Listing tasks

Learn to print tasks formatted with listTasks.

Listing tasks: the listTasks method

Now we will add a listTasks method to TodoList that prints the tasks in a readable form. This method walks tasks with forEach and prints each task's format() output.

fun listTasks() {
    tasks.forEach { println(it.format()) }
}

Thanks to this method the main code stays simple: list.listTasks() writes all tasks in one line. Inside the forEach lambda each element is known as it.

it and forEach

Inside the forEach lambda the single parameter is automatically named it. In tasks.forEach { println(it.format()) }, it is the current Task; it.format() gives its readable output.

tasks.forEach { task ->
    println(task.format())
}

You can also give an explicit name: { task -> ... }. In nested lambdas where it is ambiguous, an explicit name improves readability.

Exercise Add and call the listTasks method

Add a fun listTasks() { tasks.forEach { println(it.format()) } } method to TodoList. In main create a TodoList, add two tasks, and print them with list.listTasks().

Show example solution

fun main() {
    val list = TodoList()
    list.addTask("Shopping")
    list.addTask("Homework")
    list.listTasks()
}

data class Task(val title: String, val done: Boolean = false) {
    fun format() = if (done) "[x] $title" else "[ ] $title"
}

class TodoList {
    val tasks = mutableListOf<Task>()
    fun addTask(title: String) {
        tasks.add(Task(title))
    }
    fun listTasks() {
        tasks.forEach { println(it.format()) }
    }
}

Test yourself

What should the return type of listTasks be?
- Unit (only prints)
- List<Task>
- Int (count)
- String
Show answer

Unit (only prints) — listTasks only prints; it returns no value, so it is Unit.
Match expression to role

listTasks()

Prints all tasks
forEach { }

Walks each task
it.format()

Task's output
println(...)

Writes to screen

Fill in the blank: walk each task

tasks.____ { println(it.format()) }

Show answer

tasks.forEach { println(it.format()) }

Next lesson: Completing tasks

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