RandomAccess

RandomAccess

总结：

1.基于 JDK 1.8 源码分析

2.作用

起到判断的作用，即当前集合是否支持随机访问
顺序表（数组形式）还是单链表存储，以便使用更合适的遍历方式
RandomAccess(如ArrayList)还是Sequence List (如LinkedList）

源码：

/*
 * @(#)RandomAccess.java	1.9 06/04/21
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN 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 &lt; 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 {
}

对于顺序表 for循环通过数组下标访问的效率要高于通过 Iterator 访问的效率；反之，对于单链表要使用 Iterator 方式循环

// ArrayList 的定义
public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

// LinkedList 的定义
public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable

总结：

所以对大量数据进行遍历时要区分当前的List的实现方式

    if (list instance of RandomAccess) {
        for(int m = 0; m < list.size(); m++){}
    }else{
        Iterator iter = list.iterator();
        while(iter.hasNext()){}
    }

博文参考：
RandomAccess接口的使用