Accessing method on private Map member in Java

I got this private member private Map<Character, String> eTM = new HashMap<>();.

I need to be able read its size from a test. I'd like to do it using Java reflection, however it seems that in order to be able to call size() I'd have to get the field as a java.lang.reflect.Field. You can't call any Map methods on that so I'd have to cast the field to "Map<Character, String>" or "HashMap<Character, String>". That doesn't seem to be possible though.

I experimented a bit to see if you could somehow access the size method on a private Map, but I don't see a way to do this. For example:

Field eTmF = Holder.class.getDeclaredField("eTM");
HashMap castTM = (HashMap<String, Character>) eTmF; //cannot cast
castTM.size();//not available

This shows zero length: Holder.class.getDeclaredClasses().length;

and unsurprisingly this has nothing to print:

Class<?>[] classes = Holder.class.getDeclaredClasses();
Arrays.stream(classes).forEach(c -> System.out.println(c.getSimpleName()));

Is this simply as far as reflection goes or is there a way?

For now I've loosened the field to "protected", but if it's possible I'd like to learn. For one thing I was hoping for hints on reflection libraries that may be able to go deeper.