RandomAccess接口

ArrayList实现了RandomAccess接口，怀着好奇的心情去看了一下RandomAccess接口，结果：

/*

* Copyright (c) 2000, 2006, Oracle and/or its affiliates. All rights reserved.

* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.

*/

package java.util;

/**

* Marker interface used by <tt>List</tt> implementations to indicate that

* they support fast (generally constant time) random access. The primary

* purpose of this interface is to allow generic algorithms to alter their

* behavior to provide good performance when applied to either random or

* sequential access lists.

*

* <p>The best algorithms for manipulating random access lists (such as

* <tt>ArrayList</tt>) can produce quadratic behavior when applied to

* sequential access lists (such as <tt>LinkedList</tt>). Generic list

* algorithms are encouraged to check whether the given list is an

* <tt>instanceof</tt> this interface before applying an algorithm that would

* provide poor performance if it were applied to a sequential access list,

* and to alter their behavior if necessary to guarantee acceptable

* performance.

*

* <p>It is recognized that the distinction between random and sequential

* access is often fuzzy. For example, some <tt>List</tt> implementations

* provide asymptotically linear access times if they get huge, but constant

* access times in practice. Such a <tt>List</tt> implementation

* should generally implement this interface. As a rule of thumb, a

* <tt>List</tt> implementation should implement this interface if,

* for typical instances of the class, this loop:

* <pre>

* for (int i=0, n=list.size(); i < n; i++)

* list.get(i);

* </pre>

* runs faster than this loop:

* <pre>

* for (Iterator i=list.iterator(); i.hasNext(); )

* i.next();

* </pre>

*

* <p>This interface is a member of the

* <a href="{@docRoot}/../technotes/guides/collections/index.html">

* Java Collections Framework</a>.

*

* @since 1.4

*/

public interface RandomAccess {

}

一大串英文+一个空接口，百思不得其姐，难道实现它就是为了挂个名？无意中一天在Collections类中看到了shuffle方法：

public static void shuffle(List<?> list, Random rnd) {

int size = list.size();

if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {//看这里

for (int i=size; i>1; i--)

swap(list, i-1, rnd.nextInt(i));

} else {

Object arr[] = list.toArray();

// Shuffle array

for (int i=size; i>1; i--)

swap(arr, i-1, rnd.nextInt(i));

// Dump array back into list

ListIterator it = list.listIterator();

for (int i=0; i<arr.length; i++) {

it.next();

it.set(arr[i]);

}

}

}

看第三行代码，首先判断list是不是RandomAccess的实例，如果是则swap，否则...

百度翻译了以下，得知shuffle=洗牌，大概猜想这个方法用来打乱list中的顺序。可以看到else里执行过程要比if里的复杂，也就是说if里的代码执行效率要高点，那么也就是说，实现RandomAccess接口是为了执行if中的代码块，也就是说告诉类用某种算法来执行程序。好心的网友给出解释：List 实现所使用的标记接口，用来表明其支持快速（通常是固定时间）随机访问。此接口的主要目的是允许一般的算法更改其行为，从而在将其应用到随机或连续访问列表时能提供良好的性能。