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# System prepended metadata

title: Lecture 9 Setup/Prep
tags: [Setup-Summer21]

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tags: Setup-Summer21
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# Lecture 9 Setup/Prep

At the end of lecture 8, we had a `LinkList` class with two methods: `addFirst` and `contains`. This lecture, we will look at two more common list methods: `addLast` and `length`. At this point, you should be able to write a basic version of these methods, so we've provided initial versions in the starter code.

Our goal for this lecture is to see how to use fields and field-mutation to improve the running time of these two methods.

## Prep

Download [the starter code for lecture 9](https://cs18-summer-2021.github.io/static/classes/9/lec09.zip). In this version, we are using an `EmptyList` class rather than `null` for the empty list, just so you see different ways of implementing lists.

- The starter code has two interfaces: `IList` and `IListInternal`. Why are there two interfaces?
- Think about adding field(s) to the existing classes that would reduce the running-time of `length` to constant. What field(s) would you add and to which classes? How will they be maintained as we add elements to the list?
- Think about how you would maintain these fields within the `remEltOnce' method.

During lecture, we will also add fields to make `addLast` run in constant time.




