文档章节

【Java集合类】ArrayList与LinkedList官方说明

YuanyuanL
 YuanyuanL
发布于 2016/07/12 13:16
字数 760
阅读 30
收藏 0

LinkedList and ArrayList are two different implementations of the List interface. LinkedListimplements it with a doubly-linked list. ArrayList implements it with a dynamically re-sizing array.

As with standard linked list and array operations, the various methods will have different algorithmic runtimes.

For LinkedList<E>

  • get(int index) is O(n/4) average
  • add(E element) is O(1)
  • add(int index, E element) is O(n/4) average
         but O(1) when index = 0 <--- main benefit of LinkedList<E>
  • remove(int index) is O(n/4) average
  • Iterator.remove() is O(1) <--- main benefit of LinkedList<E>
  • ListIterator.add(E element) is O(1) <--- main benefit of LinkedList<E>

Note: O(n/4) is average, O(1) best case (e.g. index = 0), O(n/2) worst case (middle of list)

For ArrayList<E>

  • get(int index) is O(1) <--- main benefit of ArrayList<E>
  • add(E element) is O(1) amortized, but O(n) worst-case since the array must be resized and copied
  • add(int index, E element) is O(n/2) average
  • remove(int index) is O(n/2) average
  • Iterator.remove() is O(n/2) average
  • ListIterator.add(E element) is O(n/2) average

Note: O(n/2) is average, O(1) best case (end of list), O(n) worst case (start of list)

LinkedList<E> allows for constant-time insertions or removals using iterators, but only sequential access of elements. In other words, you can walk the list forwards or backwards, but finding a position in the list takes time proportional to the size of the list. Javadoc says "operations that index into the list will traverse the list from the beginning or the end, whichever is closer", so those methods are O(n/4) on average, though O(1) for index = 0.

ArrayList<E>, on the other hand, allow fast random read access, so you can grab any element in constant time. But adding or removing from anywhere but the end requires shifting all the latter elements over, either to make an opening or fill the gap. Also, if you add more elements than the capacity of the underlying array, a new array (1.5 times the size) is allocated, and the old array is copied to the new one, so adding to an ArrayList is O(n) in the worst case but constant on average.

So depending on the operations you intend to do, you should choose the implementations accordingly. Iterating over either kind of List is practically equally cheap. (Iterating over an ArrayList is technically faster, but unless you're doing something really performance-sensitive, you shouldn't worry about this -- they're both constants.)

The main benefits of using a LinkedList arise when you re-use existing iterators to insert and remove elements. These operations can then be done in O(1) by changing the list locally only. In an array list, the remainder of the array needs to be moved (i.e. copied). On the other side, seeking in a LinkedList means following the links in O(n), whereas in an ArrayList the desired position can be computed mathematically and accessed in O(1).

Another benefit of using a LinkedList arise when you add or remove from the head of the list, since those operations are O(1), while they are O(n) for ArrayList. Note that ArrayDeque may be a good alternative to LinkedList for adding and removing from the head, but it is not a List.

Also, if you have large lists, keep in mind that memory usage is also different. Each element of a LinkedList has more overhead since pointers to the next and previous elements are also stored. ArrayLists don't have this overhead. However, ArrayLists take up as much memory as is allocated for the capacity, regardless of whether elements have actually been added.

The default initial capacity of an ArrayList is pretty small (10 from Java 1.4 - 1.8). But since the underlying implementation is an array, the array must be resized if you add a lot of elements. To avoid the high cost of resizing when you know you're going to add a lot of elements, construct the ArrayList with a higher initial capacity.

It's worth noting that Vector also implements the List interface and is almost identical toArrayList. The difference is that Vector is synchronized, so it is thread-safe. Because of this, it is also slightly slower than ArrayList. So as far as I understand, most Java programmers avoid Vector in favor of ArrayList since they will probably synchronize explicitly anyway if they care about that.

© 著作权归作者所有

共有 人打赏支持
YuanyuanL

YuanyuanL

粉丝 152
博文 320
码字总数 187682
作品 0
济南
部门经理
Java高级篇 -- List选择及优化

在java编程中,我们常常使用到java自带的集合类List 以下为几点简单的优化建议: 1.Vector还是ArrayList Vector有其特有有点,其每个方法都为同步方法【synchronized】,所以是线程安全的,在...

YOTOO
2014/05/05
0
0
关于java.util.Vector 或 java.util.Hashtable类过时的讨论

某些高级IDE在检测代码成熟问题时,会报告集合是否过时的问题。目前过时的集合类有两个java.util.Vector 和 java.util.Hashtable 。 Vector的api描述是:从jdk 1.2版本开始,该类被修正为实现...

Barudisshu
2013/09/10
0
2
一份关于 Java、Kotlin 与 Android 的学习笔记

JavaKotlinAndroidLearn 这是一份关于 Java 、Kotlin 、Android 的学习笔记,既包含对基础知识点的介绍,也包含对一些重要知识点的源码解析,笔记的大纲如下所示: Java 重拾Java(0)-基础知...

叶应是叶
08/08
0
0
Vector、ArrayList、LinkedList 有什么区别?

版权声明:本文供交流学习,能够帮助到你是我最大的荣幸! https://blog.csdn.net/u014231523/article/details/82086185 这个问题主要是考察集合框架的问题,主要考察三者之间的设计区别,以...

兴国First
08/26
0
0
115个Java面试题和答案——终极列表(上)

本文我们将要讨论Java面试中的各种不同类型的面试题,它们可以让雇主测试应聘者的Java和通用的面向对象编程的能力。下面的章节分为上下两篇,第一篇将要讨论面向对象编程和它的特点,关于Jav...

LCZ777
2014/04/23
0
0

没有更多内容

加载失败,请刷新页面

加载更多

win32截屏并rgb24转yuv420

//最终f的内存布局为BGRA格式,需要保证buf长度足够(>w*h*4)void ScreenCap(void* buf, int w, int h){ HWND hDesk = GetDesktopWindow(); HDC hScreen = GetDC(hDesk); ......

styleman
50分钟前
1
0
php输出mysql取出的中文为??的问题

解决方法: @ $db=new mysqli(DB_HOST,DB_USER,DB_PASSWORD,DB_DB); $db->query("set names utf8");//添加此语句,可以解决问题...

Aomo
今天
1
2
白话SpringCloud | 第五章:服务容错保护(Hystrix)

前言 前一章节,我们知道了如何利用RestTemplate+Ribbon和Feign的方式进行服务的调用。在微服务架构中,一个服务可能会调用很多的其他微服务应用,虽然做了多集群部署,但可能还会存在诸如网...

oKong
今天
2
0
【解惑】领略Java内部类的“内部”

内部类有两种情况: (1) 在类中定义一个类(私有内部类,静态内部类) (2) 在方法中定义一个类(局部内部类,匿名内部类) 1、私有内部类 —— 在方法之间定义的内部类,非静态 我们首先看看类中...

偶尔诗文
今天
1
0
sqlserver 2008 r2 直接下载地址(百度云)

之前下载的sqlserver2008发现不能附加,就卸载了,重新找到了sqlserver2008R2的百度云资源 卸载sqlserver2008还是有点麻烦,不过就是需要删除注册表中的信息 自己来回卸载了3次终于重装sqlse...

dillonxiao
今天
3
0

没有更多内容

加载失败,请刷新页面

加载更多

返回顶部
顶部