower type bound.<\/p> \n <pre class=\"highlight\"><code><span class=\"k\">trait <span class=\"nc\">Node<span class=\"o\">[<span class=\"kt\">+B<span class=\"o\">] <span class=\"o\">{ <span class=\"k\">def <span class=\"n\">prepend<span class=\"o\">[<span class=\"kt\">U <span class=\"k\">&gt;: <span class=\"kt\">B<span class=\"o\">](<span class=\"n\">elem<span class=\"k\">: <span class=\"kt\">U<span class=\"o\">) <span class=\"o\">} <span class=\"k\">case <span class=\"k\">class <span class=\"nc\">ListNode<span class=\"o\">[<span class=\"kt\">+B<span class=\"o\">](<span class=\"n\">h<span class=\"k\">: <span class=\"kt\">B<span class=\"o\">, <span class=\"n\">t<span class=\"k\">: <span class=\"kt\">Node<span class=\"o\">[<span class=\"kt\">B<span class=\"o\">]) <span class=\"k\">extends <span class=\"nc\">Node<span class=\"o\">[<span class=\"kt\">B<span class=\"o\">] <span class=\"o\">{ <span class=\"k\">def <span class=\"n\">prepend<span class=\"o\">[<span class=\"kt\">U <span class=\"k\">&gt;: <span class=\"kt\">B<span class=\"o\">](<span class=\"n\">elem<span class=\"k\">: <span class=\"kt\">U<span class=\"o\">) <span class=\"k\">= <span class=\"nc\">ListNode<span class=\"o\">[<span class=\"kt\">U<span class=\"o\">](<span class=\"n\">elem<span class=\"o\">, <span class=\"k\">this<span class=\"o\">) <span class=\"k\">def <span class=\"n\">head<span class=\"k\">: <span class=\"kt\">B <span class=\"o\">= <span class=\"n\">h <span class=\"k\">def <span class=\"n\">tail <span class=\"k\">= <span class=\"n\">t <span class=\"o\">} <span class=\"k\">case <span class=\"k\">class <span class=\"nc\">Nil<span class=\"o\">[<span class=\"kt\">+B<span class=\"o\">]() <span class=\"k\">extends <span class=\"nc\">Node<span class=\"o\">[<span class=\"kt\">B<span class=\"o\">] <span class=\"o\">{ <span class=\"k\">def <span class=\"n\">prepend<span class=\"o\">[<span class=\"kt\">U <span class=\"k\">&gt;: <span class=\"kt\">B<span class=\"o\">](<span class=\"n\">elem<span class=\"k\">: <span class=\"kt\">U<span class=\"o\">) <span class=\"k\">= <span class=\"nc\">ListNode<span class=\"o\">[<span class=\"kt\">U<span class=\"o\">](<span class=\"n\">elem<span class=\"o\">, <span class=\"k\">this<span class=\"o\">) <span class=\"o\">}<br \/>\u73b0\u5728\u518d\u6765\u770b\u521a\u624d\u7684\u95ee\u9898<br \/><br \/><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/code>\u5982\u679cgeneric\u7684\u7c7b\u578b\u662fAnimal\u7684\u8bdd\uff0cListNode\u5c31\u53d8\u6210\u5982\u4e0b<\/pre> \n <pre>case class ListNode[Animal](h: Animal, t: Node[Animal]) extends Node[Animal] {\n  def prepend[U &gt;: Animal](elem: Animal) = ListNode[Animal](elem, this)\n  def head: Animal = h\n  def tail = t\n}<\/pre> \n <pre>\u5982\u679cgeneric\u7684\u7c7b\u578b\u662fCat\u7684\u8bdd\uff0cListNode\u5c31\u53d8\u6210\u5982\u4e0b<br \/>case class ListNode[Cat](h:Cat, t: Node[Cat]) extends Node[Cat] {\n  <strong>def prepend[U &gt;: Cat](elem:Cat) = ListNode[Cat](elem, this)<\/strong>\n  def head: Cat= h\n  def tail = t\n}<\/pre> \n <pre class=\"highlight\"><span style=\"color: #ff0000; font-size: 18px\"><strong><code><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"kt\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"kt\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"o\"><span class=\"k\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"n\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"k\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"kt\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"o\"><span class=\"k\"><span class=\"o\"><span class=\"o\">ListNode[Cat] \u8fd8\u662f ListNode[Animal]\u7684\u5b50\u7c7b<br \/>addDogToAnimal(ListNode(Cat(), Nil()))\u7684\u65f6\u5019<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/code><code><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"kt\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"kt\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"o\"><span class=\"k\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"n\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"k\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"kt\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"o\"><span class=\"k\"><span class=\"o\"><span class=\"o\">\uff0c<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/code>ListNode[Cat]\u7684prepend\u65b9\u6cd5\u53ef\u4ee5\u63a5\u53d7\u6240\u6709U &gt;: Cat \u7684\u5bf9\u8c61<br \/>\u6240\u4ee5prepend\u65b9\u6cd5\u53ef\u4ee5\u63a5\u53d7Animal\u7684\u5bf9\u8c61\u4f5c\u4e3a\u53c2\u6570\u3002Dog\u4e5f\u662f\u4e00\u79cdAnimal\uff0c\u6240\u4ee5<\/strong><\/span><span style=\"font-size: 14pt\"><strong><span style=\"color: #ff0000\">animalNode.prepend(Dog())\u662f\u6ca1\u6709\u95ee\u9898\u7684<br \/>\u5728\u8fd9\u91cc\u4ee5\u4e00\u4e2a<a href=https:\/\/www.cppentry.com style=text-decoration:underline;font-size:14px;color:#F70968; target=_blank>java<\/a>\u5f00\u53d1\u8005\u6765\u8bf4\uff0c\u4f1a\u89c9\u5f97\u5f88\u4e0d\u5408\u7406\u3002\u660e\u660e\u662fcat\u7c7b\u578b\u7684ListNode\uff0c\u600e\u4e48\u53ef\u4ee5\u52a0\u5165dog\u3002\u4f46\u8fd9\u4e5f\u662fscala\u548c<a href=https:\/\/www.cppentry.com style=text-decoration:underline;font-size:14px;color:#F70968; target=_blank>java<\/a>\u7684\u4e0d\u540c\u5440\u3002\u5509<\/span><\/strong><\/span><\/pre> \n <pre class=\"highlight\"><span style=\"color: #ff0000; font-size: 18px\"><code><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"kt\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"kt\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"o\"><span class=\"k\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"n\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"k\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"o\"><span class=\"k\"><span class=\"n\"><span class=\"o\"><span class=\"kt\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"k\"><span class=\"kt\"><span class=\"o\"><span class=\"k\"><span class=\"nc\"><span class=\"o\"><span class=\"kt\"><span class=\"o\"><span class=\"n\"><span class=\"o\"><span class=\"k\"><span class=\"o\"><span class=\"o\"><strong><br \/><br \/><\/strong><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/code><\/span><\/pre> \n <p>&nbsp;<\/p> \n <p><span style=\"color: #ff0000\"><span style=\"color: #000000\">&nbsp;<\/span><\/span><\/p> \n <p><span style=\"color: #ff0000\"><strong>\u3000\u3000<\/strong><\/span><\/p>\n<\/div>","orderid":"0","title":"Scala\u7c7b\u578b\u7cfb\u7edf(sudden thought)","smalltitle":"","mid":"0","fname":"Scala","special_id":"0","bak_id":"0","info":"0","hits":"9247","pages":"1","comments":"0","posttime":"2017-10-16 18:18:32","list":"1508149112","username":"admin","author":"","copyfrom":"","copyfromurl":"","titlecolor":"","fonttype":"0","titleicon":"0","picurl":"https:\/\/www.cppentry.com\/upload_files\/","ispic":"0","yz":"1","yzer":"","yztime":"0","levels":"0","levelstime":"0","keywords":"<A HREF='https:\/\/www.cppentry.com\/do\/search.php?type=keyword&keyword=Scala' target=_blank>Scala<\/A> <A HREF='https:\/\/www.cppentry.com\/do\/search.php?type=keyword&keyword=%C0%E0%D0%CD' target=_blank>\u7c7b\u578b<\/A> <A HREF='https:\/\/www.cppentry.com\/do\/search.php?type=keyword&keyword=%CF%B5%CD%B3' target=_blank>\u7cfb\u7edf<\/A> <A HREF='https:\/\/www.cppentry.com\/do\/search.php?type=keyword&keyword=sudden' target=_blank>sudden<\/A> <A HREF='https:\/\/www.cppentry.com\/do\/search.php?type=keyword&keyword=thought' target=_blank>thought<\/A>","jumpurl":"","iframeurl":"","style":"","template":"a:3:{s:4:\"head\";s:0:\"\";s:4:\"foot\";s:0:\"\";s:8:\"bencandy\";s:0:\"\";}","target":"0","ip":"113.108.110.181","lastfid":"0","money":"0","buyuser":"","passwd":"","allowdown":"","allowview":"","editer":"","edittime":"0","begintime":"0","endtime":"0","description":"Scala\u7c7b\u578b\u7cfb\u7edf(sudden thought)","lastview":"1712668893","digg_num":"1398","digg_time":"0","forbidcomment":"0","ifvote":"0","heart":"","htmlname":"","city_id":"0"},"page":"1"}