Java1.8的优化关注点

怎么执行的散列化？

Treeify 的时机和要求是什么？

1. 数组中的元素数据至少为64个
2. 链表元素数量至少为8个。

    /**
     * The smallest table capacity for which bins may be treeified.
     * (Otherwise the table is resized if too many nodes in a bin.)
     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
     * between resizing and treeification thresholds.
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

        /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     */
    static final int UNTREEIFY_THRESHOLD = 6;

HashMap 和HashTable有什么区别？

多线程下 HashMap不安全，
HashTable不允许有空值。
迭代方式，HashTable使用Enumeration， HashMap使用Iterator
扩容方式不同。
HashTable继承于Dictionary， HashMap继承于AbstractMap类。

怎样让HashMap也变得线程安全

        Map<Integer, Integer> map = new HashMap<>();
        Map<Integer, Integer> syncronizedMap = Collections.synchronizedMap(map);
        // 返回一个SynchronizedMap 




        // 第二种方式， 使用ConcurrentHashMap  
        ConcurrentHashMap<Integer, Integer> chp = new ConcurrentHashMap<>();

高并发情况下集合有哪些问题？

第一代线程安全集合类： Vector，HashTable 使用synchronized 修饰了方法；
第二代线程非安全集合类： ArrayList HashMap， 线程不安全，但性能更好。 想要安全怎么办？ Collections.synchronized(list);
第三代线程安全集合类：
java.util.concurrent.*;

ConcurrentHashMap;
CopyOnWriteArrayList
CopyOnWriteArraySet 注意 不是 CopyOnWriteHashSet

底层多采用Lock锁（，