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

If you want something terse and relatively safe, you can just shift compile-time type checking to run-time:

static final Map map = MapUtils.unmodifiableMap(
    String.class, Integer.class,
    "cat",  4,
    "dog",  2,
    "frog", 17
);

This implementation should catch any errors:

import java.util.HashMap;

public abstract class MapUtils
{
    private MapUtils() { }

    public static  HashMap unmodifiableMap(
            Class keyClazz,
            Class valClazz,
            Object...keyValues)
    {
        return Collections.unmodifiableMap(makeMap(
            keyClazz,
            valClazz,
            keyValues));
    }

    public static  HashMap makeMap(
            Class keyClazz,
            Class valClazz,
            Object...keyValues)
    {
        if (keyValues.length % 2 != 0)
        {
            throw new IllegalArgumentException(
                    "'keyValues' was formatted incorrectly!  "
                  + "(Expected an even length, but found '" + keyValues.length + "')");
        }

        HashMap result = new HashMap(keyValues.length / 2);

        for (int i = 0; i < keyValues.length;)
        {
            K key = cast(keyClazz, keyValues[i], i);
            ++i;
            V val = cast(valClazz, keyValues[i], i);
            ++i;
            result.put(key, val);
        }

        return result;
    }

    private static  T cast(Class clazz, Object object, int i)
    {
        try
        {
            return clazz.cast(object);
        }
        catch (ClassCastException e)
        {
            String objectName = (i % 2 == 0) ? "Key" : "Value";
            String format = "%s at index %d ('%s') wasn't assignable to type '%s'";
            throw new IllegalArgumentException(String.format(format, objectName, i, object.toString(), clazz.getSimpleName()), e);
        }
    }
}