Correct mapping for a Java HashMap

I had a look at the mapping ES craeted outomatically for one of my indeces,
and found something that's not quite right:

....
"annotations" : {
"properties" : {
"ids" : {
"properties" : {
"hashMap" : {
"properties" : {
"N_10290607" : {
"type" : "double"
},
"A_1038408" : {
"type" : "double"
},
"A_11585994" : {
"type" : "double"
},
"B_1245677" : {
"type" : "double"
},
"B_1269810" : {
"type" : "double"
},
"C_15680034" : {
"type" : "double"
},
"N_1654171" : {
"type" : "double"
},
...

I use Gson to convert Java classes to Json and then directly put them into
ES. One of the classes I use has a HashMap<String, Double> as a central
piece, where it stores its key-value pairs. It's really just that: the keys
are stings, the values doubles. ES however creates a mapping for every key
of the hashmap as you can see above.

Does someone here knoe what mapping I need to define to avoid that?

Also: Can I change this mapping on the already existing index, or do I have
to create a new index, then create a mapping, then copy over the data from
the old index?

Many Thanks!

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