Introduction to Kotlin (for Java Developers) - Cool Features - 7. Null safety.kt

class MyClass { // This is very helpful for preventing NPEs more easily.
    var field1: String = "hello" // null value NOT allowed
    var field2: String? = null // null value allowed

    fun myMethod() {
        // field1 = null // COMPILER ERROR
        field2 = "world"
        field2 = null
    }

    fun myMethod2() {
        // println(field2.length) // COMPILER ERROR

        // "Safe call" ( ?. )
        val len = field2?.length // null if field2 is null (otherwise equal to field2's length)
        println(len)
        /*
            Note: if we have a chain such as "bob?.department?.head?.name",
            the chain will return null if any of the properties are null.
         */

        // "Elvis operator" ( ?: )
        val myString = field2 ?: "some value" // "Give me field2's value if it's not null, otherwise use another value"
        val myStringLength = field2?.length ?: 5

        // "Asserted call" ( !! )
        val lengthOfField2 = field2!!.length // "I'm sure field2 is NOT null and even if it is, I'm OK with getting a NPE"

        // Smart cast
        val str: String? = null
        // println(str.length) // COMPILER ERROR
        if (str != null) {
            // If "str" is not null, Kotlin automatically knows that it will have "length", lowercase(), etc.
            println("${str.length} ${str.lowercase()} ${str.uppercase()}")
        }
    }
}