JDK源码学习 —— 集合框架抽象类:AbstractMap

读完 Map 接口之后，就可以去读 Map 的抽象类了，抽象类一般实现了接口最通用的功能，这样具体的实现类就不用再重复去实现了。

类注释

This class provides a skeletal implementation of the Map interface, to minimize the effort required to implement this interface.

AbstractMap 是 Map 接口的一个基本实现，最大程度减少 Map 接口实现类需要做的工作。

①：To implement an unmodifiable map, the programmer needs only to extend this class and provide an implementation for the entrySet method, which returns a set-view of the map's mappings.②： Typically, the returned set will, in turn（依次、轮流）, be implemented atop AbstractSet. This set should not support the add or remove methods, and its iterator should not support the remove method.

①： 如果要实现一个不可变的 Map 类，开发者只需要继承 AbstractMap ，并且提供 entrySet 方法的实现，该方法返回 Map 的集合视图【public abstract Set<Entry<K,V>> entrySet();】。

② ：通常，返回的集合将依次在 AbstractSet 上实现，这个实现不支持 add 或 remove 方法，并且其迭代器不支持 remove 方法。【也就是集合本身不支持添加/删除元素，迭代器不支持删除元素】

To implement a modifiable map, the programmer must additionally override this class's put method (which otherwise throws an UnsupportedOperationException), and the iterator returned by entrySet(). iterator() must additionally implement its remove method.

如果要实现一个可变的 Map，开发者必须额外覆盖 put 方法，否则会抛出 UnsupportedOperationException 异常。并且迭代器由 entrySet() 方法返回，迭代器也需要额外实现 remove 方法。

The programmer should generally provide a void (no argument) and map constructor, as per the recommendation in the Map interface specification.
The documentation for each non-abstract method in this class describes its implementation in detail. Each of these methods may be overridden if the map being implemented admits a more efficient implementation.

开发者通常根据 Map 规范提供两种构造函数：