JDK源码-RandomAccess

RandomAccess

• List实现了该接口,说明该实现类的数据可以进行随机访问,不需要保证顺序性.比如ArrayList实现了该接口,LinkedList没有实现该接口.
• 随机访问列表使用循环遍历，顺序访问列表使用迭代器遍历。

JDK定义

/**
 * 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
 */

• List的实现类使用的标记接口,一般来标识该实现类能够支持快速随机访问.
• 接口存在的目的:继承该接口的实现类,能够允许一般的算法来更改他们的行为,yibian在随机或者循环访问列表时,能够提供更好的性能.
• 对于随机访问的列表(ArrayList)的操作方法应用到顺序访问的列表(LinkedList),将会产生二次项行为(个人理解应该是额外的操作吧).所以,对不同访问类型的列表,我们需要采用最为合适的算法.
• 总结来讲,随机访问类型的列表进行loop遍历的时间更短,顺序访问类型的列表通过Iterator进行遍历的时间更短.
  参考文章:RandomAccess 接口使用