How to optimize imports automatically after each save in IntelliJ IDEA

In version 'Ultimate 2016.2' it is:

File > Other Settings > Default Settings > Auto Import > Optimize imports on the fly

IntelliJ IDEA offers an option to Optimize imports on the fly in Settings | Editor | General | Auto Import and Optimize imports option in the Commit Project dialog.

The mentioned answer works, but there is also the Intellij "save actions" plugin available from the JetBrains plugin repository that does that (as well as other things): Intellij Save Actions Plugin.

Installation

File > Settings > Plugins > Browse repositories... > Search 'Save Actions' > Category 'Code tools'

Disclaimer

I'm the plugin developer

First step:

CTRL + SHIFT + A or CMD + SHIFT + A (for mac)

Second step:

Search Optimize imports

Third step:

Switch ON the option