Java EnumMap

EnumMap is one of the specialized implementation of Map interface for use with enum type keys, introduced in Java 1.5 with enumeration type. All of the keys in an enum map must come from a single enum type that is specified, explicitly or implicitly, when the map is created. Enum maps are represented internally as arrays. This representation is extremely compact and efficient. Programmer often use HashMap to store enum type, because they are unaware about this little gem.

Enum constants are unique and have predefined length, you can’t create new enum at run time. It allows java designer to make highly optimized EmumMap. Enum maps are maintained in the natural order of their keys (the order in which the enum constants are declared)

Java EnumMap class Declaration

public class EnumMap<K extends Enum<K>, V> extends AbstractMap<K, V>
      implements java.io.Serializable, Cloneable

Features:

1. Java EnumMap inherits AbstractMap class.
2. All keys of each EnumMap instance must be keys of a single enum type.
3. Java EnumMap does not permits null key, but permits multiple null values.
4. Enum maps are maintained in the natural order of their keys as the order in which the enum constants are declared).
5. Iterators returned by the collection views are weakly consistent: they will never throw ConcurrentModificationException and they may or may not show the effects of any modifications to the map that occur while the iteration is in progress.
6. Java EnumMap is not synchronized.
7. Java EnumMap is not thread-safe. You can make thread-safe by doing Collections.synchronizedMap(new EnumMap<EnumKey, V>(…));
8. Java EnumMap implementation provides constant-time performance for the basic operations (like get, and put)
9. Java EnumMap performance is better than HashMap.

Let’s create a small example so that we can analyze it more

Create an ENUM:

public enum Activity {
       APPROVED, DENIED, CANCELLED
}

Create one class where we can do some CRUD operation with EnumMap

import java.util.EnumMap;
import java.util.Map;

public class EnumMapDemo {

       public static void main(String[] args) {

             Map<Activity, String> enumMap = new EnumMap<>(Activity.class);

             enumMap.put(Activity.APPROVED, "Approved");
             enumMap.put(Activity.DENIED, "Denied");
             enumMap.put(Activity.CANCELLED, "Cancelled");

             System.out.println("All key-value in enum map : " + enumMap);

             System.out
                          .println("Getting value for key = " + Activity.CANCELLED + " is : " + enumMap.get(Activity.CANCELLED));

             // checking if EnumMap contains a particular key
             System.out.println("Does enum map has :" + Activity.DENIED + " " + enumMap.containsKey(Activity.DENIED));

             // checking if EnumMap contains a particular value
             System.out.println("Does enum map has Cancelled value: " + enumMap.containsValue("Cancelled"));
             System.out.println("Removing key = " + Activity.APPROVED + " from enum map and value is : "
                                      + enumMap.remove(Activity.APPROVED));
       }
}

Result:

All key-value in enum map: {APPROVED=Approved, DENIED=Denied, CANCELLED=Cancelled}

Getting value for key = CANCELLED is: Cancelled

Does enum map has: DENIED true

Does enum map has cancelled value: true

Removing key = APPROVED from enum map and value is: Approved

Let’s see the approaches to iterate an EnumMap:

import java.util.EnumMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

public class EnumMapDemo {

       public static void main(String[] args) {

             Map<Activity, String> enumMap = new EnumMap<>(Activity.class);

             enumMap.put(Activity.APPROVED, "Approved");
             enumMap.put(Activity.DENIED, "Denied");
             enumMap.put(Activity.CANCELLED, "Cancelled");

             // iterate EnumMap using forEach

             for (Activity key : enumMap.keySet()) {
                    System.out.println(key + ":" + enumMap.get(key));
             }

             // iterate EnumMap using Iterator

             Set<Entry<Activity, String>> set = enumMap.entrySet();

             Iterator<Entry<Activity, String>> itr = set.iterator();

             while (itr.hasNext()) {
                    Map.Entry<Activity, String> map = itr.next();
                    System.out.println(map.getKey() + ":" + map.getValue());
             }

             // iterate EnumMap using java 8 for each and lambda expression

             enumMap.forEach((k, v) -> {
                    System.out.println(k + " :" + v);
             });
       }
}

Now let’s discuss Difference between EnumMap and HashMap in Java

diference