How can I initialise a static Map?

Question

How would you initialise a static Map in Java?

Method one: static initialiser
Method two: instance initialiser (anonymous subclass) or some other m

Answer 1

I prefer using a static initializer to avoid generating anonymous classes (which would have no further purpose), so I'll list tips initializing with a static initializer. All listed solutions / tips are type-safe.

Note: The question doesn't say anything about making the map unmodifiable, so I will leave that out, but know that it can easily be done with Collections.unmodifiableMap(map).

First tip

The 1st tip is that you can make a local reference to the map and you give it a SHORT name:

private static final Map myMap = new HashMap<>();
static {
    final Map m = myMap; // Use short name!
    m.put(1, "one"); // Here referencing the local variable which is also faster!
    m.put(2, "two");
    m.put(3, "three");
}

Second tip

The 2nd tip is that you can create a helper method to add entries; you can also make this helper method public if you want to:

private static final Map myMap2 = new HashMap<>();
static {
    p(1, "one"); // Calling the helper method.
    p(2, "two");
    p(3, "three");
}

private static void p(Integer k, String v) {
    myMap2.put(k, v);
}

The helper method here is not re-usable though because it can only add elements to myMap2. To make it re-usable, we could make the map itself a parameter of the helper method, but then initialization code would not be any shorter.

Third tip

The 3rd tip is that you can create a re-usable builder-like helper class with the populating functionality. This is really a simple, 10-line helper class which is type-safe:

public class Test {
    private static final Map myMap3 = new HashMap<>();
    static {
        new B<>(myMap3)   // Instantiating the helper class with our map
            .p(1, "one")
            .p(2, "two")
            .p(3, "three");
    }
}

class B {
    private final Map m;

    public B(Map m) {
        this.m = m;
    }

    public B p(K k, V v) {
        m.put(k, v);
        return this; // Return this for chaining
    }
}