12.1 Lists and Sets in Java

In this section, we will learn about how to use Java's built-in List and Set data structures as well as build our own ArraySet.

Getting Started

In this course, we've already built two kinds of lists: AList and SLList. We also built an interface List61B to enforce specific list methods AList and SLList had to implement. You can find the code at the following links:

This is how we might use List61B type:

List61B<Integer> L = new AList<>();

Lists in Real Java Code

We built a list from scratch, but Java provides a built-in List interface and several implementations, e.g. ArrayList. Remember, since List is an interface we can't instantiate it! We must instantiate one of its implementations.

To access this, we can use the full name ('canonical name') of classes/interfaces:

java.util.List<Integer> L = new java.util.ArrayList<>();

However, this is a bit verbose. Instead, we can import java libraries:

import java.util.List;
import java.util.ArrayList;

public class Example {
    public static void main(String[] args) {
        List<Integer> L = new ArrayList<>();


Sets are a collection of unique elements - you can only have one copy of each element. Unlike Lists, there is also no sense of order: you can't index into a set, nor can you control where each element is inserted into the set.

Java Sets

Java has the Set interface along with implementations, e.g. HashSet. Remember to import them if you don't want to use the full name!

import java.util.Set;
import java.util.HashSet;

Example use:

Set<String> s = new HashSet<>();
System.out.println(s.contains("Tokyo")); // true

Python Equivalent

In python, we simply call set(). To check for contains we don't use a method but the keyword in. Here's an example:

s = set()
print("Tokyo" in s) // True

DIY: ArraySet

Our goal is to make our own set, ArraySet, with the following methods:

  • add(value): add the value to the set if not already present

  • contains(value): check to see if ArraySet contains the key

  • size(): return number of values

If you would like to try it yourself, find 'Do It Yourself' ArraySet starter code here. In the lecture clip below, Professor Hug goes develops the solution:

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