Map<String, Map<String, Double>> twoLevelMap = new HashMap<>();
// now put "foo", "bar" -> 3.14159 into the map
twoLevelMap.computeIfAbsent("foo", key -> new HashMap<String, Double>())
    .put("bar", 3.14159);
// now get the value from the map
System.out.println(twoLevelMap.get("foo").get("bar"));