The instance initialiser is just syntactic sugar in this case, right? I don’t see why you need an extra anonymous class just to initialize. And it won’t work if the class being created is final.
You can create an immutable map using a static initialiser too:
public class Test {
private static final Map<Integer, String> myMap;
static {
Map<Integer, String> aMap = ....;
aMap.put(1, "one");
aMap.put(2, "two");
myMap = Collections.unmodifiableMap(aMap);
}
}