Calculate words frequencies in declarative way

WordsFrequencies.java

import java.io.BufferedReader;
import java.io.Reader;
import java.util.Map;
import java.util.function.Function;
import java.util.regex.Pattern;

import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;

public class WordsFrequencies {
	private static final Pattern WORDS_REGEX = Pattern.compile("[^\\w]+");
	private static final List<String> IGNORED_WORDS = Arrays.asList(
			"a", "an", "the", "in", "on", "at", "is", "are", "am", "as", "of", "if", "so");
	private final BufferedReader reader;

	public WordsFrequencies(Reader reader) {
		this.reader = reader instanceof BufferedReader
				? (BufferedReader) reader
				: new BufferedReader(reader);
	}

	public Map<String, Long> get() {
		return reader.lines()
				.map(String::toLowerCase)
				.flatMap(WORDS_REGEX::splitAsStream)
				.filter(w -> !IGNORED_WORDS.contains(w))
				.collect(groupingBy(Function.identity(), counting()));
	}
}