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    Kotlin
    Java

    • Practice with Interfaces : 41

    • Implementing Interfaces : 40

    • Using Interfaces : 39

    • Working with Exceptions : 38

    • Throwing Exceptions : 37

    • Catching Exceptions : 36

    • References and Polymorphism : 35

    • References : 34

    • Data Modeling 2 : 33

    • Equality and Object Copying : 32

    • Polymorphism : 31

    • Inheritance : 30

    • Data Modeling 1 : 29

    • Companion Objects : 28

    • Encapsulation : 27

    • Constructors : 26

    • Objects, Continued : 25

    • Introduction to Objects : 24

    • Compilation and Immutability : 23

    • Practice with Collections : 22

    • Maps and Sets : 21

    • Lists and Type Parameters : 20

    • Imports and Libraries : 19

    • Multidimensional Arrays : 18

    • Practice with Strings : 17

    • null : 16

    • Algorithms and Strings : 15

    • Strings : 14

    • Functions and Algorithms : 13

    • Practice with Functions : 12

    • More About Functions : 11

    • Errors and Debugging : 10

    • Functions : 9

    • Practice with Loops and Algorithms : 8

    • Algorithms I : 7

    • Loops : 6

    • Arrays : 5

    • Compound Conditionals : 4

    • Conditional Expressions and Statements : 3

    • Operations on Variables : 2

    • Variables and Types : 1

    • Hello, world! : 0

    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!

    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 Java 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!

    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!

    // The Wide World of Comparable

    Solve: Last Odd Interface (Practice)

    Created By: Geoffrey Challen
    / 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

    Solve: Running Total Interface

    Created By: Geoffrey Challen
    / Version: 2021.10.0

    Create a public class RunningTotal that implements the following 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.