java8新特性|java 8 lambda遍历和增强for循环的效率哪个快（） java8|java|java8遍历

前面讲list转树形结构的时候，由于业务原因无法使用最快的方法，导致很慢，今天就研究在循环上提升速度。
Java8 lambda遍历出来后有人说它快有人说它要慎用？到底怎么回事让我们来一探究竟吧！看不懂可以先看后面的分析
测试代码：

package cn.cncommdata.zhonglvreport; import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.Objects; public class ForiForeachJava8ForeachTest {public static void main(String[] args) { // 预热 List tmpList = listDog(10); long time1 = System.nanoTime(); testFori(tmpList); long time2 = System.nanoTime(); testForeach(tmpList); long time3 = System.nanoTime(); testJava8ForEach(tmpList); long time4 = System.nanoTime(); System.out.println("for2\t\t\t\t" + (int) (time2 - time1) / 1000.0 + " ms"); System.out.println("增强for\t\t\t\t" + (int) (time3 - time2) / 1000.0 + " ms"); System.out.println("java8 foreach\t\t" + (int) (time4 - time3) / 1000.0 + " ms"); List list = Arrays.asList(10, 50, 250, 1000, 2000, 3000, 5000, 10000, 20000); for (int i = 0; i < list.size(); i++) { test(list.get(i)); } }public static void test(int size) { System.out.println("-----------次数:" + size + "------------"); List list = listDog(size); long nanoTime = System.nanoTime(); testFori(list); long nanoTime1 = System.nanoTime(); testForeach(list); long nanoTime2 = System.nanoTime(); testJava8ForEach(list); long nanoTime3 = System.nanoTime(); testJava8ForEachStream(list); long nanoTime4 = System.nanoTime(); System.out.println("fori\t\t\t\t" + (int) (nanoTime1 - nanoTime) / 1000.0 + " ms"); System.out.println("增强for\t\t\t\t" + (int) (nanoTime2 - nanoTime1) / 1000.0 + " ms"); System.out.println("java8 foreach\t\t" + (int) (nanoTime3 - nanoTime2) / 1000.0 + " ms"); System.out.println("java8 Stream foreach\t\t" + (int) (nanoTime4 - nanoTime3) / 1000.0 + " ms"); System.out.println(); }/** * 初始化list * * @param size int * @return list */ public static List listDog(int size) { List list = new ArrayList<>(); for (int i = 0; i < size; i++) { list.add(new Dog(i + 1, "dog " + (i + 1))); } return list; }/** * 测试fori * * @param list List */ public static void testFori(List list) { for (int i = 0; i < list.size(); i++) { list.get(i).hashCode(); } }/** * 测试增强for循环 * * @param list List */ public static void testForeach(List list) { for (Dog dog : list) { dog.hashCode(); } }/** * 测试java8的foreach * * @param list List */ public static void testJava8ForEach(List list) { list.forEach(dog -> dog.hashCode()); }/** * 测试java8的foreach * * @param list List */ public static void testJava8ForEachStream(List list) { list.stream().forEach(dog -> dog.hashCode()); } }/** * 测试实体类，用来计算hashCode */ class Dog { private int age; private String name; public Dog(int age, String name) { this.age = age; this.name = name; }public int getAge() { return age; }public void setAge(int age) { this.age = age; }public String getName() { return name; }public void setName(String name) { this.name = name; }@Override public int hashCode() { int result = 17; result = 31 * result + age; result = 31 * result + (name == null ? 0 : name.hashCode()); return result; }@Override public boolean equals(Object obj) { if (obj == this) { return true; } if (!(obj instanceof Dog)) { return false; } Dog dog = (Dog) obj; return dog.age == this.age && Objects.equals(dog.name, this.name); } }

运行结果：

-------不预热的情况下------- for213.5 ms 增强for162.5 ms java8 foreach41539.1 ms java8 Stream foreach 2365.4 ms -----------预热情况次数:10------------ fori3.6 ms 增强for4.7 ms java8 foreach3.8 ms java8 Stream foreach 6.9 ms-----------预热情况次数:50------------ fori9.7 ms 增强for13.1 ms java8 foreach8.6 ms java8 Stream foreach 11.1 ms-----------预热情况次数:250------------ fori50.6 ms 增强for63.4 ms java8 foreach32.1 ms java8 Stream foreach 34.5 ms-----------预热情况次数:1000------------ fori205.9 ms 增强for255.9 ms java8 foreach101.9 ms java8 Stream foreach 74.7 ms-----------预热情况次数:2000------------ fori142.1 ms 增强for322.8 ms java8 foreach164.5 ms java8 Stream foreach 153.5 ms-----------预热情况次数:3000------------ fori193.9 ms 增强for167.5 ms java8 foreach136.8 ms java8 Stream foreach 118.0 ms-----------预热情况次数:5000------------ fori154.1 ms 增强for125.2 ms java8 foreach98.7 ms java8 Stream foreach 73.8 ms-----------预热情况次数:10000------------ fori263.6 ms 增强for196.3 ms java8 foreach131.5 ms java8 Stream foreach 163.1 ms-----------预热情况次数:20000------------ fori417.3 ms 增强for411.6 ms java8 foreach593.4 ms java8 Stream foreach 476.9 ms

【java8新特性|java 8 lambda遍历和增强for循环的效率哪个快（）】分析：我这里测试用了4种for循环
从第一个结果来看在jvm不预热的情况下lambda遍历方式反而花费的时间更长（jvm预热可以单独去查看是什么意思）
从后面的结果来看在jvm预热的情况下，并且数据量大的情况下lambda遍历方式花费的时间少一些，我这里没用使用Stream接口里面的方法，所以foreach比Stream().forEach快一些