标签：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。
本文出自 “没有水勒鱼” 博客，请务必保留此出处http://javaqun.blog.51cto.com/10687700/1703132
标签：randomaccess 接口
原文地址：http://javaqun.blog.51cto.com/10687700/1703132
