CS 3100: Program Design and Implementation II

Lecture 36: Kotlin: A Better Java

Learning Objectives

After this lecture, you will be able to explain the benefits of these Kotlin features:

Nullable types
Named and default arguments
Top-level functions
Type inference
Conditional expressions

if expressions
switch expressions

Semester Grades: Labs

Grade	Total Points	Individual Assignments	Group Assignments	Exams	Labs	Participation
A	≥900	≥240 (80%)	≥160 (80%)	≥280 (70%)	≥11 completed	≥40 (80%)
B	≥800	≥210 (70%)	≥140 (70%)	≥220 (55%)	≥9 completed	≥25 (50%)
C	≥700	≥180 (60%)	≥120 (60%)	≥200 (50%)	≥7 completed	—
D	≥600	—	—	—	—	—
F	<600 or fails to meet above minimums

There are 14 labs; each is worth 5 points, with the total capped at 50 points (allowing you to miss a few without penalty).

For example, if you attend 10 labs, you get 50 points and can earn a B (possibly a B+).

Poll: Why do/don't you attend class/lecture when ungraded?

Text espertus to 22333 if the
URL isn't working for you.

https://pollev.com/espertus

This poll is anonymous but counts toward participation.

Poll: What's your least favorite thing about Java?

Text espertus to 22333 if the
URL isn't working for you.

https://pollev.com/espertus

This is a word cloud poll, so answer briefly, with hyphens separating words (e.g., null-pointers)

Common Criticisms of Java

Verbosity and boilerplate code
Everything must be in a class
Lack of null-safety
Checked exceptions

What Is Kotlin?

Created by JetBrains (makers of IntelliJ IDEA), released 2016
Designed to be fully interoperable with Java — runs on the JVM
Goals: fix Java's pain points while staying familiar to Java developers
In 2017, Google named it a first-class language for Android
Today used in Android, backend (Spring), and multiplatform development

Java and Kotlin on the JVM

Java

static int cube(int x) {
    return x * x * x;
}
public static void main(String[] args) {
    System.out.println(cube(3));
}

Kotlin

fun cube(x: Int) = x * x * x

fun main() {
    println(cube(3))
}

⬇️ compiler ⬇️

Byte Code

static cube(I)I        public static main(Ljava/lang/String;)V
    ILOAD 0                L0
    ILOAD 0                LINENUMBER 8 L0
    IMUL                   GETSTATIC System.out : Ljava/io/PrintStream;
    ILOAD 0                ICONST_3
    IMUL                   INVOKESTATIC Main.cube (I)I
    IRETURN                INVOKEVIRTUAL java/io/PrintStream.println (I)V

JVM ➡️27

What differences can you spot?

Java

public class MyClass {
  static int cube(int x) {
    return x * x * x;
  }

  public static void main(String[] args) {
    System.out.println(cube(3));
  }
}

Kotlin

fun cube(x: Int) = x * x * x

fun main() {
  println(cube(3))
}

Some Differences

Java

public class MyClass {
  static int cube(int x) {
    return x * x * x;
  }

  public static void main(String[] args) {
    System.out.println(cube(3));
  }
}

Kotlin

fun cube(x: Int) = x * x * x

fun main() {
  println(cube(3))
}

No semicolons
Code outside class (top-level functions)
Visibility defaults to public
fun keyword
Different parameter lists
- name followed by : and type
- all types are capitalized (objects)
A function body can be an expression
Return type can be inferred

Transforming describeItems()

Java

// describeItems(1, "rock") => "1 rock"
// describeItems(2, "rock") => "2 rocks"
static String describeItems(
    int quantity, String item) {
  String description = quantity + " " + item;
  if (quantity != 1) {
    return description + "s";
  } else {
    return description;
  }
}

public static void main(String[] args) {
  System.out.println(describeItems(1, "rock"));
  System.out.println(describeItems(2, "rock"));
}

Kotlin

// describeItems(1, "rock") => "1 rock"
// describeItems(2, "rock") => "2 rocks"
fun describeItems(
    quantity: Int, item: String): String {
  val description = quantity.toString() + " " + item
  if (quantity != 1) {
    return description + "s"
  } else {
    return description
  }
}

fun main() {
  println(describeItems(1, "rock"))
  println(describeItems(2, "rock"))
}

Kotlin variables are declared with

val (immutable)
var (mutable)

https://pl.kotl.in/KhDNBlWcH

Transformation

fun describeItems(
    quantity: Int, item: String): String {
  val description =
    quantity.toString() + " " + item
  if (quantity != 1) {
    return description + "s"
  } else {
    return description
  }
}

fun describeItems(
    quantity: Int, item: String) =
  "$quantity $item" +
  if (quantity != 1) "s" else ""

Changes:

String templates
Return type omitted
Return statement omitted
if-expression value used

You Transform 3-Argument Max

Java

static int max(int a, int b, int c) {
  if (a >= b && a >= c) {
    return a;
  } else if (b >= c) {
    return b;
  } else {
    return c;
  }
}

Kotlin

fun max(a: Int, b: Int, c: Int): Int {
  if (a >= b && a >= c) {
    return a
  } else if (b >= c) {
    return b
  } else {
    return c
  }
}

https://pl.kotl.in/vbnzB1wzd

Transformation

Before

fun max(a: Int, b: Int, c: Int): Int {
  if (a >= b && a >= c) {
    return a
  } else if (b >= c) {
    return b
  } else {
    return c
  }
}

After

fun max(a: Int, b: Int, c: Int) =
  if (a >= b && a >= c) a
  else if (b >= c) b
  else c

Nullable Types

In Java, any reference can be null:

String name = null;
System.out.println(name.length()); // NullPointerException at runtime

In Kotlin, non-nullable is the default:

var name: String = null  // does not compile
var name: String? = null // ok — question mark indicates nullability

Safe Call Operator `?.`

Kotlin won't let you call methods on a nullable type without handling the null case:

var name: String? = null
println(name.length)  // does not compile

The safe call operator returns null instead of throwing an exception:

println(name?.length)  // prints null

Chains short-circuit at the first null:

println(user?.address?.city)  // prints null if any part is null

Exercise: Nullable Types

Java

record Client(PersonalInfo personalInfo) {}
record PersonalInfo(String email) {}
interface Mailer {
  void sendMessage(String email, String message);
}

void sendMessageToClient(
    @Nullable Client client,
    @Nullable String message,
    @NotNull Mailer mailer) {

  // Try to extract email address
  if (client == null) return;
  PersonalInfo info = client.personalInfo();
  if (info == null) return;
  String email = info.email();

  if (email == null || message == null) return;

  mailer.sendMessage(email, message);
}

Kotlin

class Client(val personalInfo: PersonalInfo?)
class PersonalInfo(val email: String?)
interface Mailer {
  fun sendMessage(email: String, message: String)
}

fun sendMessageToClient(
    client: Client?,
    message: String?,
    mailer: Mailer
) {
  // Extract email address or set to null
  val email = TODO()

  // Can't send if there is no email or message
  if (email == null || message == null) {
    return
  }

  mailer.sendMessage(email, message)
}

https://pl.kotl.in/toXWp7eW9

Transformation

Java

void sendMessageToClient(
    @Nullable Client client,
    @Nullable String message,
    @NotNull Mailer mailer) {

  // Try to extract email address
  if (client == null) return;
  PersonalInfo info = client.personalInfo();
  if (info == null) return;
  String email = info.email();

  if (email == null || message == null) return;

  mailer.sendMessage(email, message);
}

Kotlin

fun sendMessageToClient(
    client: Client?,
    message: String?,
    mailer: Mailer
) {
  val email = client?.personalInfo?.email
  if (email == null || message == null) {
    return
  }
  mailer.sendMessage(email, message)
}

Kotlin Koan: Smart Casts

interface Expr
class Num(val value: Int) : Expr
class Sum(val left: Expr, val right: Expr) : Expr

fun eval(expr: Expr): Int =
    when (expr) {
        is Num -> TODO()
        is Sum -> TODO()
        else -> throw IllegalArgumentException(
            "Unknown expression")
    }

https://play.kotlinlang.org/koans/Classes/Smart%20casts/Task.kt

Kotlin Koan: Sealed Classes

fun eval(expr: Expr): Int =
        when (expr) {
            is Num -> TODO()
            is Sum -> TODO()
        } a + (b ?: 0) + (c ?: 0)

interface Expr
class Num(val value: Int) : TODO()
class Sum(val left: Expr, val right: Expr) : TODO()

https://play.kotlinlang.org/koans/Classes/Sealed%20classes/Task.kt

The Elvis Operator `?:`

The Elvis operator returns a default value when the left side is null:

val name: String? = null
println(name ?: "Unknown")  // Unknown

val length = name?.length ?: 0  // 0

This replaces the common null-check pattern:

// Without Elvis
val display = if (name != null) name else "Unknown"

// With Elvis
val display = name ?: "Unknown"

Why "the Elvis Operator"?

Graphic showing that a rotated '?:' looks like Elvis Presley's hair and eyes

Elvis Operator: Example

Java

static int add3(int a, Integer b, Integer c) {
  return a
    + (b != null ? b : 0)
    + (c != null ? c : 0);
}

Kotlin

fun add3(a: Int, b: Int?, c: Int?): Int {
  var result = a
  if (b != null) {
    result = result + b
  } else {
    result = result + 0
  }
  if (c != null) {
    result = result + c
  } else {
    result = result + 0
  }
  return result
}

https://pl.kotl.in/GpeGlDC96

Transformation

Before

fun add3(a: Int, b: Int?, c: Int?): Int {
  var result = a
  if (b != null) {
    result = result + b
  } else {
    result = result + 0
  }
  if (c != null) {
    result = result + c
  } else {
    result = result + 0
  }
  return result
}

After

fun add3(a: Int, b: Int?, c: Int?) =
    a + (b ?: 0) + (c ?: 0)

Extension Functions

Can you add a new method to an existing class whose source code you don't have?

Not in Java, but in Kotlin...

fun String.truncate(maxLength: Int): String {
    return if (this.length <= maxLength) this else take(maxLength - 3) + "..."
}

// Hint: Use the String method reversed()
fun String.isPalindrome(): Boolean = TODO()

fun main() {
    val shortUsername = "KotlinFan42"
    val longUsername = "JetBrainsLoverForever"

    println("Short username: ${shortUsername.truncate(15)}")
    println("Long username:  ${longUsername.truncate(15)}")

    // println("wow".isPalindrome()) // true
    // println("woo".isPalindrome()) // false
}

https://pl.kotl.in/RWcXCXM3p

Poll: Which Kotlin feature excites you most?

A. Null safety (no more NullPointerExceptions)

B. Default and named arguments (no more overloads)

C. Data classes (no more boilerplate)

D. Extension functions (add methods to existing classes)

E. Smart casts (no more explicit casting)

F. String templates (no more concatenation)

G. Conditional expressions (if/when)

H. I don't like any of them

Text espertus to 22333 if the
URL isn't working for you.

https://pollev.com/espertus

Poll: How do you feel about Java?

Text espertus to 22333 if the
URL isn't working for you.

https://pollev.com/espertus

Bonus Slide

Futurama hypnotoad with caption 'ALL HAIL KOTLIN'

Lecture 36: Kotlin: A Better Java​

Learning Objectives​

Semester Grades: Labs​

Poll: Why do/don't you attend class/lecture when ungraded?​

Poll: What's your least favorite thing about Java?​

Common Criticisms of Java​

What Is Kotlin?​

Java and Kotlin on the JVM​

What differences can you spot?​

Some Differences​

Transforming describeItems()​

Transformation​

You Transform 3-Argument Max​

Transformation​

Nullable Types​

Safe Call Operator ?.​

Exercise: Nullable Types​

Transformation​

Kotlin Koan: Smart Casts​

Kotlin Koan: Sealed Classes​

The Elvis Operator ?:​

Why "the Elvis Operator"?​

Elvis Operator: Example​

Transformation​

Extension Functions​

Poll: Which Kotlin feature excites you most?​

Poll: How do you feel about Java?​

Bonus Slide​

Lecture 36: Kotlin: A Better Java

Learning Objectives

Semester Grades: Labs

Poll: Why do/don't you attend class/lecture when ungraded?

Poll: What's your least favorite thing about Java?

Common Criticisms of Java

What Is Kotlin?

Java and Kotlin on the JVM

What differences can you spot?

Some Differences

Transforming describeItems()

Transformation

You Transform 3-Argument Max

Transformation

Nullable Types

Safe Call Operator `?.`

Exercise: Nullable Types

Transformation

Kotlin Koan: Smart Casts

Kotlin Koan: Sealed Classes

The Elvis Operator `?:`

Why "the Elvis Operator"?

Elvis Operator: Example

Transformation

Extension Functions

Poll: Which Kotlin feature excites you most?

Poll: How do you feel about Java?

Bonus Slide