TopTen.java

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class TopTen {

    public static void main(String[] args) {
        Arrays.stream(args)
                .flatMap(TopTen::fileLines)
                .flatMap(line -> Arrays.stream(line.split("\\b")))
                .map(word -> word.replaceAll("[^a-zA-Z]", ""))
                .filter(word -> word.length() > 0)
                .map(word -> word.toLowerCase())
                .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
                .entrySet().stream()
                .sorted((a, b) -> -a.getValue().compareTo(b.getValue()))
                .limit(10)
                .forEach(e -> System.out.format("%s = %d%n", e.getKey(), e.getValue()));
    }

    private static Stream<String> fileLines(String path) {
        try {
            return Files.lines(Paths.get(path));
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

}