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    Kotlin
    Java

    • Algorithms and Lists : 44

    • Lambda Expressions : 43

    • Anonymous Classes : 42

    • 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

    Lambda Expressions

    fun interface Modify {
    fun modify(value: Int): Int
    }
    val first = Modify { value -> value + 1 }
    val second = Modify { value -> value - 10 }
    println(first.modify(10))
    println(second.modify(3))

    We saw how anonymous classes could encapsulate reusable logic and allow us to build, for example, a general purpose counting method. But their syntax still left a lot to be desired! In this lesson we’ll improve on that by introducing lambda expressions. Awesome! Let’s start.

    As a reminder, this is an advanced topic. We’re introducing it because you will see lambda expressions in real Java code, particularly when working with user interfaces.

    Warm Up
    Warm Up

    But let’s warm up with a classic practice problem on software testing! This is similar to the problem you’ll need to solve for this lesson’s homework.

    Created By: Geoffrey Challen
    / Version: 2020.10.0

    Create a method named test accepting a single parameter: an instance of ArraySum.

    Each ArraySum provides a method sum that accepts an Array<Int> and returns the sum of the values as an Int, or 0 if the array is null. However, some ArraySum implementations are broken! Your job is to identify all of them correctly.

    To do this you should use assert to test various inputs. Here's an example:

    Your function does not need to return a value. Instead, if the code is correct no assertion should fail, and if it is incorrect one should.

    As you design test inputs, here are two conflicting objectives to keep in mind:

    • Less is more. The fewer inputs you need to identify all broken cases, the more readable your test suite will be.
    • Think defensively. At the same time, you want to anticipate all of the different mistakes that a programmer might make. You've probably made many of these yourself! Examples include forgetting to check for null, off-by-one errors, not handling special cases properly, etc.

    Good luck and have fun!

    Functional Programming
    Functional Programming

    So far we’ve introduced Kotlin as an object-oriented programming language. However, Kotlin also supports other programming styles. One powerful and interesting style of programming is known as functional programming:

    In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions.

    Let’s examine the Wikipedia definition together and contrast it with Java’s object-oriented style:

    First-Class Functions
    First-Class Functions

    One characteristic of true functional programming languages is that functions (or methods) are first-class citizens. They can be stored in variables and passed to and returned from other functions, just like any other kind of data.

    Unlike Java, Kotlin does support first-class functions. However, for this lesson we’ll focus on functional programming approaches that interoperate with Java. Specifically, declaring and implementing so-called functional interfaces. We may return to functional programming in Kotlin in future lesson.

    Getting to Lambda Expressions
    Getting to Lambda Expressions

    Let’s look at our first lambda expression in Kotlin.

    fun interface Modify {
    fun modify(value: Int): Int
    }
    // first stores a reference to a function that returns its argument increased by one
    val first = Modify { value -> value + 1 }
    println(first.modify(7))

    We accomplish this by combining two things we already know—interfaces and anonymous classes—with some new Kotlin syntax. Let’s see how, step by step.

    But first, let’s state our goal.

    Functional Interfaces
    Functional Interfaces

    Our first ingredient is called a functional interface. A functional interface is any old interface, but with one restriction: it can only provide one method. We’ll see why in a minute. In Kotlin we also mark it with the fun keyword.

    Other than that, there are no restrictions on what a functional interface looks like. Here’s one:

    fun interface Modify {
    fun modify(value: Int): Int
    }

    Here’s another:

    fun interface Filter {
    fun accept(first: Int, second: Int): Boolean
    }

    Anonymous Classes
    Anonymous Classes

    Next, we need a way to create something that implements a functional interface on the fly. But wait—we already know how to do that! It’s called an anonymous object:

    fun interface Filter {
    fun accept(first: Int, second: Int): Boolean
    }
    val bothPositive = object : Filter {
    override fun accept(first: Int, second: Int) = first > 0 && second > 0
    }
    val bothNegative = object : Filter {
    override fun accept(first: Int, second: Int) = first < 0 && second < 0
    }

    Lambda Expressions
    Lambda Expressions

    We are so close now. Imagine that we want to save into a variable a method that increments an Int by one. Here’s what it looks like given what we already know. First we need our functional interface, and then an anonymous class to implement it correctly:

    fun interface Modify {
    fun modify(value: Int): Int
    }
    val first = object : Modify {
    override fun modify(value: Int) = value + 1
    }
    println(first.modify(7))

    But we can do better! Let’s see how:

    fun interface Modify {
    fun modify(value: Int): Int
    }
    val first = object : Modify {
    override fun modify(value: Int) = value + 1
    }
    println(first.modify(7))

    Solve: Adder with Lambda (Practice)

    Created By: Geoffrey Challen
    / Version: 2020.10.0

    Declare a function named adder. adder takes a single Int parameter and returns a method that implements the Modify functional interface:

    The returned "function" should implement modify so that it adds the value passed to adder. So, for example:

    The correct solution to this problem is a single line lambda expression!

    Array Counting with Lambdas
    Array Counting with Lambdas

    To finish up, let’s return to our example from last time that used anonymous classes to count arrays in different ways. We’ll reimplement it using lambdas and show how much cleaner and more direct this syntax is.

    // Array Counting with Lambdas

    Solve: Test Array Max

    Created By: Geoffrey Challen
    / Version: 2021.10.0

    Create a method named test that accepts a single parameter: an instance of ArrayMax.

    Each ArrayMax provides a method max that accepts an IntArray and returns the maximum of the values as an Int. If the array is null or empty max should throw an IllegalArgumentException.

    However, some ArrayMax implementations are broken! Your job is to identify all of them correctly. To do this you should use assert to test various inputs. (Do not throw other kinds of exceptions on failure, or use require or check.)

    Here's an example:

    Your function does not need to return a value. Instead, if the code is correct no assertion should fail, and if it is incorrect one should.

    As you design test inputs, here are two conflicting objectives to keep in mind:

    • Less is more. The fewer inputs you need to identify all broken cases, the more readable your test suite will be.
    • Think defensively. At the same time, you want to anticipate all the different mistakes that a programmer might make. You've probably made many of these yourself! Examples include forgetting to check for null, off-by-one errors, not handling special cases properly, etc.

    You'll also need to think about how to use try-catch to handle places where the code should throw an exception, how to ensure that it does, and how to make sure it throws the right type.

    Good luck and have fun!

    More Practice

    Need more practice? Head over to the practice page.