The set method of the ToDoStorage class can throw ToDoAlreadyExistException:

operator fun set(name: String, todo: ToDo): Boolean {
    if (todos.contains(name)) {
        throw ToDoAlreadyExistException()
    }
    todos[name] = todo

    return true
}

It is also important to make sure that this exception will be thrown when it is needed. For this, we can write one more test that checks whether ToDoAlreadyException is thrown when a user tries to store a todo with a key that already exists.

We need to add a dependency to the kotlintest library (https://github.com/kotlintest/kotlintest) to be able to use the shouldThrow function. After integrating this library, your dependencies section in the build.gradle file will look as follows:

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:0.23.0'
    implementation "org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.6.1"
    testCompile 'org.jetbrains.spek:spek-api:1.1.5'
    testRuntime 'org.jetbrains.spek:spek-junit-platform-engine:1.1.5'
    testCompile 'org.junit.platform:junit-platform-runner:1.0.0'
    testCompile 'io.kotlintest:kotlintest-runner-junit5:3.1.7'
}

Let's add a test that checks whether ToDoAlreadyException is thrown:

on("set a todo with a key that already stored") {
    it ("should throw ToDoAlreadyExistException") {
        shouldThrow<ToDoAlreadyExistException> {
            val todo = ToDo("name", "content")
            storage.set("name", todo)
        }
    }
}

After running this, you will see the following window: