why do i get `> java.lang.NullPointerException ...

# getting-started

[Smallville7123]

why do i get > java.lang.NullPointerException (no error message) for v[0]!!::class.objectInstance

[serebit]

Read the docs for objectInstance and it'll tell you why

[Smallville7123]

ok

[Smallville7123]

The instance of the object declaration, or null if this class is not an object declaration.

[serebit]

yep. is v[0] an object, or is it an instance of a class?

[Smallville7123]

im passing it f which is val f = arrayListOf<a>()

[Smallville7123]

which is

private class a {
    var empty : Int = 0
}

[serebit]

that... doesn't answer my question

[serebit]

if you're just starting out with kotlin, you should try the koans to get a handle on things before jumping in and trying random things like this

[Smallville7123]

v[0] is an instance of a class

[serebit]

so then objectInstance is going to be null, and if you don't handle that before passing it to a Java function, it might throw a NullPointerException.

[Smallville7123]

why would it be null?

[serebit]

The instance of the object declaration, or null if this class is not an object declaration.

[serebit]

it says it right there. if it's an object, it'll return the object, if it's a class, it'll return null.

[Smallville7123]

ok, so how do i return the class itself?

[Smallville7123]

for example, v.add(a() as E)

[Smallville7123]

doing v.add(v[0]!!.javaClass as E) gives me java.lang.Class cannot be cast to preprocessor.utils.core.a

[karelpeeters]

You won't get back an E, you'll get back a Class<E>.