新ConcurrentHashMap的新ConcurrentHashMap

Question

我正在尝试初始化ConcurrentHashMap of ConcurrentHashMap

private final ConcurrentHashMap<
    String, 
    ConcurrentHashMap<String, Double>
> myMulitiConcurrentHashMap = new ConcurrentHashMap<
    String, 
    new ConcurrentHashMap<String, Double>()
>();

但是javac给了

HashMapper.java:132: error: illegal start of type
    new ConcurrentHashMap<String, Double>()
    ^
HashMapper.java:132: error: '(' or '[' expected
    new ConcurrentHashMap<String, Double>()
        ^
HashMapper.java:132: error: ';' expected
    new ConcurrentHashMap<String, Double>()

指向第二个new。

如何正确初始化myMulitiConcurrentHashMap new？

Answer 1

顺便说一下，Java 7现在有了更简洁的语法(“菱形”)：

private final 
   ConcurrentHashMap<String, ConcurrentHashMap<String, Double>>
      myMulitiConcurrentHashMap =
         new ConcurrentHashMap<>();

您也应该能够在左手边使用接口：

private final 
   ConcurrentMap<String, ConcurrentMap<String, Double>>
      myMulitiConcurrentHashMap =
         new ConcurrentHashMap<>();

Answer 2

您不初始化内部ConcurrentHashMap<String, Double>；只需执行以下操作：

new ConcurrentHashMap<
    String, 
    ConcurrentHashMap<String, Double>
>();

Answer 3

泛型类型参数正是-类型。

拥有一个Map<String, new SomeType()>是没有意义的。

只需编写第二个参数的类型即可。

换句话来说，您正在创建一个new ConcurrentHashMap<K, V>()，它可以在稍后保存多个内部映射。