IdentityHashMap in Java is a specific class similar to HashMap, except that it uses the reference equation (==) when comparing data (keys and values).

Simply put: Java IdentityHashMap uses reference equals (==) instead of object equals() when comparing keys and values.

When comparing keys, HashMap uses the equals() method, while IdentityHashMap uses the equals (==) reference. In HashMap, two keys k1 and k2 are considered equal only if (k1==null? k2==null:k1.equal(k2)).  In IdentityHashMap, two keys are considered equivalent if and only if (k1==k2).

IdentityHashMap in Java has been added to JDK 1.4 and is included in the java.util.IdentityHashMap package. This lesson is rarely used in the program.

IdentityHashMap Hierarchy on Java

The Java IdentityHashMap class extends the AbstractMap class and implements the map interface (using a hash table). The IdentityHashMap Java hierarchy chart is shown in the following figure

Class Identity StatementHashMap

IdentityHashMap is a generic class that can be explained as follows:

Class IdentityHashMap<K,V>
extended AbstractMap<K,V>
implemented Folder<K,V>

Here K defines the key type and V the value type. The API documentation explicitly states that IdentityHashMap is not intended for general use.

IdentityHashMap Features

There are several important features of IdentityHashMap in Java that you should take into account. They’re identical:

1. IdentityHashMap allows you to store data as key value pairs.

2. Only one null key can be added to IdentityHashMap, but many null values can be added.

3. Like HashMap, IdentityHashMap does not support insertion sequences. There is no guarantee that the items will be removed in the same order in which we have placed them.

4. It’s out of sync. That means he’s not sure about the stitches. This allows two or more threads to access the same IdentityHashMap object at the same time.

We can synchronize IdentityHashMap with the method of the SynchronizedMap() class of Collections. The synchronization syntax is as follows:

Folder synchronizedMap = Collections.synchronizedMap (IdentityHashMap) ;

From now on, no two or more streams can access the same card object at the same time.

5. The iterator returned by IdentityHashMap fails quickly. This means that all structural changes in the IdentityHashMap, such as adding or deleting items during iteration, result in an exception called ConcurrentModificationException.

Design IdentityHashMap Class

Java IdentityHashMap has the following three public vendors

  • Public IdentityHashMap()
  • Public IdentityHashMap(int ems)
  • public identityHashMap

The first two designers are standard designers used to implement general purpose maps. You create an empty IdentityHashMap with a maximum expected size of 21 (default) and an ems. The maximum expected size of the RMS is the maximum number of records that can be retained.

A third constructor creates an IdentityHashMap that contains the same entries as those in the mmap.

Identification methodsHassaMar to Java

The IdentityHashMap methods are exactly the same as the HashMap methods. So let’s perform some operations based on the IdentityHashMap methods and investigate the difference between HashMap and IdentityHashMap.

Java IdentityHashMapExample programs

Let’s make a program in which we see the difference between HashMap and IdentityHashMap. For a better understanding, please refer to the following source code.

Program source code 1 :

Import java.util.HashMap;
Import java.util.IdentityHashMap ;

public class IdentityHashMapEx1 [
public static empty head(String[] args)
// Create a HashMap
HashMap<String, String> hmap = new HashMap<>();
String s1 = new String(R);
String s2 = new String(R) ;

// Adding records in HashMap.
hmap.put(s1, red) ;
hmap.put(s2, red) ;

System.out.println(Keys available in the HashMap HashMap: +hmap.keySet());
System.out.println(Values available in the HashMap HashMap: +hmap.values()) ;

// Create an IdentityHashMap object.
IdentityHashMap<String, String> demap = new IdentityHashMap<>();
demap.put(s1, Red);
demap.put(s2, Red) ;

System.out.println(Key available in the IdentityHashMap: +ihmap.keySet());
System.out.println(Values available in the IdentityHashMap: +ihmap.values());

A way out:
The keys available in HashMap hmap : A] The available values in the HashMap : Red.

The keys available in IdentityHashMap : R, R] Values available in IdentityHashMap : Red, red.


1. The previous program creates an empty HashMap. Then two separate lines s1 and s2 are generated, each pointing to R. They are used as keys with corresponding R values and place the s1 and s2 keys with their values in the hash card.

2. The keys are then retrieved using the keySet() method and displayed on the console. You will see that only one R-key is displayed on the console.

3. The values are extracted with the values() method and displayed in the console. The red value is displayed on the screen.

HashMap uses the equals() method to compare keys. Since s1 and s2 both contain the same content (i.e. red), the equivalent() method reflects the truth.

If you try to place s2 with the corresponding value, it will replace the value. This means that only one data record (a key value pair) is stored.

4. Now create an empty IdentityHashMap demo card. Both s1 and s2 are inserted into the IdentityHashMap object with their values.

5. Next, the keySet() method is used to obtain the keys. The two buttons, each with an R value, are displayed on the screen.

6. The value() method is used to obtain values. Both values, each containing a red value, are displayed on the screen. As you can see along the way.

Indeed, IdentityHashMap uses the == operator to compare keys. Since s1 and s2 are two different objects (although they contain the same value in red), the == operator returns false.

When trying to place s2 with its value in the IdentityHashMap, the second key value pair is treated as another key value pair and added to the IdentityHashMap.

Although Java IdentityHashMap implements the card interface, this class still violates the card’s general agreement to use the equals() method when comparing key objects.

It is therefore not intended for general use. IdentityHashMap can only be used if good reference semantics are needed.

2. Let’s look at a sample program based on the following IdentityHashMap methods. We will perform the following operations. Look at the source code.

Program source code 2 :

import java.util.IdentityHashMap

public class IdentityHashMapEx2 [
public static gap main(String[] args)
string s1 = new string(R) ;
string s2 = new string(G) ;

// Create an IdentityHashMap object.
IdentityHashMap<String, String> demap = new IdentityHashMap<>() ;
demap.put(s1, Red) ;
demap.put(s2, Green) ;

System.out.println(Key available in the IdentityHashMap: +ihmap.keySet());
System.out.println(Values available in the IdentityHashMap: +ihmap.values()) ;

// Insert other keys and values.
demap.well(P, pink);
demap.well(B, black);
demap.well(W, white);
demap.well(O, orange) ;

System.out.println(IdentityHashMap contains the P-key: +ihmap.containsKey(P));
System.out.println(IdentityHashMap contains the orange value: +ihmap.containsValue(Orange));
System.out.println(A sequence of commands in ihnap: +ihmap.entrySet());

System.out.println(The value corresponding to the W key: +ihmap.get(W));
System.out.println(Is IdentityHashMap demap empty: +ihmap.isEmpty());
System.out.println(After using the clear() method, ihmap contains: +ihmap);

A way out:
The keys available in IdentityHashMap : R, G] The values available in IdentityHashMap : Red, green.

IdentityHashMap contains the key P: true
IdentityHashMap contains the value Orange: true

A set of cards included in the erap : R=Red, P=Pink, O=Orange, W=White, B=Black, G=Green] The value corresponding to the W White key
IdentityHashMap is empty: false
After using the method clear(), erappen: {}

When using IdentityHashMap in Java

IdentityHashMap can be used as :

  • We want to make a deep copy.
  • We want to support proxy objects.
  • We just want to keep a zero key on the map.
  • We don’t need the order in which the records are inserted.
  • We don’t work in a multi-wire environment.

We hope this tutorial has covered the important points related to IdentityHashMap in Java using sample programs. I hope you understand the subject.
Thanks for reading!

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