Print All Values In A Format Showing Their Key

Java Code:

public static void main(String[] args) {
HashMap<Integer, String> stuff = new HashMap<>();
stuff.put(1, "Book");
stuff.put(2, "Pancake");
stuff.put(3, "Waffle");

for(Map.Entry thing : stuff.entrySet())
System.out.println(thing.getKey() + ": " + thing.getValue());
} mh_sh_highlight_all('java');

So I make a HashMap which is pretty simple. My book showed me how I would print all the values in a format showing their key a ": " and the the actual value. What I don't understand is why the type element for the for each loop is the interface, Map.Entry.