Could you explain:

Optional<Integer> res = ransom.entrySet().stream()
.map(e -> magazine.getOrDefault(e.getKey(), 0) - e.getValue())
.filter(e -> e < 0)
.findFirst();

How did you know how the order of the .entrySet, .stream, .map, etc...?

I understand what the Optional class does, I am just not experienced with this syntax.