Kotlin
Java

Arrays
Java

Created By: Geoffrey Challen
/ Updated: 2021-08-30

Today we're going to learn some exciting new things, and also start to leverage what we already know to build our first small useful programs.

Sequential Data

Until now we've been limited to working with single data values. That's all that our variables could store!

But a lot of data around us can't be reduced to a single data value. So throughout our journey we'll periodically upgrade our ability to work with data in our programs, allowing us to program more of the real world around us.

Our first step is to consider series: 0 or more values of the same type arranged in some order. For example, consider the three individual values 1, 2, and 4. I can consider them unordered as three separate values. Or, I can order them, in several different ways:

  • 1, 2, 4
  • 1, 4, 2
  • 2, 1, 4
  • 2, 4, 1
  • 4, 2, 1
  • 4, 1, 2

Notice that from 3 different values I can create 6 different unique orderings. And the order matters! If you were choosing an introductory CS course, you'd prefer 1, 2, 4 to 4, 2, 1. (Not like there is anything wrong with CS 421. It's just not an introductory course.)

Data Structures

Explain how sequences act as data structures. Identify the metadata: the position of each value. Distinguish between the values themselves and their position. Might be fun to use props.

Putting multiple values in order is our first example of a data structure. Data structures bring structure to data. Putting values in a sequence is just one way that we can structure them. We'll explore others later in the course.

Note also that the structure introduces additional data about the data that it stores. We sometimes calls this metadata, since it's data about data! In the case of a sequence, the metadata is the position of each value in the series. Placing the values 1, 2, and 4 in the order 4, 2, 1, associates a position with each value: 4 is first, 2 is second, 1 is third.

Java Arrays

In Java one of the ways that we can store sequential data is using an array. An array orders zero or more values. Let's dive into to declaring and initializing our first array and then unpack what is happening slowly.

Walk through each part of the array declaration and initialization, slowly and deliberately.

This is not that different than what we've seen before. We see an assignment operator, meaning that there is a variable on the left and what is being assigned to it on the right. On the left, instead of int, we see int[], which indicates that values will store a series of int values.

The right side is a bit more mysterious: new int[8]. new is not going to make full sense for a few weeks, but for now you can think of it as indicating that we are creating a new array. int is the type, and the value 8 which appears inside square brackets sets the size.

In Java, we cannot change the size of an array after it has been created. Later in the semester we'll look at other sequential data structures that are more flexible. But, they are also built on top of arrays, so you'll be able to understand exactly how they work.

We're also going to see something a bit strange if we try and print values:

We'll show you how to display the values stored inside Java arrays in just a minute. But just be prepared for this confusing display if you try and directly print the value of a variable that stores an array.

Array Indexing

We've created our first array! But how do we actually store and retrieve the data inside it? Let's go over that together:

Go through array indexing carefully in this example, and setting and retrieving array values. Also discuss default values for empty arrays.

Arrays put values in order. So, to access the data inside them we need to tell Java what position to either save or retrieve. Here's an example of changing the first value of an array:

Here's an example changing the second value:

And the last (or fourth) value:

Notice something strange? Array indices start at 0. To a computer scientist, 0 means first, 1 means second, and 3 means fourth. This may seem strange to you when you are getting started, but it quickly becomes second nature:

Go through array indexing carefully in this example, and setting and retrieving array values. Also discuss default values.

Out of Bounds

What happens if we use a bad position value? Let's find out!

Present ArrayIndexOutOfBoundsExceptions. Don't get into too many details yet, since students don't understand exceptions. Point out that array lengths cannot change.

Array Literals

Just like we could set a variable with a literal, Java also has array literals. They look like this:

Present a few examples of array literals.

You'll notice above that when we initialize an array with a literal we don't need to specify a size. Java sets the size of the array based on the number of values in the literal.

Default Values

Finally, you may wonder what values Java uses when we create an empty array. Use the playground below to find out! (Or consult this reference.)

Sources of Confusion

Let's walk through some of the possible bits of confusion regarding arrays.

Go through some of the confusing bits related to array usage:

  • Difference between brackets during initialization and assignment (size v. position)
  • Left and right side of array assignment
  • Assigning single values v. the entire array
  • Curly braces for array literals
  • Anything else that might confuse use

Digital Joys

We're just getting started working with data. But even being able to work with series of data using arrays opens up some exciting possibilities. This is largely due to digitization: the conversion of data to numeric format so that computers can work with it. Let's look at examples of digitization that produce sequential or linear data.

Discuss your favorite example of digitization. Music? DNA? Temperature? Pick something strictly linear—no additional dimensions yet.

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

Solution Walkthrough

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

Solution Walkthrough