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@title[Kotlin vs Java]

Kotlin vs Java

Why Kotlin?

Modern, Less code, More readable, Smarter

@title[Kotlin fixes]

Kotlin fixes a series of issues that Java suffers from

@ul

@ulend

What Kotlin has that Java does not 🍒

@ul

@ulend

More what Kotlin has that Java does not

What Java has that Kotlin does not

@ul

  • Checked exceptions
  • Primitive types that are not classes
  • Static members
  • Non-private fields
  • Wildcard-types
  • Ternary-operator a ? b : c

@ulend

Kotlin is 100% interoperable with Java

Kotlin java

@title[Syntax]

Kotlin

fun main(args: Array<String>) {
  println("Hello, ${args[0]}")
}

Java

public static final void main(String[] args) {
  System.out.println(String.format("Hello, %s", args[0]))
}

@title[Kotlin variable syntax]

val & var Properties

// Explicit
const val hello: String = "World" // Compile time constant
val hello: String = "World" // Runtime constant
var android: String = "Kotlin"

// Implicit
val hello = "World" // Runtime constant
var android = "Kotlin"

// Nullable ?
var name: String? = null

// Lateinit
lateinit var name: String // Assign later in code, can't be null

Basic types

@title[Kotlin vs Java syntax]

Dry, clean, readable

Kotlin

val file = File()

Java

public static final File file = new File()

Kotlin visibility modifiers

Primary constructor

class Person(val name: String) { // Primary constructor in header
    constructor(name: String, parent: Person) : this(name) { // Secondary constructor
        ...
    }
}

Open & Override

Classes and functions are by default always final and must have open modifier to be inherited or overridden.

open class A {
    open fun foo(i: Int = 10) { }
}
class B : A() {
    override fun foo(i: Int) {  }  // no default value allowed
}

Functions

fun multiply(multiplicator: Int, multiplicand: Int): Int {
    return multiplicator * multiplicand
}

Usage

multiply(1,2)
multiply(multiplicator = 1, multiplicand = 2)
multiply(multiplicand = 2, multiplicator = 1)

@title[Null safe quote]

"I call it my billion-dollar mistake. It was the invention of the null reference in 1965."

Sir Tony Hoare

Null safe Java

public String getLastFour(Employee employee) {
    if(employee != null) {
        Address address = employee.getPrimaryAddress();
        if(address != null) {
            ZipCode zip = address.getZipCode();
            if(zip != null) {
                return zip.getLastFour()
            }
        }
    }
    throw new Exception("Missing data");
}

Null safe Java 8

public String getLastFour(Optional<Employee> employee) {
    return employee.flatMap(employee -> employee.getPrimaryAddress())
                   .flatMap(address -> address.getZipCode())
                   .flatMap(zip -> zip.getLastFour())
                   .orElseThrow(() -> new FMLException("Missing data"));
}

Null safe Kotlin 🚀

Can be as one line single expression

fun getLastFour(employee: Employee?) = 
  employee?.address?.zip?.lastFour 
    ?: throw Exception("Missing data")

Null safe Kotlin

var a: String = "abc"
a = null // compilation error

var b: String? = "abc"
b = null // ok

Usage

val l = a.length // ok
val l = b.length // error: variable 'b' can be null

Checking for null in conditions

Explicitly check if b is null

val l = if (b != null) b.length else -1

Safe call operator, ?.

b?.length

Elvis operator

?:

val name = bob?.department?.head?.name ?: "Unknown"

The !! Operator

Throws an NPE if b is null

val l = b!!.length

Safe & smart cast

Safe casts that return null

val aInt: Int? = a as? Int

Smart cast, no ((String)x).length

fun demo(x: Any) {
    if (x is String) {
        print(x.length) // x is automatically cast to String
    }
}

Singletons & Extensions

MySingleton.kt

object MySingleton { // Note object
    fun add(number: Int): Int = 10 + number
}

MySingleton+Extensions.kt

fun MySingleton.multiply(multiplicand: Int, multiplicator: Int) -> Int {
    return multiplicand * multiplicator
}

Usage

val sum = MySingleton.add(2)
val product = MySingleton.multiply(1, 2)

Range expressions

for (i in 1..4) print(i)     // prints "1234"
for (i in 4..1)              // prints nothing
for (i in 4 downTo 1)        // prints "4321"
for (i in 1..4 step 2)       // prints "13"
for (i in 4 downTo 1 step 2) // prints "42"
for (i in 1 until 10)        // i in [1, 10), 10 is excluded

Data Classes

We frequently create classes whose main purpose is to hold data. In such a class some standard functionality and utility functions are often mechanically derivable from the data.

data class Person(val name: String, val age: Int)

equals(), hashCode(), toString(), copy()

+++

Data Classes and Destructuring Declarations

val jane = Person("Jane", 35) 
val (name, age) = jane
println("$name, $age years of age") // prints "Jane, 35 years of age"

+++

Java POJO

data class Person(val name: String, val age: Int)
public class Person {
   private String name;
   private int age = 0;

   public Person(String name, int age) {
       this.name = name;
       this.age = age;
   }

   public String getName() {
       return name;
   }

   public void setName(String name) {
       this.name = name;
   }

   public int getAge() {
       return age;
   }

   public void setAge(int age) {
       this.age = age;
   }

   @Override
   public boolean equals(Object o) {
       if (this == o) return true;
       if (o == null || getClass() != o.getClass()) return false;

       Person person = (Person) o;

       if (name != null ? !name.equals(person.name) : person.name != null) return false;
       if (age != 0 ? age != person.age : person.age != 0) return false;
   }

   @Override
   public int hashCode() {
       int result = name != null ? name.hashCode() : 0;
       result = 31 * result + age;
       return result;
   }

   @Override
   public String toString() {
       return "Person{" +
               "name='" + name + '\'' +
               ", age='" + age + '\'' +
               '}';
   }
}

Other cool stuff 🍿

Traversing a map/list of pairs

for ((k, v) in map) {
    println("$k -> $v")
}

+++

Execute if not null

val value = ...

value?.let {
    ... // execute this block if not null
}

+++

Return on when statement

fun transform(color: String): Int {
    return when (color) {
        "Red" -> 0
        "Green" -> 1
        "Blue" -> 2
        else -> throw IllegalArgumentException("Invalid color param value")
    }
}

or

fun transform(color: String) = when (color) {
    "Red" -> 0
    "Green" -> 1
    "Blue" -> 2
    else -> throw IllegalArgumentException("Invalid color param value")
}

+++

'if' expression

fun foo(i: Int) {
    val result = if (i == 1) {
        "one"
    } else if (i == 2) {
        "two"
    } else {
        "three"
    }
}

+++

Typealias

typealias JsonData = String
typealias StatusCode = Int
typealias Error = String

typealias Success = (JsonData) -> Unit
typealias Error = (StatusCode, Error) -> Unit

+++

Generics

inline fun <reified T : Any> loadStyle(inputStream: InputStream): T {
  return Gson().fromJson(InputStreamReader(inputStream), object : TypeToken<T>() {}.type)
}

Kotlin Android

Java

public class MainActivity extends AppCompatActivity {
 
   @Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
       Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
       setSupportActionBar(toolbar);
   }
}

+++

Kotlin

No findViewById or Butterknife needed

class MainActivity : AppCompatActivity() {
 
   override fun onCreate(savedInstanceState: Bundle?) {
       super.onCreate(savedInstanceState)
       setContentView(R.layout.activity_main)
       setSupportActionBar(toolbar)
   }
}

@title[Links]

Frost Mobile Guide

Frost Android Kotlin Style Guide

Lets see some code online

@title[The End]

Ok 👋