How to use double brace initialization for Map of Map

Question

I do understand that double brace initialization has its own hidden cost, still is there a possible way to initialize Map<String,Map<String,String>>().

What i tried:

Map<String, Map<String, String>> defaultSourceCode = new HashMap<String, Map<String, String>>(){
            {"a",new HashMap<String, String>(){{"c","d"}}}
        };

I know it is a bad practice but as for experiment i am trying it.

Reference and Motivation: Arrays.asList also for maps?

Answer 1

You can use Map.of() from java9 that returns an immutable map:

Map<String, Map<String, String>> map = Map.of("a", Map.of("c", "d"));

Or Map.ofEntries :

Map<String, Map<String, String>> map1 = Map.ofEntries(
        Map.entry("a", Map.of("c", "d"))
);

Answer 2

Almost everything is fine, you just have to use method calls in double braces:

Map<String, Map<String, String>> defaultSourceCode = new HashMap<String, Map<String, String>>(){
    {put("a",new HashMap<String, String>(){{put("c","d");}});}
};

But this answer describes, why you shouldn't do that.