字符串作为HashMap中的键 [英] String as a key in HashMap

问题描述

我已经看到了，只有String被用作HashMap中的一个键。虽然put（）方法将Object作为参数，但它的重要性如何。如果任何其他对象也可以用作Key？
请提供答案。

解决方案

任何提供 hashCode ）是地图中完美的关键人物：请参阅理解equals和hashCode在HashMap中的工作方式。

另外，正如@Jon所说，映射中的所有键都应该是

编辑：当然，您需要实现 equals（）和 hashcode（）。我认为与其他问题链接的标题明确表示。但是 hashcode（）的愚蠢实现只会给你带来退化的 HashMap ，这表现很差。

编辑2：正如@Adrian在他的回答中提到的，泛型将帮助您限制地图的键和值的类型。

参考文献：

• Effective Java Programming Language Guide，关于equals（）和hashCode（）的示例章节
Java理论与实践：散列

• Equals and HashCode

I had seen, only the String is used as a key in HashMap.Although the put() method takes Object as a parameter.What is the significant of it.If any other object can also used as a Key or not? Please provide the answers.

解决方案

Any object that provides a meaningful implementation of hashCode() is a perfect key candidate in a map: see Understanding the workings of equals and hashCode in a HashMap.

Also, as @Jon mentioned, all keys in your map should be of the same type.

EDIT: Of course, you need to implement both equals() and hashcode() . I thought the title of the link to the other question made that clear. But a dumb implementation of hashcode() will just bring you a degenerate HashMap which performance is poor.

EDIT2: As @Adrian mentioned in his answer, generics will help you constrain the type of keys and values for the map.

References:

• Effective Java Programming Language Guide, sample chapter about equals() and hashCode()
• Java theory and practice: Hashing it out
• Equals and HashCode

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