Difference Between IdentityHashMap, WeakHashMap, and EnumMap in 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());
        }
    }
}

// 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);
    }
}

// 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());
        }
    }
}