Basics

Sections

---

• Variables
• Nullibility
• Strings
• Casting
• When / Switch
• Loops
• Collections

---

Variables

Java

final int a = 0;
final int b = 1;
    
int c = 0;
c = 1;

Kotlin

//val is immutable
val a: Int = 0
val b = 1 //Int is implied
   
//var is mutable
var c: Int = 0
c = 1

---

Nullibility

Java

//Simple Synthax
Date today = null;
long t1 = today.getTime(); //NullPointerException

Kotlin

//Simple Synthax
val today: Date? = null
//do
val t1 = today?.time //t1 (Long?) = null
val t2 = today?.time ?: 0L //t2 (Long) = 0L
	
//do not
val t4: Long = today.time //Compilation Error!
val t3 = today!!.time //NullPointer Exception DO NOT USE '!!'

Java - Android example

//Examples
TextView profileFirstName = (TextView) findViewById(R.id.first_name);
TextView homePhoneNumber = (TextView) findViewById(R.id.home_number);
    
Profile profile = dataSource.getProfile();
    
if(profile != null){
	profileFirstName.setText(profile.getFirstName());
	if (profile.getNumbers() != null) {
		homePhoneNumber.setText(profile.getNumbers().getHomeNumber());
	} else {
		homePhoneNumber.setText("N/A");
	}
} else {
	profileFirstName.setText("Default");
}

Kotlin - Android example

val profileFirstname = findViewById(R.id.first_name) as TextView //use 'as' to cast
val homePhoneNumber = findViewById(R.id.home_number) as TextView

val profile = dataSource.getProfile()
	
profileFirstname.text = profile?.firstName ?: "Default" //Optionals with Elvis expression
homePhoneNumber.text = profile?.numbers?.homeNumber ?: "N/A" //Optionals Chaining! 

---

Strings

Java

final String firstName = "Taras";
final String lastName = "Koshkin";
final String fullName = firstName + " " + lastName;
final String initials = firstName.substring(0, 1) + lastName.substring(0, 1);

Kotlin

val firstName = "Taras"
val lastName = "Koshkin"
val fullName = "$firstName $lastName" 
val initials = "${firstName.substring(0, 1)}${lastName.substring(0, 1)}" //expressions within text

Casting

Java

EditText editText = (EditText) findViewById(R.id.edit_text);

if (activity instanceof AppCompatActivity) {
    ((AppCompatActivity) activity).onBackPressed();
}

Kotlin

val editText = findViewById(R.id.edit_text) as EditText


if (activity is AppCompatActivity) {
    activity.onBackPressed() //Activity is automatically Smart Casted into AppCompatActivity
}

When / Switch

Java

int x =  ...; // value
String result;

switch (x) {
    case 1:
        result = "primary number";
        break;
    case 3:
        result = "primary number";
        break;
    case 7:
        result = "primary number";
        break;
    default:
        if (x >= 4 && x <= 6) {
            result = "from 4 to 6";
            break;
        } else if (x < 8 && x > 12) {
            result = "not from 8 to 12";
            break;
        } else {
            result = isOdd(x) ? "add number" : "even number";
            break;
        }
}

//What if it's not a simple int?

if (bodyPart instanceof Arm) {
	...
} else if (bodyPart instanceof Leg) {
	...
} else {
    ...
}

Kotlin

val x: Int = ...

val result = when (x) {
    1, 3, 7 -> "primary number"
    in 4..6 -> "from 4 to 6"
    !in 8..12 -> "not from 8 to 12"
    else -> if (isOdd(x)) "odd number" else "even number"
}

when(bodyPart) { //It's a lot more versatile
	is Arm -> ...
	is Leg -> ...
	else -> ...
}

For Loops

Java

for (int i = 1; i <= 10; i++) System.out.print(i);
for (int i = 1; i <= 10; i += 2) System.out.print(i);
for (int i = 10; i > 0; i--) System.out.print(i);
for (int i = 10; i > 0; i -= 2) System.out.print(i);

ArrayList<String> collection = new ArrayList<>();
HashMap<String, String> map = new HashMap<>();

for (String item : collection) System.out.print(item);
for (String item : collection) System.out.print(collection.indexOf(item) + " " + item);
for (Map.Entry<String, String> entry : map.entrySet())
    System.out.print(entry.getKey() + " " + entry.getValue());

Kotlin

for (i in 1..10) print(i)
for (i in 1..10 step 2) print(i)
for (i in 10 downTo 1) print(i)
for (i in 10 downTo 1 step 2) print(i)

val collection = ArrayList<String>()
val map = HashMap<String, String>()

for (item in collection) print(item)
for ((index, item) in collection.withIndex()) print("$index $item")

for ((key, value) in map) print("$key $value")

Collections

Java

List<String> list = Arrays.asList("one", "two", "three");
Map<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);
map.put("three", 3);

for (String item : list) {
    System.out.print(item);
}
for (String item : list) {
    if (item.equals("two")) {
        System.out.print(map.get(item));
    }
}

Kotlin

val list = arrayListOf("one", "two", "three") //or you can use listOf()
val map = hashMapOf("one" to 1, "two" to 2, "three" to 3) //or you can use mapOf()

list.forEach { print(it) } //by default it is the list item
list.filter { it != "two" } //skips "two"
    .map { map[it] } //changes the object from "one" to 1
    .forEach { print(it) } //will print >> 12