目录
一、简介二、遍历方式1、ArrayList遍历方式（1）for循环遍历（2）foreach循环遍历（3）Iterator迭代器遍历2、LinkedList遍历方式（1）for循环遍历（2）foreach循环遍历（3）Iterator迭代器遍历3、HashSet遍历方式（1）foreach循环遍历（2）Iterator迭代器遍历4、HashMap遍历方式（1）entrySet遍历（2）Iterator迭代器遍历5、LinkedHashMap遍历方式（1）entrySet遍历（2）Iterator迭代器遍历三、性能比较一、简介
List、Set都继承Collection接口，Map不是。
List：元素有序存储，元素可重复，取出来的顺序可能和放入的顺序不同，支持for循环和迭代器遍历；
Set：元素无序存储，且唯一，不能包含重复的元素，不支持for循环遍历，支持迭代器遍历；
Map：元素无序存储，key值唯一不能重复，value值可重复，支持迭代器遍历；
List、Set、Map实现类
List：ArrayList、LinkedList、Vector
Set：HashSet、TreeSet、LinkedHashSet
Map：HashMap、TreeMap、HashTable、LinkedHashMap
线程安全 / 线程不安全
线程安全：Vector、HashTable
线程不安全：ArrayList、LinkedList、HashSet、LinkedHashSet、HashMap、TreeMap、LinkedHashMap
下面我们只拿出ArrayList、LinkedList、HashSet、HashMap、LinkedHashMap来讲解遍历方式以及遍历性能比较。
二、遍历方式
1、ArrayList遍历方式
ArrayList有三种遍历方式：for循环遍历、foreach循环遍历、Iterator迭代器遍历。
（1）for循环遍历
ArrayList<String> lists = new ArrayList<String>();for(int i=0;i<lists.size();i++){String line = lists.get(i);}
（2）foreach循环遍历
ArrayList<String> lists = new ArrayList<String>();for(String str : lists){String line = str;}
（3）Iterator迭代器遍历
ArrayList<String> lists = new ArrayList<String>();Iterator<String> iterator = lists.iterator();while (iterator.hasNext()){String line = iterator.next();}
2、LinkedList遍历方式
LinkedList有三种遍历方式：for循环遍历、foreach循环遍历、Iterator迭代器遍历。
（1）for循环遍历
LinkedList<String> lists = new LinkedList<String>();for(int i=0;i<lists.size();i++){String line = lists.get(i);}
（2）foreach循环遍历
LinkedList<String> lists = new LinkedList<String>();for(String str : lists){String line = str;}
（3）Iterator迭代器遍历
LinkedList<String> lists = new LinkedList<String>();Iterator<String> iterator = lists.iterator();while (iterator.hasNext()){String line = iterator.next();}
3、HashSet遍历方式
HashSet有两种遍历方式：foreach循环遍历、Iterator迭代器遍历。
（1）foreach循环遍历
HashSet<String> hashSets = new HashSet<String>();for(String str : hashSets){String line = str;}
（2）Iterator迭代器遍历
HashSet<String> hashSets = new HashSet<String>();Iterator<String> iterator = hashSets.iterator();while (iterator.hasNext()){String line = iterator.next();}
4、HashMap遍历方式
HashMap有三种遍历方式：keySet循环遍历、entrySet遍历、Iterator迭代器遍历。
下面我们只讲解entrySet遍历和Iterator迭代器遍历。
（1）entrySet遍历
HashMap<String, String> hashMaps = new HashMap<String, String>();for(Map.Entry<String, String> entry : hashMaps.entrySet()){String line = entry.getKey();}
（2）Iterator迭代器遍历
HashMap<String, String> hashMaps = new HashMap<String, String>();Iterator iterator = hashMaps.entrySet().iterator();while (iterator.hasNext()){Map.Entry<String, String> entry = (Map.Entry<String, String>)iterator.next();String line = entry.getKey();}
5、LinkedHashMap遍历方式
LinkedHashMap有三种遍历方式：keySet循环遍历、entrySet遍历、Iterator迭代器遍历。
下面我们只讲解entrySet遍历和Iterator迭代器遍历。
（1）entrySet遍历
LinkedHashMap<String, String> linkedHashMaps = new LinkedHashMap<String, String>();for(Map.Entry<String, String> entry : linkedHashMaps.entrySet()){String line = entry.getKey();}
（2）Iterator迭代器遍历
LinkedHashMap<String, String> linkedHashMaps = new LinkedHashMap<String, String>();Iterator iterator = linkedHashMaps.entrySet().iterator();while (iterator.hasNext()){Map.Entry<String, String> entry = (Map.Entry<String, String>)iterator.next();String line = entry.getKey();}
三、性能比较
不同数量级的性能差异是比较大的，下面我们分别在30、100、1000、10000、100000数量级进行性能比较。
完整代码如下：
package com.example.springbootdemo.util;import java.util.*;public class Test {public static void main(String[] args) {compare();}private static ArrayList<String> lists = new ArrayList<String>();private static LinkedList<String> linkedLists = new LinkedList<String>();private static HashSet<String> hashSets = new HashSet<String>();private static HashMap<String, String> hashMaps = new HashMap<String, String>();private static LinkedHashMap<String, String> linkedHashMaps = new LinkedHashMap<String, String>();private static void compare(){compareInit(100000);compare1();}private static void compareInit(int count){lists.clear();linkedLists.clear();hashMaps.clear();hashSets.clear();linkedHashMaps.clear();String str = "abcdefg_";String one = "";for(int i=0;i<count;i++){one = str + i;lists.add(one);linkedLists.add(one);hashSets.add(one);hashMaps.put(one, one);linkedHashMaps.put(one, one);}}private static final String listFor= "ArrayList forduration";private static final String listForeach = "ArrayList foreachduration";private static final String listIterator= "ArrayList Iteratorduration";private static final String linkedListFor = "LinkedList for duration";private static final String linkedListForeach= "LinkedList foreachduration";private static final String linkedListIterator = "LinkedList Iterator duration";private static final String hashSetForeach = "HashSet foreach duration";private static final String hashSetIterator = "HashSet Iterator duration";private static final String hashMapEntry= "HashMap entryduration";private static final String hashMapIterator = "HashMap Iterator duration";private static final String linkedHashMapEntry = "LinkedHashMap entry duration";private static final String linkedHashMapIterator = "LinkedHashMap Iterator duration";private static void compare1(){for(int i=0;i<5;i++){System.out.println("------------------------------");listOne();listTwo();listThree();linkedListOne();linkedListTwo();linkedListThree();hashSetOne();hashSetTwo();hashMapOne();hashMapTwo();linkedHashMapOne();linkedHashMapTwo();System.out.println();}}private static void listOne(){String line = "";long start = System.nanoTime();for(int i=0;i<lists.size();i++){line = lists.get(i);}long end = System.nanoTime();print(start, end, listFor);}private static void listTwo(){String line = "";long start = System.nanoTime();for(String str : lists){line = str;}long end = System.nanoTime();print(start, end, listForeach);}private static void listThree(){String line = "";long start = System.nanoTime();Iterator<String> iterator = lists.iterator();while (iterator.hasNext()){line = iterator.next();}long end = System.nanoTime();print(start, end, listIterator);}private static void linkedListOne(){String line = "";long start = System.nanoTime();for(int i=0;i<linkedLists.size();i++){line = linkedLists.get(i);}long end = System.nanoTime();print(start, end, linkedListFor);}private static void linkedListTwo(){String line = "";long start = System.nanoTime();for(String str : linkedLists){line = str;}long end = System.nanoTime();print(start, end, linkedListForeach);}private static void linkedListThree(){String line = "";long start = System.nanoTime();Iterator<String> iterator = linkedLists.iterator();while (iterator.hasNext()){line = iterator.next();}long end = System.nanoTime();print(start, end, linkedListIterator);}private static void hashSetOne(){String line = "";long start = System.nanoTime();for(String str : hashSets){line = str;}long end = System.nanoTime();print(start, end, hashSetForeach);}private static void hashSetTwo(){String line = "";long start = System.nanoTime();Iterator<String> iterator = hashSets.iterator();while (iterator.hasNext()){line = iterator.next();}long end = System.nanoTime();print(start, end, hashSetIterator);}private static void hashMapOne(){String line = "";long start = System.nanoTime();for(Map.Entry<String, String> entry : hashMaps.entrySet()){line = entry.getKey();}long end = System.nanoTime();print(start, end, hashMapEntry);}private static void hashMapTwo(){String line = "";long start = System.nanoTime();Iterator iterator = hashMaps.entrySet().iterator();while (iterator.hasNext()){Map.Entry<String, String> entry = (Map.Entry<String, String>)iterator.next();line = entry.getKey();}long end = System.nanoTime();print(start, end, hashMapIterator);}private static void linkedHashMapOne(){String line = "";long start = System.nanoTime();for(Map.Entry<String, String> entry : linkedHashMaps.entrySet()){line = entry.getKey();}long end = System.nanoTime();print(start, end, linkedHashMapEntry);}private static void linkedHashMapTwo(){String line = "";long start = System.nanoTime();Iterator iterator = linkedHashMaps.entrySet().iterator();while (iterator.hasNext()){Map.Entry<String, String> entry = (Map.Entry<String, String>)iterator.next();line = entry.getKey();}long end = System.nanoTime();print(start, end, linkedHashMapIterator);}private static void print(long start, long end, String tip){System.out.println(tip + " = [" + ((double)((end - start)/1000))/1000 + "]ms");}}
我们经过多轮测试，取相对合理的结果进行展示，单位为：毫秒（ms）
单个类型不同遍历方式性能比较总结：
ArrayList：三种遍历方式性能差距不大，数量级较小时，for循环遍历更优，数量级较大时，Iterator迭代器遍历方式性能更优；
LinkedList：三种遍历方式中for循环遍历性能最差，其他两种方式性能差距比较小，但是Iterator迭代器遍历方式性能更优；
HashSet：两种遍历方式性能差距不大，但是Iterator迭代器遍历方式性能更优；
HashMap：两种遍历方式性能差距不大，但是Iterator迭代器遍历方式性能更优；
LinkedHashMap：两种遍历方式性能差距不大，但是Iterator迭代器遍历方式性能更优；
整体性能比较总结：
同等数量级，ArrayList的遍历性能更优；
