测试hashmap 内存消耗情况两种遍历方式耗时情况

川长思鸟来 2022-08-22 11:24 175阅读 0赞

package test;  
import java.util.HashMap;  
import java.util.Iterator;  
import java.util.Map;  
import java.util.Set;  
  
/\*\*  
 \* 测试hashmap 内存消耗情况  
 \* hashmap 两种遍历方式耗时情况  
 \* key value 允许为null值 相同key只会保留最后一个值  
 \* @author Administrator  
 \*  
 \*/  
public class TestHashMap  
\{  
    public static void main(String\[\] args)  
    \{  
        try  
        \{  
            /\*\*HashMap添加100万条记录消耗内存\*/  
              Integer a = 1;  
              long start = 0;  
              long end = 0;  
              // 先垃圾回收  
              System.gc();  
              start = Runtime.getRuntime().freeMemory();  
              System.out.println("freeMemory1:"+Runtime.getRuntime().freeMemory());  
              HashMap map1 = new HashMap();  
              for (int i = 0; i < 100000; i++) \{  
                  map1.put(i, a);  
              \}  
              // 快要计算的时,再清理一次  
              System.gc();  
              System.out.println("freeMemory2:"+Runtime.getRuntime().freeMemory());  
                
              end = Runtime.getRuntime().freeMemory();  
              System.out.println("一个HashMap对象占内存:" + (end - start)/1024/1024+"M");  
                
              start = Runtime.getRuntime().freeMemory();  
              System.out.println("freeMemory3:"+Runtime.getRuntime().freeMemory());  
            Map<String,String> map = new HashMap<String,String>();  
            for(long i=0;i<100000;i++)  
            \{  
                map.put("key"+i, "value"+i);      
            \}  
            //遍历方式一开始时间  
            long startdate = System.currentTimeMillis();  
            System.out.println("开始时间1:"+startdate);  
            Set set1 = map.keySet();  
            for(Iterator iter = set1.iterator();iter.hasNext();)\{  
                String key = iter.next().toString();  
//                System.out.println("key::"+key);  
                String value = map.get(key);  
//                System.out.println("value:"+value);  
            \}  
          //遍历方式一截止时间  
            long enddate = System.currentTimeMillis();  
            System.out.println("截止时间1:"+enddate);  
            long hs1 =enddate - startdate;  
            System.out.println("总耗时1:"+hs1);  
              
            //遍历方式二开始时间  
            System.out.println("开始时间2:"+System.currentTimeMillis());  
            startdate = System.currentTimeMillis();  
            Set set2 = map.entrySet();  
            for(Iterator iter = set2.iterator();iter.hasNext();)\{  
                Map.Entry entry = (Map.Entry)iter.next();  
                String key = entry.getKey().toString();  
//                System.out.println("key::"+key);  
                String value = entry.getValue().toString();  
//                System.out.println("value:"+value);  
            \}  
          //遍历方式二截止时间  
            enddate =System.currentTimeMillis();  
            System.out.println("截止时间2:"+enddate);  
            long hs2 = enddate -startdate;  
            System.out.println("总耗时2:"+hs2);  
              
            System.out.println("时间差:"+(hs2-hs1));  
              
            // 快要计算的时,再清理一次  
            System.gc();  
            end = Runtime.getRuntime().freeMemory();  
            System.out.println("freeMemory4:"+Runtime.getRuntime().freeMemory());  
            System.out.println("一个HashMap对象占内存2:" + (end - start)/1024/1024+"M");  
             
        \}  
        catch (Exception e)  
        \{  
            // TODO Auto-generated catch block  
            e.printStackTrace();  
        \}  
    \}  
\}