Here is an example:

    val a: Int = 6
    val b = a.toLong()
    val c = a as Long

what is difference between .toLong() and as Long keyword? And what is better way to use

CodePudding user response：

a.toLong() calls the toLong method on a, so it will do whatever the toLong method does. This method is usually implemented with native code. For example, on the JVM, it can be implemented with the i2l JVM instruction.

The as operator on the other hand, does the following, according to the language spec:

This expression perform a runtime check whether the runtime type of E is a subtype of T and throws an exception otherwise.

So as only does a check at runtime. The "conversion" happens at compile time only, by means of the language mandating that the type of the expression e as T must be T.

As far as Kotlin's type system is concerned, Int is not a subtype of Long, so this check will always fail, and this expression will always throw an exception. Note that this is different from Java's type system. where the primitive int is a subtype of the primitive long.

CodePudding user response：

Using as casts that object to that type. It only works when it actually is that type. You'll see that your code actually crashes there where you try to cast the Int to a Long. The toLong() function actually turns it into a Long.