Java for循环写法效率

import java.util.ArrayList;
import java.util.List;

public class Main {

    private static void test(List<Integer> list) {
        long start = 0L;
        long end = 0L;
        // ==============================
        start = System.currentTimeMillis();
        for (Integer i : list) {

        }
        end = System.currentTimeMillis();
        System.out.println(end - start);
        // ==============================
        start = System.currentTimeMillis();
        for (int i=0;i<list.size();i++) {
            Integer j=list.get(i);
        }
        end = System.currentTimeMillis();
        System.out.println(end - start);

        // ==============================
        start = System.currentTimeMillis();
        for (int i=0,k=list.size();i<k;i++) {
            Integer j=list.get(i);
        }
        end = System.currentTimeMillis();
        System.out.println(end - start);
    }

    public static void main(String[] args) {
        List<Integer> list=new ArrayList<>();
        for(int i=0;i<20000000;i++){
            list.add(i);
        }
        test(list);
    }
}

迭代器：85ms(用了Iterator的遍历，Iterator是需要考虑同步的)
下标访问：60ms(ArrayList的效率要比LinkedList的效率要高，因为Array擅长随机访问)
优化的下标访问：59ms

来源：CSDN

作者：wanhf11

链接：https://blog.csdn.net/qq_17612199/article/details/51611049