说明：我所涉及到的试验代码，均是针对于1百万条记录，我的硬件配置如下：CPUP4 1.5G,256M DDR的内存。

如果要试验其代码，要采用这种方式运行：java -Xms128M -Xmx250M TestMap

1，对内存容量的要求

Map对象高于List。因为Map除了Value外还需要一个Object的Key，从而增大了Map的容量。试验代码如下：

import java.util.*;

public class TestMap {

public static void main(String[] args) {

System.out.println("begin>>>>>>>>>>>>>>");

long sm = Runtime.getRuntime().totalMemory();

System.out.println("Begin Memory :" + sm);

Map m = new Hashtable();

//LinkedList tmpList = new LinkedList();

for (int i=0;i<1000000;i++) {

m.put(""+i,"i=" + i);

//Integer integer = new Integer(i);

//tmpList.add(integer);

}

long em = Runtime.getRuntime().totalMemory();

System.out.println("End Memory :" + em);

System.out.println("End-Start Memory :" + (em - sm));

//walkList(tmpList);

System.out.println("<<<<<<<<<<<<<<<

//find(m,"985757");

walkMap(m);

//List tmpList = new LinkedList();

//tmpList.addAll(m.values());

}

}

其对比结果如下：1百万条记录Map大概需要70M额外空间，而List不需要，其额外空间为0

2.遍历比较

我在试验中，发现我们经常采用遍历List的方式是一个错误的遍历方式，其性能其差无比。我们经常遍历List代码实例如下：

for(int i = 0;i

Object obj = list.get(i);

}

如果List的遍历方式换一种方式，其性能和Map差不多，我们将上面的代码进行扩充，从而来比较Map和List的遍历速度：

import java.util.*;

/**

* @author whxu

* @version 1.0.0 2003-4-22 AM

*/

public class TestMap {

public static void main(String[] args) {

System.out.println("begin>>>>>>>>>>>>>>");

long sm = Runtime.getRuntime().totalMemory();

System.out.println("Begin Memory :" + sm);

Map m = new Hashtable();

//LinkedList tmpList = new LinkedList();

for (int i=0;i<1000000;i++) {

m.put(""+i,"i=" + i);

//Integer integer = new Integer(i);

//tmpList.add(integer);

}

long em = Runtime.getRuntime().totalMemory();

System.out.println("End Memory :" + em);

System.out.println("End-Start Memory :" + (em - sm));

//walkList(tmpList);

System.out.println("<<<<<<<<<<<<<<<

//find(m,"985757");

walkMap(m);

//List tmpList = new LinkedList();

//tmpList.addAll(m.values());

}

public static void find(Map m,String key) {

long st = System.currentTimeMillis();

Object obj = m.get(key);

System.out.println("Object:" + obj);

long et = System.currentTimeMillis();

System.out.println("Start Time:" + st);

System.out.println("End Time:" + et);

System.out.println("Use Time:" + (et - st));

}

public static void walkMap(Map m) {

long st = System.currentTimeMillis();

for(Iterator it = m.values().iterator();it.hasNext();) {

Object obj = it.next();

}

long et = System.currentTimeMillis();

System.out.println("WalkMap Start Time:" + st);

System.out.println("WalkMap End Time:" + et);

System.out.println("WalkMap Use Time:" + (et - st));

}

public static void walkList(LinkedList l) {

long st = System.currentTimeMillis();

Object obj = null;

//System.out.println(l.get(3434));

/*这种遍历方式的性能其差无比，建议不要使用

for(int i=0;i

obj = l.get(i);

}

*/

for(Iterator it = l.iterator();it.hasNext();) {

obj = it.next();

}

long et = System.currentTimeMillis();

System.out.println("WalkList Start Time:" + st);

System.out.println("WalkList End Time:" + et);

System.out.println("WalkList Use Time:" + (et - st));

}

}

3.随机查找。如果Map知道了Key，List知道了index，其性能在一百万条数据中无多大差别。其实例代码如下：

import java.util.*;

/**

* @author whxu

* @version 1.0.0 2003-4-22 AM

*/

public class TestMap

{

public static void main(String[] args)

{

System.out.println("begin>>>>>>>>>>>>>>");

long sm = Runtime.getRuntime().totalMemory();

System.out.println("Begin Memory :" + sm);

Map m = new Hashtable();

LinkedList tmpList = new LinkedList();

for (int i=0;i<1000000;i++) {

//m.put(""+i,"i=" + i);

tmpList.add("i=" + i);

}

long em = Runtime.getRuntime().totalMemory();

System.out.println("End Memory :" + em);

System.out.println("End-Start Memory :" + (em - sm));

//walkList(tmpList);

System.out.println("<<<<<<<<<<<<<<<

//find(m,"985757");

find(tmpList,1000000-1);

//walkMap(m);

//List tmpList = new LinkedList();

//tmpList.addAll(m.values());

}

public static void find(Map m,String key) {

long st = System.currentTimeMillis();

Object obj = m.get(key);

System.out.println("Find Map Object:" + obj);

long et = System.currentTimeMillis();

System.out.println("Find Map Start Time:" + st);

System.out.println("Find Map End Time:" + et);

System.out.println("Find Map Use Time:" + (et - st));

}

public static void find(List l,int index) {

long st = System.currentTimeMillis();

Object obj = l.get(index);

System.out.println("Find List Object:" + obj);

long et = System.currentTimeMillis();

System.out.println("Find List Start Time:" + st);

System.out.println("Find List End Time:" + et);

System.out.println("Find List Use Time:" + (et - st));

}

public static void walkMap(Map m) {

long st = System.currentTimeMillis();

for(Iterator it = m.values().iterator();it.hasNext();) {

Object obj = it.next();

}

long et = System.currentTimeMillis();

System.out.println("WalkMap Start Time:" + st);

System.out.println("WalkMap End Time:" + et);

System.out.println("WalkMap Use Time:" + (et - st));

}

public static void walkList(LinkedList l) {

long st = System.currentTimeMillis();

Object obj = null;

//System.out.println(l.get(3434));

/*for(int i=0;i

obj = l.get(i);

}

*/

for(Iterator it = l.iterator();it.hasNext();) {

obj = it.next();

}

long et = System.currentTimeMillis();

System.out.println("WalkList Start Time:" + st);

System.out.println("WalkList End Time:" + et);

System.out.println("WalkList Use Time:" + (et - st));

}

}

posted on 2007-06-02 11:35 分享爱的空间 阅读(1296) 评论(0)  编辑  收藏