Control Flow

If Expressions

In Java you can use conditional to easily assign value A or B to an object.

int x = isEven() ? 4 : 3; // java

var x = if (isEven()) 4 else 3 // kotlin

But java is much simpler in this approach isn’t it?

Yes at this point that is true. But the if expression is much more powerful than a simple condition. For example can use else if or if you have a more complex logic u can use blocks to make the code more readable.

var x = if (isEven()) {
    print("chose even")
    4
} else if (isOdd()) {
    print("chose odd")
    3
} else {
    print("chose the answer")
    42
}

When Expressions

Kotlins when is similar to Javas switch (switch does not exist in Kotlin) but much more powerful.

// java

switch (x) {
    case 1:
        System.out.print("x == 1");
        break;
    case 2:
        System.out.print("x == 2");
        break;
    default:
        System.out.print("x is neither 1 nor 2")
}

// kotlin

when (x) {
    1 -> print("x == 1")
    2 -> print("x == 2")
    else -> { // Note the block
        print("x is neither 1 nor 2")
    }
}

⚠ when matches all arguments until one is satisfied. Every argument after the match will be ignored.

when is even more powerful in its conditions:

when (x) {
    1, 2 -> print("x == 1 or x == 2")
    in 3..10 -> print("x is somewhere between 3 and 10")
    in !11..15 -> print("x is not valid")
    in validNumbers -> print("x is valid")
    else -> print("none of the above")
}

Also type checks are easily possible:

when (x) {
    is String -> saveValue(parseInt(x))
    is Int -> saveValue(x)
    else -> throw Exception("Uncool")
}

Like in if expressions you can also use when’s return values.

var str = when {
    x is String -> x
    x is Float -> ((Int)x).toString()
    else -> x.toString()
}

when can also replace a if-else-if chain.

when {
    x.isOdd() -> print("x is odd")
    x.isEven() -> print("x is even")
    else -> print("x is funny")
}

Loops

for loops

Like in C# you can use a simple human readable for-each loop.

for(item in collection) { // note that you don't need val or var
    print(item)
}

Note that the {} are not required here like in simple if statements. You can even use more complex expressions for the for loop.

for(i in 1..10)
for(i in 10 downTo 0 step 2)

Ranges are compiled to index based loops.

Furthermore you can iterate over index and value at the same time (for + foreach combinded).

for ((index, value) in array.withIndex()) {
    println("the element at $index is $value")
}

There are also some tricks for even easier loops.

while loops

While loops work as usual.

continue and break in loops

see official docs

Smart Cast

Using the is in an condition like if or when the variable is casted to the value in the next context:

// Kotlin

var s : Any = "abc"
if (s is String)
    println(s.length)

See more