How to Initialize a Static Map in Java: Static Initializers vs. Instance Initializers?

Initialising a Static Map in Java

Initialising a static map in Java involves two main approaches: static initialisers and instance initialisers.

Static Initialisers:

Static initialisers allow you to initialise the map directly within the class definition. This is achieved using a static block, as shown in the following example:

public class Test {
    private static final Map<Integer, String> myMap = new HashMap<>();

    static {
        myMap.put(1, "one");
        myMap.put(2, "two");
    }
}

Pros:

• Concise syntax
• Initialisation occurs at class loading time
• The map is final and cannot be modified once set

Cons:

• Not suitable for immutable maps
• Can be difficult to read and maintain

Instance Initialisers (Anonymous Subclass):

Instance initialisers provide an alternative syntax for initialising static members. This approach uses an anonymous subclass that overrides the default constructor:

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

Pros:

• Syntactic sugar for static initialisers
• Can be used to create immutable maps

Cons:

• Requires an extra anonymous class
• Not compatible with final classes

Additional Notes:

• The instance initialiser approach is considered a syntactic sugar for static initialisers.
• Immutable maps can be created using a static initialiser by first creating a mutable map and then making it unmodifiable using the Collections.unmodifiableMap() method.

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