KotlinCS 124 LogoJava

Implementing Interfaces

import kotlin.random.Random
import java.util.Arrays
data class Counter(private var value: Int) : Comparable<Counter> {
fun up() {
value++
}
fun down() {
value--
}
override fun compareTo(other: Counter): Int {
return value - other.value
}
}
val counters = Array<Counter>(8) { Counter(Random.nextInt(32)) }
println(counters.contentToString())
Arrays.sort(counters)
println(counters.contentToString())

Welcome back! Next we continue our journey with interfaces. In the last lesson we examined how to use interfaces. Now we’ll look at how to provide them in our own classes, and the kind of amazing things that this can unleash. Let’s go!

Debugging Challenge
Debugging Challenge

But let’s start with some debugging practice!

Implementing Interfaces
Implementing Interfaces

Last time we focused on using interfaces. In this lesson we’ll discuss implementing them ourselves. Along the way we’ll also discuss more about exactly how interfaces are so powerful.

Interfaces as Contract
Interfaces as Contract

As we begin to focus on using interfaces, it makes sense to think about an interface as a contract. When you implement an interface in one of your classes, you agree not only to provide certain methods, but also that these methods will do certain things!

Let’s return to our favorite interface—Comparable—for an example of how to read an interface like a contract. Keep in mind that we can use this interface in Kotlin even though it is provided by Java!

https://www.youtube.com/watch?v=n9JwqEP9arE

Now, let’s put what we’ve learned to use by designing a new class and making it Comparable!

// Implementing Comparable

Interfaces as Abstraction Barrier
Interfaces as Abstraction Barrier

Another important way to think about interfaces is as something called an abstraction barrier. An abstraction barrier separates two parts of a program or system in ways that allow them to develop independently. Again, let’s return to Comparable to discuss exactly how that works!

https://www.youtube.com/watch?v=Va9sQzzd-sU
// The Wide World of Comparable

Practice: Last Odd Interface

Created By: CS 124 Staff
/ Version: 2020.10.0

Create a public class LastOdd that implements the following interface:

Using v. Implementing
Using v. Implementing

This is a distinction that can be tricky for people. So let’s go through an example together and discuss the differences.

// Using v. Implementing

Practice: Running Total Interface

Created By: CS 124 Staff
/ Version: 2021.10.0

Create a public class RunningTotal that implements the following interface:

Homework: Movable Location

Created By: CS 124 Staff
/ Version: 2022.10.0

Create a class named Location. Location should define a primary constructor that accepts two fields: a latitude and longitude position, as Double values, in that order. Your constructor should reject invalid latitude and longitude values by throwing an IllegalArgumentException. Valid longitude values are between -180.0 and 180.0, inclusive, while valid latitude values are between -90.0 and 90.0.

Both latitude and longitude properties should be externally readable, but only internally writeable. You may need to review the private set syntax.

Your class should also implement the IMovableLocation interface:

CS People: Rachel Kroll
CS People: Rachel Kroll

Rachel Kroll may not be a household name. But she’s both an incredibly talented system administrator and a prolific author, having authored over 1000 posts on “software, technology, sysadmin war stories, and more.” Her writing is invariably both technically sharp, and infused with the wry humor of someone who has had a great many opportunities to observe the foibles of both people and machines. Not all of her writing will necessary make sense to you at first—but it will be there waiting once you have more experience and can appreciate it better, and likely largely as relevant in a decade as it is now.

Rachel Kroll has a few talks up online, but they’re long. So instead, please just read and enjoy this short piece commenting on code complexity. “Code runs on people. Please keep it simple.”

More Practice

Need more practice? Head over to the practice page.

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