分析轮子（七）- RandomAccess.java

1：还是先上一个类的继承关系比较图吧！

2：看一下 RandomAccess.java 的源码，空空如也，什么都没有，那她有什么用处呢？

/**
 * 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 {
}

有点磕磕巴巴，阅读了源码中的注释，大概讲解了一下 RandomAccess.java 接口的作用，它是一个标记接口，表示实现它的类支持快速随机访问，通过上图的对比我们看到实现List接口的类，有些是支持快速随机访问的，有些不支持，怎么标记哪？就是用 RandomAccess.java 接口来标记，这样有什么好处呢？可以在通用的实现List集合遍历的时候算法中，针对实现 RandomAccess.java 接口的集合，可以有选择性的使用性能更好的遍历方式，我也实验了一把，继续往下看吧！

3：简单的集合遍历性能对比小栗子，代码比较简单，可以调整参数自行玩一玩

/**
 * @description：测试循环方式的性能
 * @author：godtrue
 * @create：2018-09-11
 */
public class TestTraverse {
    /**
     * 开始循环的基值
     */
    private static final int START_LOOP = 1;

    /**
     * 结束循环的基值
     * 我的机器 1亿 次就卡死了，我就实验下 1千万 次吧！
     */
    private static final int END_LOOP = 10000000;

    /**
    *
    *@description: 测试入口，主方法
    *@param args
    *@return: void
    *@author: godtrue
    *@createTime: 2018-09-11
    *@version: v1.0
    */
    public static void main(String[] args) {
        /**
         * 注意：
         * 1：测试时，一个个来跑，避免相互影响
         * 2：可以逐渐，将 TestTraverse.END_LOOP 调高，用于观察不同量级的循环的运行结果
         */
        traverse(genArrayList());
        //traverse(genLinkedList());
    }

    /**
    *
    *@description: 遍历 list 集合，这里能体现到 RandomAccess 接口的作用，可以选择不同的遍历集合的方式
    *@param list
    *@return: void
    *@author: godtrue
    *@createTime: 2018-09-11
    *@version: v1.0
    */
    private static void traverse(List list){
        if(list instanceof RandomAccess){
            traverseByLoop(list);
        }else {
            traverseByIterator(list);
        }
    }

    /**
    *
    *@description: 循环遍历 list 集合
    *@param list
    *@return: void
    *@author: godtrue
    *@createTime: 2018-09-11
    *@version: v1.0
    */
    private static void traverseByLoop(List list){
        long startTime = System.currentTimeMillis();
        for(int i=0,sum=list.size();i<sum;i++){
            list.get(i);
        }
        System.out.println("exe traverseByLoop cost time : "+(System.currentTimeMillis()-startTime));
    }

    /**
    *
    *@description: 迭代遍历 list 集合
    *@param list
    *@return: void
    *@author: godtrue
    *@createTime: 2018-09-11
    *@version: v1.0
    */
    private static void traverseByIterator(List list){
        long startTime = System.currentTimeMillis();
          for (Iterator i=list.iterator(); i.hasNext(); ){
              i.next();
          }
        System.out.println("exe traverseByIterator cost time : "+(System.currentTimeMillis()-startTime));
    }

    /**
    *
    *@description:  生成 ArrayList 数据信息
    *@param
    *@return: java.util.List<java.lang.String>
    *@author: godtrue
    *@createTime: 2018-09-11
    *@version: v1.0
    */
    private static List<String> genArrayList(){
        long startTime = System.currentTimeMillis();
        List<String> list = new ArrayList<String>();
        for(int i=TestTraverse.START_LOOP;i<TestTraverse.END_LOOP;i++){
            list.add(String.valueOf(i));
        }
        System.out.println("exe genArrayList cost time : "+(System.currentTimeMillis()-startTime));
        return list;
    }

    /**
    *
    *@description: 生成 LinkedList 数据信息
    *@param
    *@return: java.util.List<java.lang.String>
    *@author: godtrue
    *@createTime: 2018-09-11
    *@version: v1.0
    */
    private static List<String> genLinkedList(){
        long startTime = System.currentTimeMillis();
        List<String> list = new LinkedList<String>();
        for(int i=TestTraverse.START_LOOP;i<TestTraverse.END_LOOP;i++){
            list.add(String.valueOf(i));
        }
        System.out.println("exe genLinkedList cost time : "+(System.currentTimeMillis()-startTime));
        return list;
    }
}

4：下面是实验环境的信息和结果

4-1）实验的硬件信息

4-2）运行时的性能指标参数

4-3）迭代遍历的运行情况

4-4）随机遍历的运行情况，对比一下，可以看出性能相差的还是蛮多的