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Difference Between IdentityHashMap, WeakHashMap, and EnumMap in Java

  • Last Updated : 07 Jan, 2021

The IdentityHashMap, WeakHashMap, and EnumMap all are the classes in java collection that implements the Map interface. But there are few differences exists between them.

1. IdentityHashMap: The IdentityHashMap implements the Map interface. It follows reference-equality in place of object-equality when comparing keys (and values). This class is used when the user requires the objects to be compared via reference. It is not synchronized and must be synchronized externally. The iterators in this class are fail-fast, throw ConcurrentModificationException in an attempt to modify while iterating.

Working of IdentityHashMap:

Java




// Java program to illustrate the
// working of IdentityHashmap
  
import java.util.*;
  
public class IteratingIdentityHashMap {
  
    public static void main(String[] args)
    {
  
        // Creating an empty IdentityHashMap
        IdentityHashMap<Integer, String> ihmap
            = new IdentityHashMap<Integer, String>();
  
        // Mapping string values to int keys
        ihmap.put(10, "Geeks");
        ihmap.put(20, "4");
        ihmap.put(30, "Geeks");
        ihmap.put(40, "Welcomes");
        ihmap.put(50, "You");
  
        // Displaying the size of IdentityHashMap
        System.out.println("IdentityHashMap size : "
                           + ihmap.size());
  
        // Displaying the IdentityHashMap
        System.out.println("Initial identity hash map: "
                           + ihmap);
  
        // Create an Iterator over the
        // IdentityHashMap
        Iterator<IdentityHashMap.Entry<Integer, String> >
            itr = ihmap.entrySet().iterator();
  
        // The hasNext() method is used to check if there is
        // a next element The next() method is used to
        // retrieve the next element
        
        while (itr.hasNext())
        {
            IdentityHashMap.Entry<Integer, String> entry = itr.next();
            
            System.out.println("Key = " + entry.getKey()
                               + ", Value = "
                               + entry.getValue());
        }
    }
}
Output
IdentityHashMap size : 5
Initial identity hash map: {10=Geeks, 40=Welcomes, 50=You, 30=Geeks, 20=4}
Key = 10, Value = Geeks
Key = 40, Value = Welcomes
Key = 50, Value = You
Key = 30, Value = Geeks
Key = 20, Value = 4

2. WeakHashMap: WeakHashMap is the implementation of the Map interface that stores only weak keys. In WeakHashMap, we can store only weak references of its key that allows a key-value pairs to be garbage collected when its key is no longer in ordinary use. WeakHashMap is the HashTable based implementation, but it is not synchronized. It allows you to store both null key and null values.



Working of WeakHashMap: 

Java




// Java program to illustrate
// the WeakHashMap
  
import java.util.Collection; 
import java.util.Map; 
import java.util.Set; 
import java.util.WeakHashMap; 
  
class WeakHashMapdemo {
    public static void main(String[] arg)
    {
        Map<Number, String> whmap = new WeakHashMap<Number, String>();
        
        whmap.put(1, "geeks");
        whmap.put(2, "4");
        whmap.put(3, "geeks");
        whmap.put(4, "welcomes");
        whmap.put(5, "you");
  
        // Displaying weak hash map
        System.out.println("WeakHashMap is : " + whmap);
  
        // Checking if "welcomes" exist
        if (whmap.containsValue("welcomes"))
            System.out.println("Yes welcomes exist");
  
        // Checking if 3 exist as a key in map
        if (whmap.containsKey(3))
            System.out.println("Yes 3 exist");
        
        // Creating set for key
        Set keyset = whmap.keySet();
        
        // Displaying key set
        System.out.println("key Set : " + keyset);
        
        Collection values = whmap.values();
        
        // Displaying values of map
        System.out.println("Values : " + values);
  
        // Removing all data
        whmap.clear();
  
        // Checking whether map is empty or not
        if (whmap.isEmpty())
            System.out.println("Empty WeakHashMap: " + whmap);
    }
}
Output
WeakHashMap is : {5=you, 4=welcomes, 3=geeks, 2=4, 1=geeks}
Yes welcomes exist
Yes 3 exist
key Set : [5, 4, 3, 2, 1]
Values : [you, welcomes, geeks, 4, geeks]
Empty WeakHashMap: {}

3. EnumMap: EnumMap is a specialized implementation of the Map interface for enumeration types. It extends AbstractMap and implements the Map interface in Java. Few important features of EnumMap are as follows: 

  • EnumMap class is a member of the Java Collections Framework and it is not synchronized.
  • EnumMap is an ordered collection, and they are maintained in the natural order of their keys(the natural order of keys means the order on which enum constant are declared inside enum type)
  • EnumMap is much faster than HashMap.
  • All keys of each EnumMap instance must be keys of a same enum type.
  • EnumMap doesn’t allow to insert null key if we try to insert the null key, it will throw NullPointerException.
  • EnumMap is internally represented as arrays therefore it gives the better performance.

Working of EnumMap:

Java




// Java program to illustrate working
// of EnumMap
  
import java.util.*;
  
class EnumMapExample {
  
    public enum Months {
        January,
        February,
        March,
        April;
    }
  
    public static void main(String[] args)
    {
        // Creating an EnumMap of the Days enum
        EnumMap<Months, Integer> enumMap = new EnumMap<>(Months.class);
  
        // Insert using put() method
        enumMap.put(Months.January, 31);
        enumMap.put(Months.February, 28);
        enumMap.put(Months.March, 31);
        enumMap.put(Months.April, 30);
  
        // Printing size of EnumMap 
        System.out.println("Size of EnumMap: "
                           + enumMap.size());
        // Printing the EnumMap
        for (Map.Entry m : enumMap.entrySet())
        {
            System.out.println(m.getKey() + " "
                               + m.getValue());
        }
    }
}
Output
Size of EnumMap: 4
January 31
February 28
March 31
April 30

Difference between IdentityHashMap, WeakHashMap, and EnumMap:

PROPERTIES           IdentityHashMap           WeakHashMap           EnumMap
ReferencesIdentityHashMap stores strong key reference. WeakHashMap stores the weak key reference.EnumMap stores the strong key reference.
Search and get the valuesIt uses equality operator (==) to search and get the values.It uses equals() method for that purpose.It also uses equals() method for that purpose.
KeysIt allows to store any type of keys.It also allows to store any type of keys.It allows to store only enum type keys.
Underlined data structureIt uses the array as an underlined data structure.It uses the HashTable as an underlined data structure.It uses the array as an underlined data structure.
IteratorIterator used in IdentityHashMap is Fail-fast.Iterator used in WeakHashMap is Fail-fast.Iterator used in EnumMap is weakly consistent.
Null ValuesIt allows to store null values.It allows to store null values.It doesn’t allow to store null values

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