Java中IdentityHashMap和HashMap有什么区别+性能比较

已发表: 2021-10-19
身份哈希图和哈希图性能比较之间的差异

有时我有一个特殊情况，我必须根据equality operator (==)比较 Map 的 Key。 相等运算符 (==) 将两个 Key 的引用（内存中的地址）作为两个不同的数字进行比较。

另一方面，HashMap 是最常用的 Java 集合框架组件，它借助equals()方法比较 Key 的唯一性。

此外， IdentityHashMap不使用来自object.hashCode()的哈希，而是使用System.identityHashCode(object) 。 我们可以将 IdentityHashMap 用于其哈希码在运行时发生变化的可变对象。

如果您想了解更多关于适用on String Object的equals()和==的信息，请遵循本教程：https://crunchify.com/how-to-override-equals-and-hashcode-method-in-java/。

演示上述 equals() 和 == 行为的基本测试：

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package crunchify . com . tutorials ;
 
import java . util . HashMap ;
import java . util . IdentityHashMap ;
import java . util . Map ;
 
/**
* @author Crunchify.com
*
*/
 
public class CrunchifyIdenityHashMapVsHashMapSample {
 
public static void main ( String [ ] args ) {
Map < String , String > crunchifyIdentityHashMap = new IdentityHashMap < String , String > ( ) ;
Map < String , String > crunchifyHashMap = new HashMap < String , String > ( ) ;
 
// Let's checkout what happens when we put Unique Key to IdentityHashMap
crunchifyIdentityHashMap . put ( "Company" , "Crunchify" ) ;
// this considered different object for == operator
crunchifyIdentityHashMap . put ( new String ( "Company" ) , "Google" ) ;
crunchifyIdentityHashMap . put ( "Company" , "Facebook" ) ;
System . out . println ( "crunchifyIdentityHashMap KeySet Size: " + crunchifyIdentityHashMap . keySet ( ) . size ( ) ) ;
 
// Let's checkout what happens when we put Unique Key to HashMap
crunchifyHashMap . put ( "Company" , "Crunchify" ) ;
// key1.equals(key2) returns true hence it removes the old value
crunchifyHashMap . put ( new String ( "Company" ) , "Google" ) ;
crunchifyHashMap . put ( "Company" , "Facebook" ) ;
System . out . println ( "crunchifyHashMap KeySet Size: " + crunchifyHashMap . keySet ( ) . size ( ) ) ;
}
}

结果：

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crunchifyIdentityHashMap KeySet Size : 2
crunchifyHashMap KeySet Size : 1

让我们在两个地图上进行性能测试：

  1. 创建 Java 类： CrunchifyIdentityHashMapVsHashMapPerformance .java
  2. startCrunchifyTest()
    • 生成数百万的随机地图大小
    • 实例化和初始化crunchifyString[]字符串数组对象，上面生成带有文本的随机数： This is Crunchify's Test # number
  3. crunchifyCompareIdentityHashMapVsHashMap(String[] crunchifyString , Map<String, Integer> crunchifyMap , String name )
    • 将所有必需的参数传递给此方法
    • crunchifyMap将具有值 IdentityHashMap / HashMap
    • 遍历 crunchifyString[] 并将值放入 M​​ap - 此操作需要一些时间
    • 遍历 crunchifyString[] 并从 Map 中获取值——这个操作需要一些时间
    • 我们将找出上述两种操作的执行时间，以便我们比较哪一种更适合上述操作？ 身份哈希映射或哈希映射
    • 打印上面的结果
  4. 执行上述任务 2 和 3 共 8 次。
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package crunchify . com . tutorials ;
 
import java . util . HashMap ;
import java . util . IdentityHashMap ;
import java . util . Map ;
import java . util . Random ;
 
/**
* @author Crunchify.com
*
*/
 
public class CrunchifyIdentityHashMapVsHashMapPerformance {
static Random rand = new Random ( ) ;
 
private static void startCrunchifyTest ( ) {
 
// Let's run test for 5 times
for ( int i = 0 ; i < 15 ; ++ i ) {
 
// Let's create random Map size which we will use in IdentityHashMap and HashMap
int randomMapSize = 1000000 + rand . nextInt ( 9000000 ) ;
String [ ] crunchifyString = new String [ randomMapSize ] ;
for ( int j = 0 ; j < randomMapSize ; ++ j )
// Assign below string to crunchifyString Object
crunchifyString [ j ] = "This is Crunchify's Test #" + j ;
 
System . out . println ( "\nIteration # " + i + " - Creating String with size: " + randomMapSize ) ;
crunchifyCompareIdentityHashMapVsHashMap ( crunchifyString , new HashMap < String , Integer > ( randomMapSize ) , "HashMap" ) ;
 
// Runs the garbage collector
System . gc ( ) ;
crunchifyCompareIdentityHashMapVsHashMap ( crunchifyString , new IdentityHashMap < String , Integer > ( randomMapSize ) ,
"IdentityHashMap" ) ;
 
// Runs the garbage collector
System . gc ( ) ;
}
}
 
/**
* @param crunchifyString
* @param crunchifyMap
*            : IdentityHashMap / HashMap
* @param name
*/
private static void crunchifyCompareIdentityHashMapVsHashMap ( String [ ] crunchifyString , Map < String , Integer > crunchifyMap ,
String name ) {
long start = System . currentTimeMillis ( ) ;
 
// put crunchifyString String[] to map
for ( int put = 0 ; put < crunchifyString . length ; ++ put )
crunchifyMap . put ( crunchifyString [ put ] , put ) ;
 
boolean result = false ;
for ( int get = 0 ; get < crunchifyString . length ; ++ get ) {
if ( crunchifyMap . get ( crunchifyString [ get ] ) ! = get )
result = true ;
}
System . out . println ( name + " time taken : \t" + ( System . currentTimeMillis ( ) - start ) / 1000. + " sec" ) ;
 
// Check for result discrepancy
if ( crunchifyMap . size ( ) ! = crunchifyString . length )
System . out . println ( "Please check size. Test failed" ) ;
if ( result )
System . out . println ( "Result failed.." ) ;
}
 
public static void main ( String [ ] args ) {
System . out . println ( "IdentityHashMap Vs. HashMap comparison Test started..." ) ;
 
// method to compare IdentityHashMap and HashMap
startCrunchifyTest ( ) ;
}
 
}

结果：

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IdentityHashMap Vs . HashMap comparison Test started . . .
 
Iteration # 0 - Creating String with size: 6964175
HashMap time taken : 3.155 sec
IdentityHashMap time taken : 1.517 sec
 
Iteration # 1 - Creating String with size: 6556459
HashMap time taken : 3.415 sec
IdentityHashMap time taken : 1.466 sec      < == IdentityHashMap gives better result for large object
 
Iteration # 2 - Creating String with size: 9567664
HashMap time taken : 4.173 sec
IdentityHashMap time taken : 2.339 sec      < == better
 
Iteration # 3 - Creating String with size: 4230755
HashMap time taken : 0.372 sec
IdentityHashMap time taken : 0.911 sec
 
Iteration # 4 - Creating String with size: 7821718
HashMap time taken : 1.096 sec
IdentityHashMap time taken : 0.812 sec
 
Iteration # 5 - Creating String with size: 8125421
HashMap time taken : 4.883 sec
IdentityHashMap time taken : 1.876 sec      < == better
 
Iteration # 6 - Creating String with size: 3166432
HashMap time taken : 0.537 sec
IdentityHashMap time taken : 0.708 sec
 
Iteration # 7 - Creating String with size: 2821415
HashMap time taken : 0.227 sec
IdentityHashMap time taken : 0.621 sec

观察：

正如您在结果中看到的那样，对于大型地图，IdentityHashMap 的性能要好得多。 为什么？ IdentityHashMap doesn't use equals() and hashcode() methods ，这被认为是非常昂贵的。

仅供参考：

我们为从 Map 中输入和获取值所做的上述操作非常占用 CPU。

IdentityHashMap 与 HashMap 测试 - CPU 使用率