FAQ: Data Structures - Iterating over a HashMap


#1

This community-built FAQ covers the “Iterating over a HashMap” exercise from the lesson “Data Structures”.

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#2

The items in the HashMap were inserted in this order…
restaurantMenu.put(“Turkey Burger”, 13);
restaurantMenu.put(“Naan Pizza”, 11);
restaurantMenu.put(“Cranberry Kale Salad”, 10);
But when we iterate through the list to print them out, the output is in this order (2nd and 3rd reversed)
A Turkey Burger costs 13 dollars.
A Cranberry Kale Salad costs 10 dollars.
A Naan Pizza costs 11 dollars.

Why doesn’t the Naan Pizza output second and the Cranberry Kale Salad output third?


#3

if you check the documentation:

https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html

you see:

This class makes no guarantees as to the order of the map

i wouldn’t bother with understanding how hash maps are implemented, just know they aren’t ordered, and don’t have to be given we access values by key


#4

If you do want to maintain the order, you can use a LinkedHashMap object instead! It has the exact same methods as the HashMap class you are currently using (it actually extends it), and it maintains the order of the components in the order that you add them!


#5

In the example it used the key “name” yet in our exercise we used the key “item”. How did the program know what we meant if we didn’t define them as such?


#6

here:

for (String item : restaurantMenu.keySet())

you define the variable, the for loop then assigns each key to this variable

so Java does a lot of lifting.