Android Studio : unmappable character for encoding UTF-8

I had the same problem because there was files with windows-1251 encoding and Cyrillic comments. In Android Studio which is based on IntelliJ IDEA you can solve it in two ways:

a) convert file encoding to UTF-8 or

b) set the right file encoding in your build.gradle script:

android {
    ...
    compileOptions.encoding = 'windows-1251' // write your encoding here
    ...

To convert file encoding use the menu at the bottom right corner of IDE. Select right file encoding first -> press Reload -> select UTF-8 -> press Convert.

Also read this Use the UTF-8, Luke! File Encodings in IntelliJ IDEA