NSDictionary（obj-c）和HashMap（java）之间性能的差异

我用这两种数据结构和这两种不同的语言做了一些性能测试。结果不是我所期望的。我认为obj-c程序会比java更快。我的测试说，Java的TreeMap比可可NSDictionary更快。用于测试的代码是：
OBJ-C的NSDictionary：NSDictionary（obj-c）和HashMap（java）之间性能的差异

#import <Foundation/Foundation.h> 
NSString * getRandomString(); 
int main (int argc, const char * argv[]) { 
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; 
    NSMutableDictionary * dict = [NSMutableDictionary dictionary]; 
    unsigned long i; 
    NSString * string1; 
    NSString * string2; 
    NSString * string3; 
    NSString * lastString; 
    //dictionary with 100'000 elements 
    srand(time(NULL)); 
    for (i=0;i<100000;i++){ 
     NSString * aString = getRandomString(); 
     [dict setObject:aString forKey:aString]; 
     if (i == 100) 
      string1 = aString; 
     if (i == 1000) 
      string2 = aString; 
     if (i == 10000) 
      string3 = aString; 
     if (i == 100000-1) 
      lastString = aString; 
    } 
    NSDate * now; 

    now = [NSDate date]; 
    [dict objectForKey:string1]; 
    NSTimeInterval interval = [now timeIntervalSinceNow]; 
    NSLog(@"%f",interval *-1000); 

    now = [NSDate date]; 
    [dict objectForKey:string2]; 
    interval = [now timeIntervalSinceNow]; 
    NSLog(@"%f",interval *-1000); 


    now = [NSDate date]; 
    [dict objectForKey:string3]; 
    interval = [now timeIntervalSinceNow]; 
    NSLog(@"%f",interval *-1000); 

    now = [NSDate date]; 
    [dict objectForKey:lastString]; 
    interval = [now timeIntervalSinceNow]; 
    NSLog(@"%f",interval *-1000); 


    [pool drain]; 
    return 0; 
} 
NSString * getRandomString(){ 
    NSString * tmp = [NSString string]; 
for (int i = 0 ; i < 10;i++){ 
    tmp = [NSString stringWithFormat:@"%@%c",tmp,rand()%128]; 
} 
return tmp; 
}

命令行输出是这样的：

2011-07-12 13:11:48.299 TestBench[1178:a0f] 0.008047 
2011-07-12 13:11:48.302 TestBench[1178:a0f] 0.005007 
2011-07-12 13:11:48.302 TestBench[1178:a0f] 0.003040 
2011-07-12 13:11:48.303 TestBench[1178:a0f] 0.003994

爪哇TreeSet中：

import java.util.Date; 
import java.util.TreeMap; 


public class Main { 
    public static void main(String[] args) { 
     String string1="",string2="",string3="",lastString=""; 
     TreeMap<String,String> map = new TreeMap<String,String>(); 
     for (long i=0;i<100000;i++){ 
      String aString = getRandomString(); 
      map.put(aString, aString); 
      if (i == 100) 
       string1 = aString; 
      if (i == 1000) 
       string2 = aString; 
      if (i == 10000) 
       string3 = aString; 
      if (i == 100000-1) 
       lastString = aString; 
     } 
     Date start,end; 

     start = new Date(); 
     map.get(string1); 
     end = new Date(); 
     System.out.println(end.getTime()-start.getTime()); 

     start = new Date(); 
     map.get(string2); 
     end = new Date(); 
     System.out.println(end.getTime()-start.getTime()); 

     start = new Date(); 
     map.get(string3); 
     end = new Date(); 
     System.out.println(end.getTime()-start.getTime()); 

     start = new Date(); 
     map.get(lastString); 
     end = new Date(); 
     System.out.println(end.getTime()-start.getTime()); 
    } 
    public static String getRandomString(){ 
     String toRet = ""; 
     for (int i=0;i<10;i++){ 
      toRet+=(char)(Math.random()*128); 
     } 
     return toRet; 
    } 

}

和命令行输出是这样的：

明显以毫秒为单位，与obj-c一样。为什么TreeMap速度如此之快？或...为什么NSDictionary如此缓慢？任何人都可以解释给我？对不起，我的英语很差非常感谢。

**ADDING QUESTION* ** * **** 我作出修改，以这样的代码：
OBJ-C

#import <Foundation/Foundation.h> 
NSString * getRandomString(); 
int main (int argc, const char * argv[]) { 
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; 
    NSMutableDictionary * dict = [NSMutableDictionary dictionary]; 
    long int i; 
    NSString * string1; 
    NSString * string2; 
    NSString * string3; 
    NSString * lastString; 
    //dictionary with 100'000 elements 
    srand(time(NULL)); 
    double sum = 0; 
    for (int j = 0 ;j<10;j++){ 
     NSDate * now; 
     now = [NSDate date]; 
     for (i=0;i<100000;i++){ 
      NSString * aString = getRandomString(); 
      [dict setObject:aString forKey:aString]; 
      if (i == 100) 
       string1 = aString; 
      if (i == 1000) 
       string2 = aString; 
      if (i == 10000) 
       string3 = aString; 
      if (i == 100000-1) 
       lastString = aString; 
     } 
     NSLog(@"Finished adding elements in %f ms",[now timeIntervalSinceNow]*-1000); 
     now = [NSDate date]; 
     for (int i = 0;i<1000000;i++) 
      [dict objectForKey:string1]; 
     for (int i = 0;i<1000000;i++) 
      [dict objectForKey:string2]; 
     for (int i = 0;i<1000000;i++) 
      [dict objectForKey:string3]; 
     for (int i = 0;i<1000000;i++) 
      [dict objectForKey:lastString]; 
     NSTimeInterval interval = [now timeIntervalSinceNow]; 
     sum+=interval; 
    } 
    NSLog(@"medium lookup time: %f ms",sum/10/4*-1000); 
    [pool drain]; 
    return 0; 
} 
NSString * getRandomString(){ 
    NSString * tmp = [NSString string]; 
    for (int i = 0 ; i < 10;i++){ 
     tmp = [NSString stringWithFormat:@"%@%c",tmp,rand()%128]; 
    } 
    return tmp; 
}

的输出：

2011-07-12 14:48:36.519 TestBench[974:a0f] Finished adding elements in 1950.287998 ms 
2011-07-12 14:48:38.722 TestBench[974:a0f] Finished adding elements in 1899.537027 ms 
2011-07-12 14:48:41.340 TestBench[974:a0f] Finished adding elements in 1939.461946 ms 
2011-07-12 14:48:43.681 TestBench[974:a0f] Finished adding elements in 1991.870999 ms 
2011-07-12 14:48:45.854 TestBench[974:a0f] Finished adding elements in 1857.455015 ms 
2011-07-12 14:48:48.636 TestBench[974:a0f] Finished adding elements in 2205.457032 ms 
2011-07-12 14:48:50.782 TestBench[974:a0f] Finished adding elements in 1866.232991 ms 
2011-07-12 14:48:53.106 TestBench[974:a0f] Finished adding elements in 1847.414017 ms 
2011-07-12 14:48:55.537 TestBench[974:a0f] Finished adding elements in 1982.506990 ms 
2011-07-12 14:49:00.629 TestBench[974:a0f] Finished adding elements in 4536.152005 ms 
2011-07-12 14:49:00.962 TestBench[974:a0f] medium lookup time: 107.704024 ms

的Java

import java.util.Date; 
import java.util.HashMap; 
import java.util.Map; 
import java.util.TreeMap; 


public class Main { 

    /** 
    * @param args 
    */ 

    public static void main(String[] args) { 
     String string1="",string2="",string3="",lastString=""; 
     Map<String,String> map = new HashMap<String,String>(); 
     long sum=0; 
     for (int j=0;j<10;j++){ 
      Date start,end; 
      start = new Date(); 
      for (long i=0;i<100000;i++){ 
       String aString = getRandomString(); 
       map.put(aString, aString); 
       if (i == 100) 
        string1 = aString; 
       if (i == 1000) 
        string2 = aString; 
       if (i == 10000) 
        string3 = aString; 
       if (i == 100000-1) 
        lastString = aString; 
      } 
      end = new Date(); 
      System.out.println("Finished adding elements in "+(end.getTime()-start.getTime())+" ms"); 
      start = new Date(); 
      for (int i = 0;i<1000000;i++) 
       map.get(string1); 
      for (int i = 0;i<1000000;i++) 
       map.get(string2); 
      for (int i = 0;i<1000000;i++) 
       map.get(string3); 
      for (int i = 0;i<1000000;i++) 
       map.get(lastString); 
      end = new Date(); 
      sum+=end.getTime()-start.getTime(); 
     } 
     System.out.println("medium lookup time: "+sum/10/4+" ms"); 
    } 
    public static String getRandomString(){ 
     String toRet = ""; 
     for (int i=0;i<10;i++){ 
      toRet+=(char)(Math.random()*128); 
     } 
     return toRet; 
    } 

}

结果为HashMap的：

Finished adding elements in 314 ms 
Finished adding elements in 275 ms 
Finished adding elements in 263 ms 
Finished adding elements in 285 ms 
Finished adding elements in 309 ms 
Finished adding elements in 284 ms 
Finished adding elements in 270 ms 
Finished adding elements in 395 ms 
Finished adding elements in 320 ms 
Finished adding elements in 1804 ms 
medium lookup time: 8 ms

结果的树图

Finished adding elements in 400 ms 
Finished adding elements in 430 ms 
Finished adding elements in 474 ms 
Finished adding elements in 581 ms 
Finished adding elements in 562 ms 
Finished adding elements in 599 ms 
Finished adding elements in 654 ms 
Finished adding elements in 625 ms 
Finished adding elements in 638 ms 
Finished adding elements in 1750 ms 
medium lookup time: 194 ms

因此，我认为NSDictionary中没有与散列函数但一棵树。为什么需要这么多时间在NSDictionary中添加元素？是否有可可与Java hashset相似的性能的地图？谢谢

来源

2011-07-12 tmnd91

map.get（）大概北京时间太快以毫秒为单位来测量。尝试调用map.get（）100万次或100万次，看看它如何表现。 – Matt

您的“添加”基准大部分时间花费在创建随机字符串上。如果您不在时间范围内，速度要快10倍。看到我更新的答案。 –

另请注意，与8 ms相比，我的平均查找时间为HashMap.get（）为0.000015 ms。 ;） –

你不能真正比较这些值，因为在Java中你有long s，而在可可你有double s。如果您要将NSTimeInterval（这是一个double）长时间，您会得到完全相同的结果。

此外，为了获得有用的结果，仅调用该方法的一次调用是不够的。制作一个循环并至少称它为几千次。

来源

2011-07-12 12:00:06 omz

明白了:)可可更快谢谢你！ – tmnd91

Java非常擅长优化不做任何事情的代码。在这方面，它通常比C/C++快得多。然而，在实际工作中，差异通常要小得多。

map.get()没有做任何事情，但new Date()确实，如果你想短期操作，您最好使用System.nanoTime()和计算未立即丢弃的结果，这是比System.currentTimeMillis()慢得多。

此外，值得运行测试约2秒钟以获得最佳结果。

import java.util.HashMap; 
import java.util.Map; 
import java.util.TreeMap; 

public class Main { 
    public static void main(String[] args) { 
     testMap(new TreeMap<String, String>()); 
     testMap(new HashMap<String, String>()); 
    } 

    private static void testMap(Map<String, String> map) { 
     System.out.println("Using " + map.getClass().getSimpleName()); 

      String[] strings = new String[100000]; 
      for (int i = 0; i < 100000; i++) 
       strings[i] = getRandomString(); 

      { 
       long start = System.nanoTime(); 
       for (int i = 0; i < 100000; i++) { 
        String aString = strings[i]; 
        map.put(aString, aString); 
       } 
       long time = System.nanoTime() - start; 
       System.out.printf("... added elements in %.6f ms%n", time/1e6); 
      } 
      String string1 = strings[100], string2 = strings[1000], string3 = strings[10000], string4 = strings[100000 - 1]; 
     final int runs = 3000000; 
     { 
      long start = System.nanoTime(); 
      for (int i = 0; i < runs; i++) 
       string1 = map.get(string1); 
      long time = System.nanoTime() - start; 
      System.out.printf("... average get() time was %.6f ms%n", time/1e6/runs); 
     } 
     { 
      long start = System.nanoTime(); 
      for (int i = 0; i < runs; i++) 
       string2 = map.get(string2); 
      long time = System.nanoTime() - start; 
      System.out.printf("... average get() time was %.6f ms%n", time/1e6/runs); 
     } 
     { 
      long start = System.nanoTime(); 
      for (int i = 0; i < runs; i++) 
       string3 = map.get(string3); 
      long time = System.nanoTime() - start; 
      System.out.printf("... average get() time was %.6f ms%n", time/1e6/runs); 
     } 
     { 
      long start = System.nanoTime(); 
      for (int i = 0; i < runs; i++) 
       string4 = map.get(string4); 
      long time = System.nanoTime() - start; 
      System.out.printf("... average get() time was %.6f ms%n", time/1e6/runs); 
     } 
    } 

    public static String getRandomString() { 
     String toRet = ""; 
     for (int i = 0; i < 10; i++) { 
      toRet += (char) (Math.random() * 128); 
     } 
     return toRet; 
    } 
}

打印

Using TreeMap 
... added elements in 85.856512 ms 
... average get() time was 0.000095 ms 
... average get() time was 0.000121 ms 
... average get() time was 0.000124 ms 
... average get() time was 0.000119 ms 
Using HashMap 
... added elements in 20.189437 ms 
... average get() time was 0.000016 ms 
... average get() time was 0.000015 ms 
... average get() time was 0.000012 ms 
... average get() time was 0.000012 ms

来源

2011-07-12 12:09:53

NSDictionary（obj-c）和HashMap（java）之间性能的差异

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