How to Initialize Static Maps in Java: Static Block vs. Anonymous Subclass?

Initialising Static Maps in Java

In Java, static maps can be initialised using two primary methods:

1. Static Initialiser

This method involves using a static block within the class declaration to initialise the map. An example of this is shown in the provided code sample:

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

Advantages:

• Simple and straightforward
• Guarantees initialisation upon class loading

Disadvantages:

• May not be suitable for complex initialisation logic

2. Instance Initialiser (Anonymous Subclass)

This method uses an anonymous subclass to initialise the map. It's written as an instance initialiser but serves the same purpose as static initialisation:

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

Advantages:

• Allows for more complex initialisation logic
• Creates an immutable map

Disadvantages:

• Requires creating an anonymous subclass
• May not be supported in all scenarios (e.g., when the class is final)

Alternatives:

• Immutable Static Maps: By initialising a map using a static block and then calling Collections.unmodifiableMap(map), you can create an immutable static map.

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