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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"HashMap的几种初始化方法","articleBody":"## ArrayList初始化方法\n1. 常规方法  \n\n``` java\nList\u003cstring\u003e list = new ArrayList\u003c/string\u003e\u003cstring\u003e();  \nlist.add(\"string1\");  \nlist.add(\"string2\");  \n//some other list.add() code......  \nlist.add(\"stringN\");\n```\n1. 使用Arrrays.asList()方法  \n\n``` java\nArrayList\u003cString\u003e list = new ArrayList(Arrays.asList(\"Tom\", \"Jerry\", \"Mike\"));  \n```\n1. 使用双括号\n\n``` java\nList list = new ArrayList\u003cString\u003e(){{  \n    add(\"A\");  \n    add(\"B\");  \n}};\n```\n## HashMap初始化\n### 非static HashMap\n1. 最简单直观的  \n\n``` java\n        Map\u003cString, Integer\u003e  myMap = new HashMap\u003cString, Integer\u003e();\n        myMap.put(\"Li Lei\", 1);\n        myMap.put(\"Han Meimei\", 2);\n```\n\n但是初始化数据多的时候，这样写会很麻烦，有没有更好的办法呢？  \n1. 双括号，匿名内部类初始化HashMap  \n\n``` java\n    Map\u003cString, Integer\u003e myMap = new HashMap\u003cString, Integer\u003e() {\n            {\n                put(\"key1\",\"value1\");\n                put(\"key2\",\"value2\");\n            }\n        };\n```\n### 初始化immutable HashMap\n1. 同上  \n2. 双括号  \n\n``` java\npublic static final HashMap hm = new HashMap(){\n{\n   put(\"key1\",\"value1\");\n   put(\"key2\",\"value2\");\n}\n};  \n```\n\n上面的代码包含了两个隐藏的东西：\n    \\* 一个匿名内部类\n    \\* 一个实例块  \n\n可以对上面的代码进行拆解\n- 拆除匿名内部类  \n\n``` java\nprivate static class MyHashMap() extends HashMap\n{\n    {\n       put(\"key1\",\"value1\");\n       put(\"key2\",\"value2\");\n    }\n}\npublic static final HashMap hm = new MyHashMap();\n```\n- 拆除代码块\n\n``` java\nprivate static class MyHashMap() extends HashMap\n{\n    public MyHashMap()\n    {\n       this.put(\"key1\",\"value1\");  // \u003c-- added 'this'\n       this.put(\"key2\",\"value2\");  // \u003c-- added 'this'\n    }\n}\npublic static final HashMap hm = new MyHashMap();  \n```\n#### immutable HashMap初始化推荐的方法:\n\n``` java\npublic class Test {\n    private static final Map\u003cInteger, String\u003e MY_MAP = createMap();\n\n    private static Map\u003cInteger, String\u003e createMap() {\n        Map\u003cInteger, String\u003e result = new HashMap\u003cInteger, String\u003e();\n        result.put(1, \"one\");\n        result.put(2, \"two\");\n        return Collections.unmodifiableMap(result);\n    }\n}\n```\n\n或者是用Google提供的[Guava](https://github.com/google/guava)  \n\n``` java\nstatic final Map\u003cInteger, String\u003e MY_MAP = ImmutableMap.of(\n    1, \"one\",\n    2, \"two\"\n);\n```\n\n---\n\n**双括号的方式不推荐使用**  \n原因是：\n1. 匿名内部类的声明方式，所以引用中持有着外部类的引用。所以当时串行化这个集合时外部类也会被不知不觉的串行化，当外部类没有实现serialize接口时，就会报错。  \n2. 这种方式其实是声明了一个继承自HashMap的子类。然而有些串行化方法，例如要通过Gson串行化为json，或者要串行化为xml时，类库中提供的方式，是无法串行化Hashset或者HashMap的子类的，从而导致串行化失败。  \n#### 参考链接\n\n**使用双括号初始化可能出现的问题**  \n1. http://www.c2.com/cgi/wiki?DoubleBraceInitialization\n2. http://stackoverflow.com/questions/6802483/how-to-directly-initialize-a-hashmap-in-a-literal-way  \n\n**具体的例子**  \n1. [HashMap Construction with initial values](https://coderanch.com/t/386333/java/java/HashMap-Construction-initial-values)\n2. [聊聊 Java 中 HashMap 初始化的另一种方式](https://my.oschina.net/leejun2005/blog/282783)  \n3. [Java的大括号语法糖](https://my.oschina.net/trydofor/blog/79222)\n","author":{"url":"https://github.com/Sitrone","@type":"Person","name":"Sitrone"},"datePublished":"2016-09-22T11:36:42.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/2/LeetcodeInJava/issues/2"}