Lists, Sets, ArraySet

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.

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 instatiate it! We must instatiate 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. In a similar way to how we import JUnit, 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. There is also no sense of order.


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

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

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


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' starter code here. In the lecture clip below, Professor Hug goes develops the solution:

Here is our code as of now:

import java.util.Iterator;

public class ArraySet<T> implements Iterable<T> {
    private T[] items;
    private int size; // the next item to be added will be at position size

    public ArraySet() {
        items = (T[]) new Object[100];
        size = 0;

    /* Returns true if this map contains a mapping for the specified key.
    public boolean contains(T x) {
        for (int i = 0; i < size; i += 1) {
            if (items[i].equals(x)) {
                return true;
        return false;

    /* Associates the specified value with the specified key in this map. */
    public void add(T x) {
        if (contains(x)) {
        items[size] = x;
        size += 1;

    /* Returns the number of key-value mappings in this map. */
    public int size() {
        return size;

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