Immutable Map in Java

• ImmutableMap, as suggested by the name, is an immutable type of Map. It means that the content of the map is fixed or constant after the declaration, that is, they are read-only.
• If any attempt made to add, delete and update elements in the Map, UnsupportedOperationException is thrown.
• An ImmutableMap does not allow the null element either.
• If any attempt made to create an ImmutableMap with a null element, NullPointerException is thrown. If any attempt is made to add a null element in Map, UnsupportedOperationException is thrown.

Advantages of ImmutableMap

• They are thread-safe.
• They are memory efficient.
• Since they are immutable, hence they can be passed over to third-party libraries without any problem.

Note that it is an immutable collection, not a collection of immutable objects, so the objects inside it can be modified.

Class Declaration:

@GwtCompatible(serializable=true,
               emulated=true)
public abstract class ImmutableMap
extends Object
implements Map, Serializable

Class hierarchy:

java.lang.Object
  ? com.google.common.collect.ImmutableMap 

Creating ImmutableMap
ImmutableMap can be created by various methods. These include:

1. From existing Map using copy() function of Guavafilter_none

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   // Below is the Java program to create ImmutableMap    import com.google.common.collect.ImmutableMap; import java.util.HashMap; import java.util.Map;    class MapUtil {        // Function to create ImmutableMap from Map     public static <K, T> void iMap(Map<K, T> map)     {         // Create ImmutableMap from Map using copyOf()         ImmutableMap<K, T> immutableMap = ImmutableMap.copyOf(map);            // Print the ImmutableMap         System.out.println(immutableMap);     }        public static void main(String[] args)     {         Map<Integer, String> map = new HashMap<Integer, String>();         map.put(1, "Geeks");         map.put(2, "For");         map.put(3, "Geeks");         iMap(map);     } }

   Output:

   {1=Geeks, 2=For, 3=Geeks}
   

2. New ImmutableMap using of() function from Guavafilter_none

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   // Below is the Java program to create ImmutableMap import com.google.common.collect.ImmutableMap; import java.util.HashMap; import java.util.Map;    class MapUtil {        // Function to create ImmutableMap     public static void createImmutableMap()     {         // Create ImmutableMap using of()         ImmutableMap<Integer, String> immutableMap = ImmutableMap.of(             1, "Geeks",             2, "For",             3, "Geeks");            // Print the ImmutableMap         System.out.println(immutableMap);     }        public static void main(String[] args)     {         createImmutableMap();     } }

   Output:

   {1=Geeks, 2=For, 3=Geeks}
   

3. Using Java 9 Factory Of() method

   In Java, use of() with Set, Map or List to create an Immutable Map.

   Please Note: The programs below are of Java 9. Hence you would need a Java 9 compiler to run them.

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   // Java code illustrating of() method to // create a ImmutableSet import java.util.*; import com.google.common.collect.ImmutableMap;    class GfG {     public static void main(String args[])     {         // non-empty immutable set         Map<Integer, String> map = Map.of(             1, "Geeks",             2, "For",             3, "Geeks");            // Let's print the set         System.out.println(map);     } }

   Output:

   {1=Geeks, 2=For, 3=Geeks}
   

4. Using Builder() from ImmutableMap

   In Guava, ImmnutableMap class provides a function Builder(). Through this function, a new ImmutableMap can be created, or
   an ImmutableMap can be created from an existing Map or both.

   • Creating a new ImmutableMapfilter_none

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     // Java code illustrating of() method to // create a ImmutableSet import java.util.*; import com.google.common.collect.ImmutableMap;    class GfG {     public static void main(String args[])     {         // non-empty immutable set         ImmutableMap<Integer, String> imap =                           ImmutableMap.<Integer, String>builder()                                                  .put(1, "Geeks")                                                  .put(2, "For")                                                  .put(3, "Geeks")                                                  .build();            // Let's print the set         System.out.println(imap);     } }

     Output:

     {1=Geeks, 2=For, 3=Geeks}
     

   • Creating an ImmutableMap from existing Mapfilter_none

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     // Java code illustrating of() method to // create a ImmutableSet import java.util.*; import com.google.common.collect.ImmutableMap;    class GfG {     public static void main(String args[])     {         // non-empty immutable set         Map<Integer, String> map = Map.of(1, "Geeks",                                           2, "For",                                           3, "Geeks");         ImmutableMap<Integer, String> imap =                         ImmutableMap.<Integer, String>builder()                                                  .putAll(map)                                                  .build();            // Let's print the set         System.out.println(imap);     } }

     Output:

     {1=Geeks, 2=For, 3=Geeks}
     

   • Creating a new ImmutableMap including the existing Mapfilter_none

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     // Java code illustrating of() method to // create a ImmutableSet import java.util.*; import com.google.common.collect.ImmutableMap;    class GfG {     public static void main(String args[])     {         // non-empty immutable set         Map<Integer, String> map = Map.of(1, "Geeks",                                           2, "For",                                           3, "Geeks");         ImmutableMap<Integer, String> imap =                       ImmutableMap.<Integer, String>builder()                                                .putAll(map)                                          .put(4, "Computer")                                            .put(5, "Portal")                                                    .build();            // Let's print the set         System.out.println(imap);     } }

     Output:

      

     {1=Geeks, 2=For, 3=Geeks, 4=Computer, 5=Portal}
     

Try to change ImmutableMap

As mentioned earlier, the below program will throw UnsupportedOperationException.

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Output :

Exception in thread "main" java.lang.UnsupportedOperationException
    at com.google.common.collect.ImmutableCollection.add(ImmutableCollection.java:218)
    at ImmutableListDemo.main(Main.java:16)

How is it different from Collections.unmodifiableMap()?

Collections.unmodifiableMap create a wrapper around the same existing Map such that the wrapper cannot be used to modify it. However, we can still change the original Map.

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Output:

{1=Geeks, 2=For, 3=Geeks, 4=Computer, 5=Portal}

If we create an ImmutableMap from an existing Map and change the existing Map, the ImmutableMap does not change because a copy is created.

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Output :

{1=Geeks, 2=For, 3=Geeks}