RandomAccess
总结:
1.基于 JDK 1.8 源码分析
2.作用
起到判断的作用,即当前集合是否支持随机访问
顺序表(数组形式)还是单链表存储,以便使用更合适的遍历方式
RandomAccess(如ArrayList)还是Sequence List (如LinkedList)
源码:
/* * @(#)RandomAccess.java 1.9 06/04/21 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.util; /** * 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 < 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 { }
对于顺序表 for循环通过数组下标访问的效率要高于通过 Iterator 访问的效率;反之,对于单链表要使用 Iterator 方式循环
// ArrayList 的定义 public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable // LinkedList 的定义 public class LinkedList<E> extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, java.io.Serializable
总结:
所以对大量数据进行遍历时要区分当前的List的实现方式
if (list instance of RandomAccess) { for(int m = 0; m < list.size(); m++){} }else{ Iterator iter = list.iterator(); while(iter.hasNext()){} }
博文参考:
RandomAccess接口的使用