Basic Grammer for Kotlin

This article introduces you to the basic Kotlin grammar.

😸 !!

!! converts T? to T. If the coverted object is null, app will throw a exeption.

val list: List<Int> = ...
val maxValue = list.max()!!
val square = maxValue * maxValue

😎 ?.let

?.let は null じゃない場合に処理を行うメソッド。

?.let is a function to process if an object is not null.

map[1]?.let { it + 5 }

🏀 init in Class

Pre-checking with init in an object disallows a malformed state:

class Age(private val ageInt: Int) {
    init {
        require(isValid()) { "Support $MIN_AGE <= age <="$MAX_AGE"
    }

    companion object {
        private const val MIN_AGE = 0
        private const val MAX_AGE = 120
    }

    fun value(): Int {
        return ageInt
    }

    private fun isValid(): Boolean {
        return ageInt in MIN_AGE..MAX_AGE
    }
}

🤔 References

• https://qiita.com/koher/items/d9411a00986f14683a3f

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