java 數據結構. 源碼閱讀

Collections工具類里的

Collections.synchronizedList

public static <T> List<T> synchronizedList(List<T> list) {
    return (list instanceof RandomAccess ?
                new SynchronizedRandomAccessList<T>(list) :
                new SynchronizedList<T>(list));
    }

##僅僅是通過判斷是否實現RandomAccess接口,而返回不同的SynchronizedList(內部)類,

##RandomAccess foreach較快, iterator較慢,差別不明顯(如i5上,本人測試,一個ArrayList,90w個object,  foreach出來是6多ms, iterator出來7ms)

##SequenceAccess foreach較慢,iterator較快.差別非常明顯. (如i5上,本人測試,一個LinkedList,10w個object,  foreach出來是200多ms, iterator出來僅4ms)

LinkedList的get(n) 方法.   源碼里寫得也有點妙,一不留意 確實是個坑.    (這是SequenceAccess 的foreach 慢的原因)

private Entry<E> entry(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException("Index: "+index+
                                                ", Size: "+size);
        Entry<E> e = header;
        if (index < (size >> 1)) {
            for (int i = 0; i <= index; i++)
                e = e.next;
        } else {
            for (int i = size; i > index; i--)
                e = e.previous;
        }
        return e;
    }

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