ArrayList

indexOf()

/**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */
    public int indexOf(Object o) {
	if (o == null) {
	    for (int i = 0; i < size; i++)
		if (elementData[i]==null)
		    return i;
	} else {
	    for (int i = 0; i < size; i++)
		if (o.equals(elementData[i]))
		    return i;
	}
	return -1;
    }

THINK:源码中对遍历null和非null做了区分处理，故怀疑null遍历时更快。demo证明在数据量小于千万级别下只有几毫秒的差异，在5千万时最大有80毫秒的差距。再多就抛OutOfMemoryError了。