What is the difference between String? and String! in Kotlin-CodePudding

I want to know the difference between String? and String! in Kotlin. I search on kotlinlang.org but did not find any information.

CodePudding user response：

Kotlin's type system differentiates between

fun testStringTypes() {
    val someStringFromJava = PlatformTypeTest().returnSomeStringCouldBeNull()
    someStringFromJav
}

As we can see from above two screenshots, IDE is reminding us this String from Java can be null.

And for String!, we can access it in different ways:

fun main() {
    val someStringFromJava = PlatformTypeTest().returnSomeStringCouldBeNull()
    var lenOfString = someStringFromJava?.length ?: 0
//    lenOfString = someStringFromJava.length // NullPointerException
//    lenOfString = someStringFromJava!!.length // NullPointerException
    println(lenOfString)
}

With code snippet above, it works fine with var lenOfString = someStringFromJava?.length ?: 0, but the other two ways will cause NPE, as explained at above.