Rename '_' variables for Java 8+ compatibility.

Compiling with JDK 8 reported:

    warning: '_' used as an identifier
    (use of '_' as an identifier might not be supported in releases after Java SE 8)

See [this thread](http://openjdk.5641.n7.nabble.com/Warning-about-single-underscore-identifier-td145974.html) for more info.

Also optimized the import statements.