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 List 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. * *The best algorithms for manipulating random access lists (such as * ArrayList) can produce quadratic behavior when applied to * sequential access lists (such as LinkedList). Generic list * algorithms are encouraged to check whether the given list is an * instanceof 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. * *
It is recognized that the distinction between random and sequential * access is often fuzzy. For example, some List implementations * provide asymptotically linear access times if they get huge, but constant * access times in practice. Such a List implementation * should generally implement this interface. As a rule of thumb, a * List implementation should implement this interface if, * for typical instances of the class, this loop: *
* for (int i=0, n=list.size(); i < n; i++) * list.get(i); ** runs faster than this loop: ** for (Iterator i=list.iterator(); i.hasNext(); ) * i.next(); ** *This interface is a member of the * * Java Collections Framework. * * @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 *@author: godtrue *@createTime: 2018-09-11 *@version: v1.0 */ private static List genArrayList(){ long startTime = System.currentTimeMillis(); List list = new ArrayList (); for(int i=TestTraverse.START_LOOP;i *@author: godtrue *@createTime: 2018-09-11 *@version: v1.0 */ private static List genLinkedList(){ long startTime = System.currentTimeMillis(); List list = new LinkedList (); for(int i=TestTraverse.START_LOOP;i 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
