本测试没有具体业务，单纯遍历，也许会有误差。
主单元测试方法
@Test
public void test(){
List<Integer> list = new ArrayList<>();
int n = 1000_0000;
for (int i = 0; i < n; i++) {
list.add(i);
}
System.out.println("======== 1 fori 推荐 ==========");
foriTest(list);
System.out.println("======== 2 iterator ==========");
iteratorTest(list);
System.out.println("======== 3 增强for ==========");
forBoostTest(list);
System.out.println("======== 4 forEach 最耗时 java8 ==========");
forEachTest(list);
System.out.println("======== 5 stream forEach 比较耗时和增强for差不多 java8 ==========");
streamForEachTest(list);
System.out.println("======== 6 parallelStream java8 ==========");
parallelStreamForEachTest(list);
}
测试1000万数据测试结果
======== 1 fori 推荐 ==========
for i time=7
======== 2 iterator ==========
iterator for time=8
iterator while time=8
======== 3 增强for ==========
for Boost time= 22
======== 4 forEach 最耗时 java8 ==========
list.forEach time=69
======== 5 stream forEach 比较耗时和增强for差不多 java8 ==========
list.stream().forEach time=23
======== 6 parallelStream forEach 并行 java8 ==========
list.parallelStream().forEach time=28
1、普通fori 方式遍历
/**
* 1
* 普通fori
* 效率最高，也是java官方推荐的用法
*
* @param list
*/
public static void foriTest(List<Integer> list) {
long before = System.currentTimeMillis();
int j = list.size();
for (int i = 0; i < j; i++) {
list.get(i);
}
long after = System.currentTimeMillis();
System.out.println("for i time=\t" + (after - before));
}
2、iterate()迭代器方式（collection 集合共有方法）
/**
* 2 iterator
* 迭代器
* @param list
*/
public static void iteratorTest(List<Integer> list) {
long before = System.currentTimeMillis();
for (Iterator<Integer> iterator = list.iterator(); iterator.hasNext(); ) {
iterator.next();
}
long after = System.currentTimeMillis();
System.out.println("iterator for time=\t" + (after - before));
before = System.currentTimeMillis();
/**
* while 循环写法
*/
Iterator<Integer> iterator = list.iterator();
while (iterator.hasNext()) {
iterator.next();
}
after = System.currentTimeMillis();
System.out.println("iterator while time=\t" + (after - before));
}
3、增强for
/**
* 3
* 增强for
* @param list
*/
public static void forBoostTest(List<Integer> list) {
long before = System.currentTimeMillis();
for (int item : list) {
// System.out.println(item);
}
long after = System.currentTimeMillis();
System.out.println("for Boost time=\t" + (after - before));
}
4、forEach() 注意java8 才支持
/**
* 4 foreach
* foreach 最慢不推荐 java8 lambda
* @param list
*/
public static void forEachTest(List<Integer> list) {
long before = System.currentTimeMillis();
list.forEach(item -> {
// System.out.println(item);
});
long after = System.currentTimeMillis();
System.out.println("list.forEach time=\t" + (after - before));
}
5、stream().forEach() 注意java8 才支持
/**
* 5 stream foreach
* stream foreach java8 特有
* @param list
*/
public static void streamForEachTest(List<Integer> list) {
long before = System.currentTimeMillis();
list.stream().forEach(item -> {
// System.out.println(item);
});
long after = System.currentTimeMillis();
System.out.println("list.stream().forEach time=\t" + (after - before));
}
6、parallelStream().forEach() 注意java8 才支持
/**
* 5 stream foreach
* stream foreach java8 特有
* @param list
*/
public static void parallelStreamForEachTest(List<Integer> list) {
long before = System.currentTimeMillis();
list.parallelStream().forEach(item -> {
// System.out.println(item);
});
long after = System.currentTimeMillis();
System.out.println("list.parallelStream().forEach time=\t" + (after - before));
}