## Mastering the IDE: Completion

In Kotlin, it's easy to explore classes using IntelliJ IDEA, which has *code
completion*. IDEA attempts to complete what you're typing and show you a list
of options. After you type the dot, IDEA suggests members which can be called
on a given element. You can also call completion explicitly by pressing
<span class="shortcut">&shortcut:CodeCompletion;</span>.

Use <span class="shortcut">&shortcut:EditorDown;</span> and
<span class="shortcut">&shortcut:EditorUp;</span> to select the required member
and press <span class="shortcut">&shortcut:EditorEnter;</span> to paste it.

<span class="shortcut">&shortcut:CodeCompletion;</span> discovers all possible
operations for a class. You can type the first letters of a member function or
property to see all the available members starting with these letters, or
simply containing these letters.

Type the identifier name `r` followed by a dot, then press
<span class="shortcut">&shortcut:CodeCompletion;</span> to see
the possible completions. Choose `r.sum()` and `s.reversed()` by typing only
the first letters and using code completion.