Maps and Sets

Created By: Geoffrey Challen
/ Updated: 2022-09-12

This lesson introduces two new extremely useful data structures: maps and sets. Together, maps and lists can be used to solve almost any problem. And sets have their uses as well. So let's get started!


Maps represent a fundamentally new data structure for us to consider. So far we've looked at data structures that put items in order—like arrays and lists. We've also discussed using higher-dimensional arrays when needed to represent higher-dimensional data.

Maps are quite different. They don't put items in order. Rather, they allow us to store and look up mappings between one thing and other.

More specifically, maps store mappings between keys and values. Each key in a map maps to just one value. However, the same value can be mapped to by multiple keys.

Map Operations

Let's make this more concrete by looking at a few examples. In Java, to create a map we'll import java.util.Map and java.util.HashMap, similarly to how we imported java.util.List and java.util.ArrayList when working with lists:

Note that, like Lists, Java Maps also utilize type parameters within the angle brackets: <String, String> in the example above. However, Maps require two type parameters: one for the key, and a second for the value. Also note that we use the diamond operator <> on the right, since the type parameters for the HashMap are the same as for the Map. Don't worry too much about the hash in HashMap yet, although we'll return to this later!

The map we created above can be used to map Strings to other Strings:

We can add mappings to our Map using put, which accepts a key as the first parameter and a value as the second parameter. We also show how a second call to put replaces the mapping for "challen", since each key in the map maps to a single value.

Note that a map can have multiple keys that map to the same value:

To retrieve values from a Map we use the get method, which accepts a single parameter: the key to look up in the Map.

get returns the value mapped to by that key, or null if that key does not exist in the Map. Sometimes we also refer to this as looking up the key in the map: so looking up the mapping for "challen" or "student1" in the example above.

Map Example

Maps are great for solving problems where we need to save and look up information based on a key. Let's look at an example that may hit close to home: Recording scores on a homework problem!

Walk through how to complete the example above by loading the highest scores for each student into the Map. Present getOrDefault along the way to simplify the process. Show how to use assert for testing.

Map Iteration

If you want to iterate over all of the mappings in a Java Map, there are a few different ways to do that:

Show how to complete the homework problem above. Feel free to cover multiple approaches!


Before we wrap up, let's briefly examine one other potentially-useful data structure: sets. A set represents an unordered collection of distinct elements. We can add and remove items from a set, but the set either contains the item or does not. Items in a set don't have an index and are not counted.

Sets are generally less useful that lists or maps. But they do come in hand sometimes, particularly when you need to record membership but don't care about counts or ordering. Let's look at an example where a set might come in handy:

Walk through an example using a Set and toLowerCase to clean the attendance String so that we can record who was at the Muppet meeting. Provide some practice with CSV-like String parsing, setting up the method properly, and using assert for testing.

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Show how to complete the homework problem above. Feel free to cover multiple approaches!

More Practice

Need more practice? Head over to the practice page.