ConcurrentHashMap&HashMap&Hashtable使用上的区别

1、ConcurrentHashMap

区别1、使用了Lock分段锁，效率不高，默认的是非公平锁，利于多线程使用。

/**
     * Creates a new {@code ReentrantReadWriteLock} with
     * the given fairness policy.
     *
     * @param fair {@code true} if this lock should use a fair ordering policy
     */
    public ReentrantReadWriteLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
        readerLock = new ReadLock(this);
        writerLock = new WriteLock(this);
    }

    public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }
    public ReentrantReadWriteLock.ReadLock  readLock()  { return readerLock; }

区别2，在hash的算法上也是区别很大

protected static int hash(int h) {
        h += h << 15 ^ -12931;
        h ^= h >>> 10;
        h += h << 3;
        h ^= h >>> 6;
        h += (h << 2) + (h << 14);
        return h ^ h >>> 16;
    }

区别三，不允许key与Value为null。在put的时候判别。