MP1: Loading and Sorting

But most of this lesson is tracing how data flows through our app, and discussing various ways in which data is stored and retrieved from Strings. We’ll also discuss how the list view component we use works. Your programming task for this lesson is fairly straightforward, and we’ll guide you to it.

Getting Started

Getting Started

For MP1 we’ll be continuing on with the project that you started working on for MP0. You’ll need the environment that you set up during that checkpoint to proceed.

Before you can get to work on MP1 in earnest, we need to install the MP1 test suites. Before we continue, please commit your work. You should make sure to commit both immediately before and immediately after we install the MP1 test suites. (If there are no changes to commit beforehand, that’s fine.)

You’ll receive the MP1 Kotlin test suites via email. Move the file to the app/src/test/kotlin/edu/illinois/cs/cs124/ay2023/mp/test/ directory in your machine project. You’ll also want to reconfigure grade.yaml in the root directory of your project to request that we grade Checkpoint 1. Let’s look at how to do both of these things together:

Serialization

Serialization

Today we’ll examine how data flows through our app. Along the way, we’ll naturally encounter a process called serialization. But what does that mean? Wikipedia defines serialization as:

JSON

JSON

If serialization is the process of converting object state into some format, what format should we use? There are many different options. But one particularly popular and ubiquitous object serialization format that we use in the MP is known as JSON: JavaScript Object Notation.

Here’s one example. This object has one field named example with the value test:

In JSON, each object is enclosed in curly braces: { and }s. Arrays are enclosed in square brackets:

Numeric literals are not quoted, but all field names and most other values are.

Note that JSON requires the entire object to be represented as a string. For actual Strings and certain other Kotlin classes there is an obvious way to do this. In other cases, it’s a bit more interesting. We’ll look at how some other Kotlin classes convert themselves to Strings in the examples below.

Why JSON?

Why JSON?

There are really two questions here. First, why serialization at all? Second, why the JSON format?

As we discussed above, serialization is one very powerful way to pass data between two different computer programs. In your app, JSON is how data about the list of courses is passed from the server to the client, which would normally be running on two different machines and communicating over a network connection. Because there are JSON libraries for almost every programming language, most can use JSON to communicate, meaning that a client written in Kotlin, or Java, or Python, or Lisp can talk to a server in Java, or Python, or Go, or Haskell. We’ll see a concrete example of serialization when we examine the data path in our app below.

Jackson

Jackson

Next, let’s examine how we can use JSON in our Kotlin programs. This is one of the many things that you should never do yourself! There are many great libraries that support JSON in Kotlin programs.

We’re going to be using a library called Jackson to assist us with serialization. Next, let’s look at how we can use Jackson to serialize and deserialize our Kotlin objects!

Serialization using Jackson

Serialization using Jackson

Let’s start with serialization. Here’s how to use Jackson to automatically generate JSON from our Kotlin objects:

Deserialization

Deserialization

Now that we have a string, let’s look at how we can deserialize that String and convert it back into a Kotlin object.

Practice with Serialization

Practice with Serialization

The following homework problem is only for practice, but it may help you understand how to complete later parts of the MP.

Data Model

Data Model

Now that we understand a bit about serialization, let’s return to our app and see it in action!

Your app works with data about courses at the University of Illinois. This involves both serializing that data to and from JSON, but also designing data models using class design features. Let’s look at the core data model you’ll be using during MP1, and examine where the data comes from and how it flows through your app.

Comparing Summaries

Comparing Summaries

Finally, let’s zero in on our first MP1 test case, and discuss how to correctly implement Comparable for your Summary model. Your default comparison should compare summaries first by their number, and next by their subject. For example, “IS 100” should be shown after “CS 100” but before “STAT 200”.

Let’s briefly examine where you need to make changes, and how to test your compareTo method.

Once you’re passing test0_SummaryComparison, it’s a small change to also complete test2_SummaryView. The video walkthroughs above should give you a hint. Also notice that, once a class implements Comparable, you can sort it using Kotlin’s built-in sorting methods, as shown below. Do not implement your own sort method! We’ll do that together later.

Your Goal Today

Your Goal Today

As a reminder, on lessons where we focus on the machine project we will not assign a homework problem! However, the lesson will usually focus on helping you complete a particular part of the MP test suite, and so we encourage you to spend time on that in lieu of a homework problem.

Right now your goal should be to

1. install the MP1 test suites
2. get your code to compile, and
3. implement summary sorting correctly—both in the Summary class and in the MainActivity, so that summaries are sorted properly when the app starts
4. at that point you’ll be passing test0_SummaryComparison and test2_SummaryView

If you get stuck, find us for help on the tutoring site or forum. If you have extra time after today’s lesson, you may want to move on and begin work on the next lesson, which also covers MP1.

MP1 Scores

MP1 Scores