map.keySet
/**Here, the loop iterates over keySet.
For each key, we get the corresponding value using Map.get.
While this is an obvious way to use all of the entries in the map,
it requires two operations for each entry — one to get the next key and one
to look up the value with get.
If we need just the keys in a map,
keySet is a good option. However, there's a faster way to
get both the keys and values. **/
for (String key : bookMap.keySet()) {
System.out.println("key: " + key + " value: " + bookMap.get(key));
}