How to Best Initialize Static Maps in Java?

Initialising Static Maps in Java: Options and Considerations

Static maps provide a convenient way to store key-value pairs within a class. When it comes to initialising static maps, Java offers two primary methods: the static initialiser and the instance initialiser (using an anonymous subclass).

Static Initialiser

The static initialiser syntax allows you to define the map and populate its values within a static block, as seen in the example below:

private static final Map<Integer, String> myMap = new HashMap<>();
static {
    myMap.put(1, "one");
    myMap.put(2, "two");
}

This method is straightforward and does not require any additional classes or syntactic sugar. However, any modifications made to the map after its initialisation will not be reflected in the static version.

Instance Initialiser (Anonymous Subclass)

The instance initialiser, on the other hand, uses an anonymous subclass to initialise the map:

private static final Map<Integer, String> myMap2 = new HashMap<>(){
    {
        put(1, "one");
        put(2, "two");
    }
};

This approach provides a compact and concise syntax for initialising the map. However, it creates an anonymous subclass that can potentially impact performance, especially in heavily nested instances.

Considerations

• The instance initialiser option should be used sparingly due to performance concerns.
• Both methods create mutable maps by default. To create an immutable map, you can use Java's Collections.unmodifiableMap() method:

private static final Map<Integer, String> myMap = Collections.unmodifiableMap(aMap);

Which Method to Choose?

The choice between the static and instance initialiser depends on the specific use case:

• For straightforward and efficient initialisation, the static initialiser is recommended.
• For very large or complex maps, where performance is a concern, the static initialiser is preferred.
• If immutability is desired, the static initialiser with Collections.unmodifiableMap() can be used.

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