KnockOn app brings double-tap to lock to most Androids. and wake to ones with OLED displays

Anyhow, the app does what it advertises - when you tap two times in a quick succession on an empty homescreen space it locks your phone, and when you double-tap on a locked screen it wakes it up (AMOLED only). In everyday usage this has proven to be somewhat faster and more convenient than finding and pressing a physical side or home key. We've already shown you how to do the tap-to-lock thingy on any Android without root, but the process is somewhat convoluted, requiring a paid NOVA launcher, and the Greenify app.

Of course, an app like KnockOn isn't without bugs and kinks, but on most phones with AMOLED panels both gestures seem to work well. With the paid Pro version you can set both actions at once, or customize the double-tap speed.