为什么函数的结果不能在Kotlin中的when语句中用作子句？

Question

I have the following (edited for length) Kotlin functions:

fun getType(obj: Any?): String {
    if (obj != null)
        println("$obj -> isArray:${isArray(obj)}")

    return when (obj) {
        null -> "null"
        [...]
        isArray(obj) -> "Array"
        else -> "Other object"
    }
}

private fun isArray(obj: Any): Any = 
        obj is Array<*> ||
        [...]
        obj is IntArray

When I assertEquals("Array", getType(intArrayOf(1,2,3,4))) I get the following output:

[I@1e25b76 -> isArray:true

org.junit.ComparisonFailure: 
Expected :Array
Actual   :Other object

So the isArray call is returning true , but it's not being recognised in the when clause. Why wouldn't it return "Array" here?

(I can work around this by putting this specific call before the when statement, but that's ugly)

Answer 1

You have two issues, first is that isArray needs to return Boolean and not Any .

The second problem is that you are using the form of when expression that has a fixed condition, obj and you are mixing that with the form of when that has no fixed condition and uses only boolean expressions. The documentation isn't clear on the difference.

Fixed condition when expression:

when(obj) {
    null -> "null"
    is Array<*>, is IntArray, is DoubleArray -> "Array"
    else -> "Other object"
}

versus what you need to use, boolean expressions in a when expression:

when {                            // <--- no (obj) here
    obj == null -> "null"
    isArray(obj) -> "Array"
    else -> "Other object"
}

Although a much simpler way to find an array is this:

obj.javaClass.isArray

So you can change your when expression to simply be:

when {
    obj == null -> "null"
    obj.javaClass.isArray -> "Array"
    else -> "Other object"
}

Answer 2

return when (obj) {
    null -> "null"
    [...]
    isArray(obj) -> "Array"
    else -> "Other object"
}

This seems to match the result of isArray(obj) (a Boolean ) to the obj itself. Therefore, it's making the comparison obj == true , which is false.

If you want a when expression that uses boolean statements, you can do:

return when {
    obj == null -> "null"
    [...]
    isArray(obj) -> "Array"
    else -> "Other object"
}

Of course, this will require you to write a lot of obj == x checks.