遍历 List 之效率

1. 常用遍历 List 之方法

 

遍历 List 方法有三：

a) for (Iterator iterator = ls.iterator(); iterator.hasNext();)

b) for (int i = 0; i < ls.size(); i++)

c) for (Object object : ls) (java5+)

常用 List 实现有二：

a) java.util.ArrayList

b) java.util.LinkedList

 

效率如何？

 

2. 遍历方法之小误差验证

 

输出：

class java.util.ArrayList
iterator-nanotime :7199195
arraylike-nanotime:1920143
foreach-nanotime :3775320
class java.util.LinkedList
iterator-nanotime :5430188
arraylike-nanotime:9148595046
foreach-nanotime :1677620

 

方法：

 

private static void nanoLoop(List ls){
	System.out.println(ls.getClass());
	{ // iterator
		long b = System.nanoTime();
		for (Iterator iterator = ls.iterator(); iterator.hasNext();) {
			Object object = (Object) iterator.next();
		}
		System.out.println("iterator-nanotime :"+(System.nanoTime()-b));
	}
	
	{ // array-like
		long b = System.nanoTime();
		for (int i = 0; i < ls.size(); i++) {
			Object object = (Object) ls.get(i);
		}
		System.out.println("arraylike-nanotime:"+(System.nanoTime()-b));
	}
	
	{ // foreach
		long b = System.nanoTime();
		for (Object object : ls) {
			Object s = (Object) object;
		}
		System.out.println("foreach-nanotime  :"+(System.nanoTime()-b));
	}
}

private static List initList(List ls,int count){
	for (int i = 0; i < count; i++) {
		ls.add("aaaa");
	}
	return ls;
}

public static void main(String[] args) {
	nanoLoop(initList(new ArrayList(),10*10000));
	nanoLoop(initList(new LinkedList(),10*10000));
}

 

 

3. 鲁莽结论

 

 * 黑盒情况下：foreach > iterator > arraylike

 * ArrayList ：arraylike>foreach >iterator

 * LinkedList：foreach >iterator >>>arraylike

 

4. 刨根问底

 

read the fk source code -_-