moomdate/java_vs_kotlin.md

## java_vs_kotlin.md

      
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              java_vs_kotlin.md
            
          
    1. Types and Operators

Java

// variable
int x = 1;

// constant
final int y = 2; 
Kotlin

var x: Int = 1
// or
var x1 = 1

// constant
val y: Int = 2 // y is set in runtime 

const val z: Int = 3 // const is used only in compile time

Calucalation time <3

Java

System.out.println(x + y * 4 / 3);
// output -> x = 3

System.out.println((x + y) * 4 / 3);
// output -> x = 4
Kotlin

println(x + y * 4 / 3)
// output -> x = 3

// here is another way to do the same job of the second calculation (x + y) * 4 / 3
// Kotlin will take care about the order
println(x.plus(y).times(4).div(3))
// output -> x = 4
Numbers in Kotlin

var x : Number = 1;

println(x.toInt())
println(x.toFloat())
println(x.toLong())

---- 

var x : Number = 1;
x = x.toFloat();
println(x);
// output -> x = 1.0
to do the same thing in Java we should -->
int x = 1;
float z = (float) x;

System.out.println(x);
System.out.println(z);
// output -> x = 1
// output -> z = 1.0
2. Nullability

Java

int x = null; // compilation error
Error:() java: incompatible types: <nulltype> cannot be converted to int
But this kind of initialization is a fact with Kotlin
Kotlin

var x: Int = null; // error
var y: Int? = null; // this would work !!

var firstname: String = null; // error
var lastname: String? = null; // this would work !!

// another example by allowing a list and the content to be nullable 
var list: List<String?> = listOf(null, null, null)
var secondList: List<String>? = null
var thirdList: List<String?> = null
thirdList = listOf(null, null, null)
let consider a function that returns the length of a provided string argument!
An experience developer would always check first whether the argument is null or not
to avoid null pointer exception when calling the length() method on that string argument.
java

// without checking of nullability 
public static int getLength(String arg){
        return arg.length();
    }


// with nullability check
public static int getLength(String arg) {
        if (arg != null) {
            return arg.length();
        }
        return -1;
    }
in Kotlin this is really easier to implement! We can avoid writing always boilerplates and theses if statements
in every single function... take a look at the below kotlin function!
kotlin

fun getLength(arg: String) : Int{
    return arg?.length ?: 0;
}

getLength("Kotlin is awesome");
// output = 17
Default arguments and values

to consider a function that would be called from an api and has an argument that can't be null!
java

   private static final int defaultValue = 1;

    public static void call(int arg) {
        // call another method with arg value
    }

    public static void call() {
        // call another method with default value --> 1
    }
kotlin

fun call(arg : Int = 1){
    // call the method with 1 as default value ! :D - super easy !
}
String templates

java

String firstname = anthony;
String lastname = nahas;
int favNumer = 12;

System.out.println("Hi, I am " + firstname + " " + lastname + " and my favorite number is " + favnumber);
Kotlin

val firstname: String = "Anthony"
val lastname: String = "Nahas"
val favNumber: Int = 12

println("Hi, I am $firstname $lastname and my favorite number is $favNumber ")
// output: Hi, I am Anthony Nahas and my favorite number is 12
Conditions

Consider that you have to check a value

if it's positiv and less then 5 // result = a;
equal 5 // result = b;
equal 6,7,8 // result = c;
equal 9 //result = d;
or between 10 and 20 // result = e
else // throw error

In java we use either if-else or switch statements

public static char checkValue(int value) {
        char result;
        if (value >= 0 && value < 5) {
            result = 'a';
        } else if (value == 5) {
            result = 'b';
        }else if (value == 6 || value == 7 || value == 8) {
            result = 'c';
        }else if (value == 9){
            result = 'd';
        } else if (value >= 10 && value <= 20){
            result = 'e';
        }else{
            throw new IllegalArgumentException();
        }
        return result;
    }
In Kotlin we use the when statement

fun check(value: Int): Char {
    var result: Char = when (value) {
        in 0..4 -> 'a'
        5 -> 'b'
        6, 7, 8 -> 'c'
        9 -> 'd'
        in 10..20 -> 'e'
        else -> throw Error()
    }

    return result
}

the Kotlin variant is 100% more elegant in my opinion!
4. Compact functions

java

// empty
kotlin

fun callUser() = println("I am calling the user...");

fun getMyFavNumber() = 12;

fun isCool() = true

5. Lambdas (like functions literals in other languages)

Kotlin

val myFavNumber: Int = 12;
val dividNumberByTwo = {myFavNumber: Int -> myFavNumber / 2} // --> 6

6. Classes

java

public class User {

    private String mUserName;
    private String mPassword;
    private int mSalt;

    int stack;

    public int getStack() {
        return mSalt * 2 + 1;
    }

    public void setStack(int stack) {
        if (mPassword != null) {
            this.mSalt = stack / 2 + mPassword.length();
        }
    }
}

kotlin

 class User {
    
    private val mUserName: String = ""
    private val mPassword: String = ""
    private val mSalt:Int = 1
    
    val stack: Int
        get(): Int = mSalt * 2 + 1
        private set(value) {mSalt = value/2 + mPassword.length}
}
7. Singletons

java

in java we should provide our own implementation of a singleton object
like wrapping the property in a synchronized satatic method within the class
but in Kotlin we can easy replace the class keyword with object
Kotlin

object User {
    
    private val mUserName: String = ""
    private val mPassword: String = ""
    private val mSalt:Int = 1
    
    val stack: Int
        get(): Int = mSalt * 2 + 1
        private set(value) {mSalt = value/2 + mPassword.length}
}

asd
NB: there are sealed and enum classes too ! please check the official reference here
8. Pairs and Triple

Java

// empty
// Pairs
val car = "bmw" to "M3"
println(car) // (bmw, M3)

// or
val car = "bmw" to "M3" to "500hp"
println(car)
((bmw, M3), 500hp)

// class exmaple
class User(val firstname: String, val lastname: String, val age: Int) {

    fun getShortInfo(): Pair<String, String> {
        return (firstname to lastname)
    }

    fun getFullInfo(): Triple<String, String, Int> {
        return Triple(firstname, lastname, age)
    }
}

9. For Loop

java

 for (int i = 0; i <= 10; i++) {
            System.out.println(i);
        }
        
0
1
2
3
4
5
6
7
8
9
10
Kotlin

for ( i  in 0..10 ){
    println(i)
}
// 012345678910
10. Labeled Breaks

Java

for (int i = 0; i <= 10; i++) {
            System.out.println(i);
            for (int j = 0; j <= 10; j++) {
                if (i > 5) {
                    break;
                }
            }
        }
loop@ for (i in 0..10) {
        for (j in 0..10) {
            if (i > 5) {
                break@loop
            }
        }
    }
even more control over the for loops in kotlin - choose which for loop to break in your if statement