test_doubles__usecase.kt

interface History {
    fun push(url: URL)
    fun pop(): URL
    fun peek(): URL
}

class Browser(
    private val history: History
) {

    var activeURL: URL? = null
        private set

    fun visit(url: URL) {
        activeURL = if (url == URL("http://default"))
            history.peek() else
            url
        history.push(activeURL!!)
    }

    fun back() {
        history.pop()
        activeURL = history.peek()
    }
}