36. Nested & Inner Classes

A class declared inside another is nested and has no link to the outer instance. Add the inner keyword and it carries a reference to the outer object, reachable via this@Outer.

class Outer {
    private val secret = "outer-data"

    // nested: NO access to Outer's members
    class Nested {
        fun describe() = "I'm nested and cannot see Outer's state"
    }

    // inner: holds a reference to the outer instance
    inner class Inner {
        fun describe() = "I can see: $secret (via this@Outer)"
    }
}

fun main() {
    println(Outer.Nested().describe())   // no Outer instance needed
    println(Outer().Inner().describe())  // needs an Outer instance
}

Nested is created straight from the class, while Inner must be created from an existing Outer instance — that is what gives it access to secret.

Running it:

$ kotlin run
I'm nested and cannot see Outer's state
I can see: outer-data (via this@Outer)

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