遍历List集合

先看效率

 

package com.xx;

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

/**
 * 遍历List集合
 * @author xxx
 *
 */
class ListTravel{
	
	// 定义开始时间、结束时间，便于比较
		private static long start_time = 0;
		private static long end_time = 0;
		
		// Iterator 迭代器
		public static void IteratorTest(List<String> list) {
			start_time = System.nanoTime();
			Iterator<String> it = list.iterator();
			while(it.hasNext()) {
				it.next();
			}
			end_time = System.nanoTime();		
			System.out.println("IteratorTest: "+ (end_time - start_time));
		}
	
		// for-each
		public static void foreachTest(List<String> list) {
			start_time = System.nanoTime();
			for(String tmp : list) {} // tmp 局部变量，集合中元素的一个副本，并不是元素本身
			end_time = System.nanoTime();		
			System.out.println("foreachTest: "+ (end_time - start_time));
		}
		
		// normal for 循环
		public static void normalTest(List<String> list) {
			start_time = System.nanoTime();
			for(int i = 0; i < list.size(); i++) {}
			end_time = System.nanoTime();		
			System.out.println("normalTest: "+ (end_time - start_time));
		}
		
		// Iterator 在for循环中使用
		public static void IteratorForTest(List<String> list) {
			start_time = System.nanoTime();
			for(Iterator<String> it = list.iterator(); it.hasNext();) {
				it.next();
			}
			end_time = System.nanoTime();
			System.out.println("IteratorTest: "+ (end_time - start_time));
		}
				
}

public class ListTravelTest {

	public static void main(String[] args) {
	
//		int i = 1000000;// 100万
//		List<String> list = new ArrayList<String>(1000000);
//		while(i-- > 0) {
//			list.add("test" + i);
//		}
		
		List<String> list = new ArrayList<String>(1000000);
		
		for(int i = 1000000; i > 0; i--) {
			list.add("test" + i);
		}
		
		ListTravel.IteratorTest(list);
		ListTravel.foreachTest(list);
		ListTravel.normalTest(list);
		ListTravel.IteratorForTest(list);
		
	}
	
}

 

其实，还有Lambda表达式方式

 

下附具体代码：

 

package com.xx;

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

/**
 * 遍历List集合
 * @author xxx
 *
 */
public class ListTravelTest2 {

	public static void main(String[] args) {
	
		List<String> list = new ArrayList<String>();
		list.add("张三");
		list.add("李四");
		list.add("王五");
		list.add("马六");
		
		// normal for循环
		System.out.println("normal for循环 遍历List:");
		for(int i = 0; i < list.size(); i++) {
			System.out.println(list.get(i));
		}
		System.out.println("-----------------------------");
		
		// for-each循环
		System.out.println("for-each循环 遍历List:");
		for(String tmp : list) {
			System.out.println(tmp);
		}
		System.out.println("-----------------------------");
		
		// lambda表达式1
		System.out.println("lambda表达式1 遍历List:");
		list.forEach(str -> System.out.println(str));
		System.out.println("-----------------------------");
		
		// lambda表达式2
		System.out.println("lambda表达式2 遍历List:");
		list.forEach(System.out :: println);
		System.out.println("-----------------------------");
		
		// Iterator迭代器
		System.out.println("Iterator迭代器 遍历List");
		Iterator<String> it = list.iterator();
		while (it.hasNext()) {
			String s = it.next();
			System.out.println(s);
		}
		System.out.println("-----------------------------");
		
		// for循环下Iterator迭代器
		System.out.println("for循环下Iterator迭代器 遍历List");
		for(Iterator<String> ite = list.iterator(); ite.hasNext();) {
			String st = ite.next();
			System.out.println(st);
		}
		System.out.println("-----------------------------");
		
	}
	
}