集合(三)

三、Map

先来讲一下Map，Map和Collection完全不是一个系列的，按理说讲完Collection的List，应该接着讲Collection的Set，但是因为Set中很多实现是基于Map来实现的，所以将Map提前。Map是一个接口，存储内容是键值对key-value，不能包含重复的键。

1.HashMap

AbstractMap是实现Map的抽象类，HashMap继承于AbstractMap。

Map的API:(JDK1.8版本新增较多，为25个详见 https://docs.oracle.com/javase/8/docs/api/index.html)

abstract void                 clear()
abstract boolean              containsKey(Object key)
abstract boolean              containsValue(Object value)
abstract Set<Entry<K, V>>     entrySet()
abstract boolean              equals(Object object)
abstract V                    get(Object key)
abstract int                  hashCode()
abstract boolean              isEmpty()
abstract Set<K>               keySet()
abstract V                    put(K key, V value)
abstract void                 putAll(Map<? extends K, ? extends V> map)
abstract V                    remove(Object key)
abstract int                  size()
abstract Collection<V>        values()

AbstractMap的API只比Map新增了两个方法，不妨：

String               toString()
Object               clone()

Map.Entry是Map类的静态接口，其API为 ：

abstract boolean     equals(Object object)
abstract K           getKey()
abstract V           getValue()
abstract int         hashCode()
abstract V           setValue(V object)

HashMap是线程不安全的，其key和value都可以是null。此外，HashMap不是有序的映射(什么是有序? 2019.1.1补：遍历的顺序和插入的顺序无直接关系)。

(1)HashMap存储情况

众所周知，哈希冲突的时候最经典的处理方法有再哈希和

基本成员域有：

    /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;

    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     */
    transient Set<Map.Entry<K,V>> entrySet;

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient int modCount;

    /**
     * The next size value at which to resize (capacity * load factor).
     *
     * @serial
     */
    // (The javadoc description is true upon serialization.
    // Additionally, if the table array has not been allocated, this
    // field holds the initial array capacity, or zero signifying
    // DEFAULT_INITIAL_CAPACITY.)
    int threshold;

    /**
     * The load factor for the hash table.
     *
     * @serial
     */
    final float loadFactor;

size是键值对的个数，loadfactor称为装载因子，其作用是为了使哈希中的存储更加均匀，减小冲突，定义如下：

loadfactor=元素个数/哈希表容量

HashMap共有四种构造函数：

public HashMap()

public HashMap(int initialCapacity)

public HashMap(int initialCapacity,float loadFactor)

public HashMap(Map<? extends K,? extends V> m)

SortedMap是排序键值对的接口，实现排序的的方法是Comparator。而NavigableMap接口继承于SortedMap，新增了一些导航方法。

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