源自：jdk1.8.0_121
HashMap继承自AbstractMap，实现了Map、Cloneable、Serializable。
HashMap内部是由数组、链表、红黑树实现的
变量// 默认大小 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 最大容量 static final int MAXIMUM_CAPACITY = 1 << 30; // 负载因子，默认0.75，当数组 static final float DEFAULT_LOAD_FACTOR = 0.75f; static final int TREEIFY_THRESHOLD = 8; static final int UNTREEIFY_THRESHOLD = 6; static final int MIN_TREEIFY_CAPACITY = 64; transient Node<K,V>[] table; transient Set<Map.Entry<K,V>> entrySet; transient int size; transient int modCount; int threshold; final float loadFactor;