java中RandomAccess接口明明是個空接口，有什麼用呢？

RandomAccess接口

一、官方描述

  首先我們先看一下這個接口在java中是怎麼描述的。

位置：rt.jar中java.util.RandomAccess

/**
 * 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>

大致的意思是說：這是一個用於List的實現類的中使用的標記接口，能夠支持快速隨機訪問（一定時間內）。此接口的主要目的是允許通用算法在應用於隨機或順序訪問列表時改變其行爲以提供良好的性能。
只在List的實現類ArrayList可以for + get（index）使用該算法執行，在LinkedList類中不行。

二、查看ArrayList和LinkedList

位置：rt.jar中java.util.ArrayList和java.util.LinkedList

2.1 ArrayList

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;
    ...
}

2.2 LinkedList

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    transient int size = 0;
    ... 
}

**由源碼可以看出：ArrayList是一個實現了List、RandomAccess、Cloneable、 java.io.Serializable等接口的類。實現可以隨機訪問、克隆及序列化的數組集合。
LinkedList則是實現了List、Deque、Cloneable、java.io.Serializable等接口的類，所以它是一個克隆、序列化的雙端連表。
**