遍历List

package cn.anzhongwei.lean.demo.list.arraylist;

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

public class Demo {
    public static void main(String[] args) {
        Object obj = new Object();
        List<Object> list = new ArrayList<>();

        int objCount = 1000;
        for (int i = 0; i < objCount; i++) {
            list.add(obj);
        }

        int testTimes = 5;
        for (int i = 0; i < testTimes; i++) {
            testFor(list);
            testForEnhanced(list);
            testForEach(list);
            testIterator(list);
            System.out.println("\n");
        }
        System.out.println("---------------------------------------------------------\n");

        List<Object> list2 = new ArrayList<>();
        for (int i = 0; i < objCount; i++) {
            list2.add(obj);
        }

        for (int i = 0; i < testTimes; i++) {
            testFor(list2);
            testForEnhanced(list2);
            testForEach(list2);
            testIterator(list2);
            System.out.println();
        }
    }

    private static void testFor(List<Object> list) {
        long startTime = 0L;
        long endTime = 0L;
        startTime = System.nanoTime();

        for (int i = 0; i < list.size(); i++) {
            Object o = list.get(i);
        }
        endTime = System.nanoTime();
        System.out.println("for所用时间(ns)      ：" + (endTime - startTime));
    }

    private static void testForEnhanced(List<Object> list) {
        long startTime = 0L;
        long endTime = 0L;
        startTime = System.nanoTime();

        for (Object o : list) {
            Object value = o;
        }

        endTime = System.nanoTime();
        System.out.println("增强for所用时间(ns)  ：" + (endTime - startTime));
    }

    private static void testForEach(List<Object> list) {
        long startTime = 0L;
        long endTime = 0L;
        startTime = System.nanoTime();

        list.forEach(o->{Object obj = o;});

        endTime = System.nanoTime();
        System.out.println("forEach所用时间(ns)  ：" + (endTime - startTime));
    }

    private static void testIterator(List<Object> list) {
        long startTime = 0L;
        long endTime = 0L;
        startTime = System.nanoTime();

        Iterator<Object> iterator = list.iterator();
        while (iterator.hasNext()) {
            Object o = iterator.next();
        }
        endTime = System.nanoTime();
        System.out.println("iterator所用时间(ns) ：" + (endTime - startTime));
    }
}