This is just a really, quick and dirty post to help resolve this issue.
You’ve just installed Eclipse (5.2.x at the time of this writing), and attempted to create your first Android Application. When clicking create, you see an error similar to this:
Error: Unable to open class file R.java
The “Build Path” for your Java environment is not setup correctly in Eclipse. To fix this, follow these steps:
1) Open the Preferences Window by clicking Window -> Preferences
2) Find the Java section and expand it
3) Select Build Path
4) Change the value of Source and output folder from Folders to Project
5) Click Apply and then OK
Your problem should be resolved!