RandomAccess接口的使用

RandomAccess在类Collections的shuffle()方法中的使用:(jdk源码如下)

/**
     * Randomly permute the specified list using the specified source of
     * randomness.  All permutations occur with equal likelihood
     * assuming that the source of randomness is fair.<p>
     *
     * This implementation traverses the list backwards, from the last element
     * up to the second, repeatedly swapping a randomly selected element into
     * the "current position".  Elements are randomly selected from the
     * portion of the list that runs from the first element to the current
     * position, inclusive.<p>
     *
     * This method runs in linear time.  If the specified list does not
     * implement the {@link RandomAccess} interface and is large, this
     * implementation dumps the specified list into an array before shuffling
     * it, and dumps the shuffled array back into the list.  This avoids the
     * quadratic behavior that would result from shuffling a "sequential
     * access" list in place.
     *
     * @param  list the list to be shuffled.
     * @param  rnd the source of randomness to use to shuffle the list.
     * @throws UnsupportedOperationException if the specified list or its
     *         list-iterator does not support the <tt>set</tt> operation.
     */
    public static void shuffle(List<?> list, Random rnd) {
        int size = list.size();
        if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
            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]);
            }
        }
    }

由以上的jdk源码可见,在对实现list接口的对象进行洗牌,打乱时,区分了该类是否是RandomAccess的实例,这样做有什么意义呢?请继续向下看:

在jdk文档中对RandomAccess接口的定义如下:

/**
 * 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}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 *
 */
public interface RandomAccess {
}

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。

将操作随机访问列表的最佳算法（如 ArrayList ）应用到连续访问列表（如 LinkedList ）时，可产生二次项的行为。如果将某个算法应用到连续访问列表，那么在应用可能提供较差性能的算法前，鼓励使用一般的列表算法检查给定列表是否为此接口的一个 instanceof ，如果需要保证可接受的性能，还可以更改其行为。

现在已经认识到，随机和连续访问之间的区别通常是模糊的。例如，如果列表很大时，某些 List 实现提供渐进的线性访问时间，但实际上是固定的访问时间。这样的 List 实现通常应该实现此接口。

JDK中推荐的是对List集合尽量要实现RandomAccess接口

如果集合类是RandomAccess的实现，则尽量用for(int i = 0; i < size; i++) 来遍历而不要用Iterator迭代器来遍历，在效率上要差一些。反过来，如果List是Sequence List，则最好用迭代器来进行迭代。

JDK中说的很清楚，在对List特别是Huge size的List的遍历算法中，要尽量来判断是属于RandomAccess(如ArrayList)还是Sequence List (如LinkedList），因为适合RandomAccess List的遍历算法，用在Sequence List上就差别很大，常用的作法就是：
    要作一个判断：

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

package testrandomaccess;  
  
import java.util.ArrayList;  
import java.util.Iterator;  
import java.util.LinkedList;  
import java.util.List;  
import java.util.RandomAccess;  
  
/** 
 * 
 * @author bolong 
 */  
public class TestRandomAccess {  
// 初始化列表  
  
    public static void initList(List list, int n) {  
        for (int i = 0; i < n; i++) {  
            list.add(i);  
        }  
    }  
//使用循环进行对列表的迭代  
  
    public static void traverseWithLoop(List list) {  
        long starttime = 0;  
        long endtime = 0;  
        starttime = System.currentTimeMillis();  
        for (int count = 0; count <= 1000; count++) {  
            for (int i = 0; i < list.size(); i++) {  
                list.get(i);  
            }  
        }  
        endtime = System.currentTimeMillis();  
        System.out.println("使用loop迭代一共花了" + (endtime - starttime) + "ms时间");  
  
    }  
//使用迭代器对列表进行迭代  
  
    public static void traverseWithIterator(List list) {  
        long starttime = 0;  
        long endtime = 0;  
        starttime = System.currentTimeMillis();  
        for (int count = 0; count <= 1000; count++) {  
            for (Iterator itr = list.iterator(); itr.hasNext();) {  
                itr.next();  
            }  
        }  
        endtime = System.currentTimeMillis();  
        System.out.println("使用Iterator迭代一共花了" + (endtime - starttime) + "ms时间");  
    }  
  
    public static void traverse(List list) {  
  
        long starttime = 0;  
        long endtime = 0;  
        if (list instanceof RandomAccess) {  
            System.out.println("该list实现了RandomAccess接口");  
            starttime = System.currentTimeMillis();  
            for (int count = 0; count <= 1000; count++) {  
                for (int i = 0; i < list.size(); i++) {  
                    list.get(i);  
                }  
            }  
            endtime = System.currentTimeMillis();  
            System.out.println("迭代一共花了" + (endtime - starttime) + "ms时间");  
        } else {  
            System.out.println("该list未实现RandomAccess接口");  
            starttime = System.currentTimeMillis();  
            for (int count = 0; count <= 1000; count++) {  
                for (Iterator itr = list.iterator(); itr.hasNext();) {  
                    itr.next();  
                }  
            }  
            endtime = System.currentTimeMillis();  
            System.out.println("迭代一共花了" + (endtime - starttime) + "ms时间");  
        }  
    }  
  
    public static void main(String[] args) {  
        ArrayList arraylist = new ArrayList();  
        LinkedList linkedlist = new LinkedList();  
        initList(arraylist, 1000);  
        initList(linkedlist, 1000);  
        traverse(arraylist);  
        traverse(linkedlist);  
        traverseWithIterator(arraylist);  
        traverseWithLoop(arraylist);  
        traverseWithIterator(linkedlist);  
        traverseWithLoop(linkedlist);  
    }  
}  

根据程序输出的结果的确证明了,arraylist等实现了RandomAccess接口的类在进行迭代时使用loop效率更高,而linkedList那些未实现该接口的类在进行迭代时使用Iterator进行迭代效率更高.