注意 ， 本文基于JDK 1.8

1.table

    /**
     * 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;

存放KV数据的数组。第一次使用的时候被初始化，根据需要可以重新resize。分配的长度总是2的幂。
2.entrySet

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

当被调用entrySet时被赋值。通过keySet()方法可以得到map key的集合，通过values方法可以得到map value的集合。
3.size

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

存放在map中的KV映射的总数。
4.modCount

    /**
     * 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;

HashMap被结构性修改的次数。（结构性修改是指改变了KV映射数量的操作或者修改了HashMap的内部结构（如 rehash）。这个用于fail-fast。
5.threshold

    /** * 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;

当需要resize时的阈值。即当HashMap中KV映射的数量（即size）超过了threshold就会resize。threshold=capacity*loadFactor。
6.loadFactor

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

装载因子。

注意，在成员变量中并没有capacity这个数据。当然capacity可以通过threshold和loadFactor计算得来。