Immutable Map in Java (using Guava & Java 9)

In this post, we will discuss various methods to create immutable map in Java.

 

Unmodifiable Maps are “read-only” wrappers over other collections. They do not support any modification operations such as add, remove and clear but their underlying collection can be changed. Maps that additionally guarantee that no change in the Collection object will ever be visible are referred to as immutable.

When you don’t expect to modify a collection, it’s a good practice to defensively copy it into an immutable collection. Immutable Maps have many advantages over their mutable siblings, like they are thread-safe, more memory-efficient and can be passed to untrusted libraries without any side effects.

 

1. Guava

The Collections framework provides unmodifiableMap() method, but it is unsafe to use as the returned map is only truly immutable if nobody holds a reference to the original collection. The returned map is also inefficient as the data structures will still have all the overhead of mutable collections, including concurrent modification checks, extra space in hash tables, etc.

Guava provides simple, easy-to-use immutable versions of each Map using ImmutableMap class. Unlike Collections’s unmodifiableMap() an instance of ImmutableMap contains its own private data and will never change. An ImmutableMap collection can be created in several ways:
 

1. Using the copyOf() method

ImmutableMap.copyOf returns an immutable map containing the same entries as specified map. It returns a NullPointerException if any key or value in map is null.

 

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Output:

java.lang.UnsupportedOperationException
{United States=Washington D.C.}

 

2. Using a Builder

Guava also provides a builder for creating immutable map instances as shown below:

 

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Output:

java.lang.UnsupportedOperationException
{United States=Washington D.C., China=Beijing}

 

3. Using the of method

ImmutableMap.of() returns an immutable map containing the given entries, in order. It throws an IllegalArgumentException if duplicate keys are provided. This works for up to 5 key/value pairs. To create a map with an arbitrary number of entries, we can use Builder’s put() method which takes key-value pairs instead of a map.

 

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Output:

java.lang.UnsupportedOperationException
{United States=Washington D.C.}

 

2. Java 9 – Collection factory methods

Several collection factory methods have been added in Java 9 that creates compact, structurally immutable instance of Map. For example,

This supports maps upto 10 key-value pairs. There is a no var-args overload of Map.of() which can handle any number of mappings. To create a map with an arbitrary number of entries, we can use

It include varargs overloads, so there is no fixed limit on the map size. This approach requires each key-value pair to be boxed. For boxing keys and values, we can use

Here’s complete usage of this method:

 
As map is structurally immutable, keys and values cannot be added, removed, or updated from it but if the contained keys or values are themselves mutable, this may cause the Map to behave inconsistently or its contents to appear to change.

 
References: Guava’s Wiki – Immutable Collections Explained

 
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