Mapの要素をフィルターするのってめんどくさいよね

Sandbox.java

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.*;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.stream.Stream;

import org.junit.Test;

public class Sandbox {
	
	@Test
	public void mapfilter() {
		
		Map<String, Integer> map = new HashMap<>();
		map.put("a", 1);
		map.put("b", 2);
		map.put("c", 3);
		
		Map<String, Integer> result = Stream.of(map)
			.map(Map::entrySet)
			.flatMap(Set::stream)
			.filter(e -> (e.getValue() % 2) != 0)
			.reduce(
				new HashMap<String,Integer>(),
				(m, e) -> {m.put(e.getKey(),e.getValue()); return m;},
				(a, b) -> {a.putAll(b); return a;}
			);
		
		assertThat(result, allOf(hasEntry("a", 1), not(hasEntry("b", 2)), hasEntry("c", 3)));
	}
}

ぐれいつ