哈希表的 Java 数组
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Java array of Hashtables
提问by Donal.Lynch.Msc
I need an array of Hashtables in a program that is storing all words from a given set of documents.
我需要一个程序中的哈希表数组，该程序存储给定文档集中的所有单词。
Index 1 of the array holds a hashtable of String -> Double which stores a word, and its count for document 1 (array index 100 = document number 100's hashtable).
数组的索引 1 包含一个 String -> Double 的哈希表，它存储一个单词，以及它对文档 1 的计数（数组索引 100 = 文档编号 100 的哈希表）。
I dont need help using this data structure, just in creating it. I declare the Hashtable Array as follows:
我不需要帮助使用这个数据结构，只是在创建它。我声明哈希表数组如下：
Hashtable<String,Double>[] h1 = new Hashtable<String,Double>[];
... but this does not compile.
...但这不能编译。
(NOTE: The Double is necessary rather than an Integer in the above declaration for later usage.)
（注意：上面声明中的 Double 是必需的，而不是 Integer 以供以后使用。）
QUESTION: How do you create an array of hashtables which stores String->Double ???
问题：如何创建存储 String->Double 的哈希表数组？
Any suggestions appreciated guys....
任何建议赞赏家伙....
回答by Fred Foo
... but this does not compile.
...但这不能编译。
That's because the array has no name, newexpects a number of elements and you can't just allocate an array of generics. Prefer a Listinstead:
那是因为该数组没有名称，new需要多个元素，而您不能只分配一个泛型数组。更喜欢一个List：
List<Hashtable<String,Double>> wordCountPerDoc
= new ArrayList<Hashtable<String,Double>>();
回答by Bala R
just use
只是使用
@SuppressWarnings("unchecked")
Hashtable<String,Double>[] h = (Hashtable<String,Double>[])new Hashtable<?,?>[10];
h[0] = new Hashtable<String, Double>();
回答by Dan
why don't you use a Map<Integer, Map<String, Double> >?
this way you don't waste space for non-existing documents, and still get O(1) retrieval.
你为什么不使用一个Map<Integer, Map<String, Double> >？这样您就不会为不存在的文档浪费空间，并且仍然可以获得 O(1) 检索。
回答by GuruKulki
you can create like this.
你可以这样创建。
Hashtable<String,Double>[] arr = new Hashtable[10];
回答by Carl Manaster
Two things: you can't declare an array with the parameterized types like that; you have to imply declare it a new Hashtable[]. And you need to give the array a length.
两件事：你不能用这样的参数化类型声明一个数组；您必须暗示将其声明为new Hashtable[]. 你需要给数组一个长度。
Mixing arrays and Collections, although possible, tends to be confusing and lead to problems in my experience; also HashMap is generally preferred to Hashtable. So I would tend to prefer a List<Map<String, Double>>for this application.
混合使用数组和集合，虽然可能，但往往会令人困惑并导致我的经验出现问题；HashMap 通常也优于 Hashtable。所以我倾向于更喜欢List<Map<String, Double>>这个应用程序。
回答by McDowell
The reasons why this is an error are covered in Angelika Langer's Generics FAQ: Can I create an array whose component type is a concrete parameterized type?
Angelika Langer 的泛型常见问题解答中介绍了这是错误的原因：我可以创建一个组件类型为具体参数化类型的数组吗？
Can I create an array whose component type is a concrete parameterized type?
No, because it is not type-safe.
Arrays are covariant, which means that an array of supertype references is a supertype of an array of subtype references. That is,
Object[]is a supertype ofString[]and a string array can be accessed through a reference variable of typeObject[].
我可以创建一个数组，其组件类型是具体的参数化类型吗？
不，因为它不是类型安全的。
数组是协变的，这意味着超类型引用数组是子类型引用数组的超类型。也就是说，
Object[]是 的超类型，String[]并且可以通过类型为 的引用变量访问字符串数组Object[]。
Arrays and generics can have odd interactions (largely due to implementation compromises to support compatibility). You may be better off (as larsmans suggested) looking at a suitable collection type such as a Listof Maps.
数组和泛型可能有奇怪的交互（主要是由于支持兼容性的实现妥协）。您可能会更好（如larsmans 建议）查看合适的集合类型，例如Listof Map。
回答by omerkudat
An array seems to be an unusual choice of structure here. Perhaps you should consider storing your hashtables in a List. It will dynamically resize for you if you don't know how many document you will have ahead of time. If you use an ArrayList, you will still have constant-time reads of random indeces (like an array.) I think it's much simpler than using an array, and you still get the generic type checking. If you choose a List, you syntax becomes:
数组在这里似乎是一种不寻常的结构选择。也许您应该考虑将哈希表存储在列表中。如果您不知道提前有多少文档，它会为您动态调整大小。如果您使用ArrayList，您仍然会获得随机 indeces（如数组）的恒定时间读取。我认为它比使用数组简单得多，并且您仍然可以进行泛型类型检查。如果选择列表，则语法变为：
List<Map<String,Double>> documentWordCounts = new ArrayList<Map<String,Double>>();
Or choose a LinkedListdepending on what kind of read/write pattern you want.
或者LinkedList根据您想要的读/写模式选择一个。
回答by user4407884
For fixed size array:
对于固定大小的数组：
Hashtable<String,Double>[] h1 = new Hashtable[]{new Hashtable< String,Double>(),new Hashtable< String,Double>(),new Hashtable< String,Double>()};
