Fixed: R cannot be resolved to a variable

I encountered this problem when I was making a new project on my eclipse workspace. I got a red underline on "R" in this part of my code

setContentView(R.layout.main);

So here's what I did to fix this issue. Go to Project Properties > Java Build Path > TickAndroid Version Checkbox

Click to enlarge.

Then if your workspace does not build automatically…

Properties again > Build Project

After that, all went back to normal. :)