迭代器和for循环的效率比较

 

实体Person类

 

package com.zhang.loop;

public class Person {  private String a;  private int b;  public Person(String a, int b) {   super();   this.a = a;   this.b = b;  }  public String getA() {   return a;  }  public void setA(String a) {   this.a = a;  }  public int getB() {   return b;  }  public void setB(int b) {   this.b = b;  }     }

 

 

 

测试ArrayList的主程序

package com.zhang.loop;

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

public class TestArrayList {
	private static final int COUNT = 800000;
	private static List<Person> persons = new ArrayList<Person>();
	
	public static void main(String[] args) {
		init();
		System.out.println("for循环测试结果："+testFor());
		System.out.println("foreach循环测试结果："+testForeach());
		System.out.println("iterator循环测试结果："+testIterator());
	}
	
	//初始化集合
	public static void init(){
		for (int i = 0; i <COUNT; i++) {
			persons.add(new Person("第"+i+"个元素",i));
		}
	}
	//for循环便利集合
	public static long testFor(){
		long start = System.nanoTime();
		Person p = null;
		for (int i = 0; i < persons.size(); i++) {
			p = persons.get(i);
		}
		return (System.nanoTime()-start)/1000;
	}
	//foreach循环便利集合
	public static long testForeach(){
		long start = System.nanoTime();
		Person p = null;
		for (Person person : persons) {
			p = person;
		}
		return (System.nanoTime()-start)/1000;
	}
	//iterator循环便利集合
	public static long testIterator(){
		long start = System.nanoTime();
		Person p = null;
		Iterator<Person> itreator = persons.iterator();
		while(itreator.hasNext()){
			p = itreator.next();
		}
		return (System.nanoTime()-start)/1000;
	}
	
}

 

 ArrayList测试结果

 

 

测试LinkedList的主程序

package com.zhang.loop;

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

public class TestArrayList {
	private static final int COUNT = 8000;
	private static List<Person> persons = new LinkedList<Person>();
	
	public static void main(String[] args) {
		init();
		System.out.println("for循环测试结果："+testFor());
		System.out.println("foreach循环测试结果："+testForeach());
		System.out.println("iterator循环测试结果："+testIterator());
	}
	
	//初始化集合
	public static void init(){
		for (int i = 0; i <COUNT; i++) {
			persons.add(new Person("第"+i+"个元素",i));
		}
	}
	//for循环便利集合
	public static long testFor(){
		long start = System.nanoTime();
		Person p = null;
		for (int i = 0; i < persons.size(); i++) {
			p = persons.get(i);
		}
		return (System.nanoTime()-start)/1000;
	}
	//foreach循环便利集合
	public static long testForeach(){
		long start = System.nanoTime();
		Person p = null;
		for (Person person : persons) {
			p = person;
		}
		return (System.nanoTime()-start)/1000;
	}
	//iterator循环便利集合
	public static long testIterator(){
		long start = System.nanoTime();
		Person p = null;
		Iterator<Person> itreator = persons.iterator();
		while(itreator.hasNext()){
			p = itreator.next();
		}
		return (System.nanoTime()-start)/1000;
	}
	
}

 

LinkedList测试结果：

 

记录的存取方式有两种：

             一种是顺序存储（数组，ArrayList..）可以根据其下标找到对应的记录

            另一种是链接存储(LinkedList..)链接存储(拿单链表为例)则必须找到其前一个记录的位置才能够找到本记录。

 

根据以上可以得到的结果是：for循环便于访问顺序存储的记录，而foreach和迭代器便于访问链接存储。