Java中ArrayList和LinkedList的遍历与性能分析
前言 通过本文你可以了解List的五种遍历方式及各自性能和foreach及Iterator的实现,加深对ArrayList和LinkedList实现的了解。下面来一起看看吧。 一、List的五种遍历方式 1、for each循环 List<Integer> list = new ArrayList<Integer>(); for (Integer j : list) { // use j } 2、显示调用集合迭代器 List<Integer> list = new ArrayList<Integer>(); for (Iterator<Integer> iterator = list.iterator(); iterator.hasNext();) { iterator.next(); } 或 List<Integer> list = new ArrayList<Integer>(); Iterator<Integer> iterator = list.iterator(); while (iterator.hasNext()) { iterator.next(); } 3、下标递增循环,终止条件为每次调用size()函数比较判断 List<Integer> list = new ArrayList<Integer>(); for (int j = 0; j < list.size(); j++) { list.get(j); } 4、下标递增循环,终止条件为和等于size()的临时变量比较判断 List<Integer> list = new ArrayList<Integer>(); int size = list.size(); for (int j = 0; j < size; j++) { list.get(j); } 5、下标递减循环 List<Integer> list = new ArrayList<Integer>(); for (int j = list.size() - 1; j >= 0; j--) { list.get(j); } List五种遍历方式的性能测试及对比 以下是性能测试代码,会输出不同数量级大小的ArrayList和LinkedList各种遍历方式所花费的时间。 package cn.trinea.java.test; import java.text.DecimalFormat; import java.util.ArrayList; import java.util.Calendar; import java.util.Iterator; import java.util.LinkedList; import java.util.List; /** * JavaLoopTest * * @author www.trinea.cn 2013-10-28 */ public class JavaLoopTest { public static void main(String[] args) { System.out.print("compare loop performance of ArrayList"); loopListCompare(getArrayLists(10000,100000,1000000,9000000)); System.out.print("rnrncompare loop performance of LinkedList"); loopListCompare(getLinkedLists(100,1000,10000,100000)); } public static List<Integer>[] getArrayLists(int... sizeArray) { List<Integer>[] listArray = new ArrayList[sizeArray.length]; for (int i = 0; i < listArray.length; i++) { int size = sizeArray[i]; List<Integer> list = new ArrayList<Integer>(); for (int j = 0; j < size; j++) { list.add(j); } listArray[i] = list; } return listArray; } public static List<Integer>[] getLinkedLists(int... sizeArray) { List<Integer>[] listArray = new LinkedList[sizeArray.length]; for (int i = 0; i < listArray.length; i++) { int size = sizeArray[i]; List<Integer> list = new LinkedList<Integer>(); for (int j = 0; j < size; j++) { list.add(j); } listArray[i] = list; } return listArray; } public static void loopListCompare(List<Integer>... listArray) { printHeader(listArray); long startTime,endTime; // Type 1 for (int i = 0; i < listArray.length; i++) { List<Integer> list = listArray[i]; startTime = Calendar.getInstance().getTimeInMillis(); for (Integer j : list) { // use j } endTime = Calendar.getInstance().getTimeInMillis(); printCostTime(i,listArray.length,"for each",endTime - startTime); } // Type 2 for (int i = 0; i < listArray.length; i++) { List<Integer> list = listArray[i]; startTime = Calendar.getInstance().getTimeInMillis(); // Iterator<Integer> iterator = list.iterator(); // while(iterator.hasNext()) { // iterator.next(); // } for (Iterator<Integer> iterator = list.iterator(); iterator.hasNext();) { iterator.next(); } endTime = Calendar.getInstance().getTimeInMillis(); printCostTime(i,"for iterator",endTime - startTime); } // Type 3 for (int i = 0; i < listArray.length; i++) { List<Integer> list = listArray[i]; startTime = Calendar.getInstance().getTimeInMillis(); for (int j = 0; j < list.size(); j++) { list.get(j); } endTime = Calendar.getInstance().getTimeInMillis(); printCostTime(i,"for list.size()",endTime - startTime); } // Type 4 for (int i = 0; i < listArray.length; i++) { List<Integer> list = listArray[i]; startTime = Calendar.getInstance().getTimeInMillis(); int size = list.size(); for (int j = 0; j < size; j++) { list.get(j); } endTime = Calendar.getInstance().getTimeInMillis(); printCostTime(i,"for size = list.size()",endTime - startTime); } // Type 5 for (int i = 0; i < listArray.length; i++) { List<Integer> list = listArray[i]; startTime = Calendar.getInstance().getTimeInMillis(); for (int j = list.size() - 1; j >= 0; j--) { list.get(j); } endTime = Calendar.getInstance().getTimeInMillis(); printCostTime(i,"for j--",endTime - startTime); } } static int FIRST_COLUMN_LENGTH = 23,OTHER_COLUMN_LENGTH = 12,TOTAL_COLUMN_LENGTH = 71; static final DecimalFormat COMMA_FORMAT = new DecimalFormat("#,###"); public static void printHeader(List<Integer>... listArray) { printRowDivider(); for (int i = 0; i < listArray.length; i++) { if (i == 0) { StringBuilder sb = new StringBuilder().append("list size"); while (sb.length() < FIRST_COLUMN_LENGTH) { sb.append(" "); } System.out.print(sb); } StringBuilder sb = new StringBuilder().append("| ").append(COMMA_FORMAT.format(listArray[i].size())); while (sb.length() < OTHER_COLUMN_LENGTH) { sb.append(" "); } System.out.print(sb); } TOTAL_COLUMN_LENGTH = FIRST_COLUMN_LENGTH + OTHER_COLUMN_LENGTH * listArray.length; printRowDivider(); } public static void printRowDivider() { System.out.println(); StringBuilder sb = new StringBuilder(); while (sb.length() < TOTAL_COLUMN_LENGTH) { sb.append("-"); } System.out.println(sb); } public static void printCostTime(int i,int size,String caseName,long costTime) { if (i == 0) { StringBuilder sb = new StringBuilder().append(caseName); while (sb.length() < FIRST_COLUMN_LENGTH) { sb.append(" "); } System.out.print(sb); } StringBuilder sb = new StringBuilder().append("| ").append(costTime).append(" ms"); while (sb.length() < OTHER_COLUMN_LENGTH) { sb.append(" "); } System.out.print(sb); if (i == size - 1) { printRowDivider(); } } } PS:如果运行报异常 其中
测试环境为Windows7 32位系统 3.2G双核CPU 4G内存,Java 7,Eclipse -Xms512m -Xmx512m 最终测试结果如下: compare loop performance of ArrayList ----------------------------------------------------------------------- list size | 10,000 | 100,000 | 1,000,000 | 10,000 ----------------------------------------------------------------------- for each | 1 ms | 3 ms | 14 ms | 152 ms ----------------------------------------------------------------------- for iterator | 0 ms | 1 ms | 12 ms | 114 ms ----------------------------------------------------------------------- for list.size() | 1 ms | 1 ms | 13 ms | 128 ms ----------------------------------------------------------------------- for size = list.size() | 0 ms | 0 ms | 6 ms | 62 ms ----------------------------------------------------------------------- for j-- | 0 ms | 1 ms | 6 ms | 63 ms ----------------------------------------------------------------------- compare loop performance of LinkedList ----------------------------------------------------------------------- list size | 100 | 1,000 | 10,000 ----------------------------------------------------------------------- for each | 0 ms | 1 ms | 1 ms | 2 ms ----------------------------------------------------------------------- for iterator | 0 ms | 0 ms | 0 ms | 2 ms ----------------------------------------------------------------------- for list.size() | 0 ms | 1 ms | 73 ms | 7972 ms ----------------------------------------------------------------------- for size = list.size() | 0 ms | 0 ms | 67 ms | 8216 ms ----------------------------------------------------------------------- for j-- | 0 ms | 1 ms | 67 ms | 8277 ms ----------------------------------------------------------------------- 第一张表为ArrayList对比结果,第二张表为LinkedList对比结果。 表横向为同一遍历方式不同大小list遍历的时间消耗,纵向为同一list不同遍历方式遍历的时间消耗。 PS:由于首次遍历List会稍微多耗时一点, 遍历方式性能测试结果分析 1、foreach介绍 foreach是Java SE5.0引入的功能很强的循环结构,
Iterator<Integer> iterator = list.iterator(); while(iterator.hasNext()) { Integer j = iterator.next(); } foreach代码书写简单,不必关心下标初始值和终止值及越界等,所以不易出错 2、ArrayList遍历方式结果分析 a. 在ArrayList大小为十万之前,五种遍历方式时间消耗几乎一样 b. 在十万以后,第四、五种遍历方式快于前三种,get方式优于Iterator方式,并且 int size = list.size(); for (int j = 0; j < size; j++) { list.get(j); } 用临时变量size取代 private class Itr implements Iterator<E> { int cursor; // index of next element to return int lastRet = -1; // index of last element returned; -1 if no such int expectedModCount = modCount; public boolean hasNext() { return cursor != size; } @SuppressWarnings("unchecked") public E next() { checkForComodification(); int i = cursor; if (i >= size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; return (E) elementData[lastRet = i]; } …… } public E get(int index) { rangeCheck(index); return elementData(index); } 从中可以看出 c. 从上可以看出即便在千万大小的ArrayList中,几种遍历方式相差也不过50ms左右,且在常用的十万左右时间几乎相等,考虑foreach的优点,我们大可选用foreach这种简便方式进行遍历。 3、LinkedList遍历方式结果分析 a. 在LinkedList大小接近一万时, 我们看看LinkedList中迭代器和 private class ListItr implements ListIterator<E> { private Node<E> lastReturned = null; private Node<E> next; private int nextIndex; private int expectedModCount = modCount; ListItr(int index) { // assert isPositionIndex(index); next = (index == size) ? null : node(index); nextIndex = index; } public boolean hasNext() { return nextIndex < size; } public E next() { checkForComodification(); if (!hasNext()) throw new NoSuchElementException(); lastReturned = next; next = next.next; nextIndex++; return lastReturned.item; } …… } public E get(int index) { checkElementIndex(index); return node(index).item; } /** * Returns the (non-null) Node at the specified element index. */ Node<E> node(int index) { // assert isElementIndex(index); if (index < (size >> 1)) { Node<E> x = first; for (int i = 0; i < index; i++) x = x.next; return x; } else { Node<E> x = last; for (int i = size - 1; i > index; i--) x = x.prev; return x; } } 从上面代码中可以看出LinkedList迭代器的 所以对于LinkedList的遍历推荐使用foreach,避免使用 4、ArrayList和LinkedList遍历方式结果对比分析 从上面的数量级来看,同样是foreach循环遍历,ArrayList和LinkedList时间差不多,可将本例稍作修改加大 但 再结合考虑空间消耗的话,建议首选ArrayList。对于个别插入删除非常多的可以使用LinkedList。 结论总结 通过上面的分析我们基本可以总结下:
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