GameUI

One way you can achieve a fog-of-war effect is by keeping a Framebuffer that you write transparent pixels to, and then draw that buffer ontop of your game view:

The steps are more or less:

1. Draw game

2. Update fog-of-war buffer

3. Draw fog-of-war ontop of the game

To make your code work, I decided to sort of rewrite it based on this answer. It works now, but really the only thing that you needed to add was self.icon instead of icon. icon gets garbage collected since there is no further reference to it, while self.icon remains. Also, it's not entirely the same as yours was, so it probably needs a bit of rewriting too.

Texture test = map[7];

map[7] returns a reference on a Texture, or default-constructs it if doesn't exist yet. If Texture doesn't have a default constructor, you get this error.

Try using map::at() instead, which throw an exception if map[7] doesn't exist. Or map::find() if you don't want exceptions. Or, well, just define Texture::Texture()...

map.emplace(count, Texture{p.path().string()}); // Is this the most efficient way?

I guess map.emplace(count, p.path().string()) should be enough, here?

They are not instances of ConcurrentHashMap, they are instances of MapEntry.

If you mean MapEntry instances, the answer is no JDK create new instances of those objects during iteration of ConcurrentHashMap and it is inevitable when you are using ConcurrentHashMap you can see that in the next method in EntityIterator class inside ConcurrentHashMap. The problem is that to mange concurrency JDK store Objects of type Node and those objects are not considered to be exported as mentioned in Documentation in the source code:

Key-value entry. This class is never exported out as a user-mutable Map.Entry (i.e., one supporting setValue; see MapEntry below), but can be used for read-only traversals used in bulk tasks. Subclasses of Node with a negative hash field are special, and contain null keys and values (but are never exported). Otherwise, keys and vals are never null.

So EntryIterator class inside ConcurrentHashMap class converts this objects into MapEntry inside next method. If you are only using your map in single thread application you can use HashMap instead.