Accumulating Map Values in Java 8

AccumulatingMapValues.java

// Uses a function instance with internal state, along with mapping the map values only.
// see: http://stackoverflow.com/questions/23213891/how-to-map-values-in-a-map-in-java-8

import com.google.common.collect.ImmutableMap;

import java.util.Map;
import java.util.function.Function;

import static java.util.stream.Collectors.toMap;

// A map instance to be accumulated
Map<String, Integer> before = new ImmutableMap.Builder<String, Integer>()
	.put("A", 1)
	.put("B", 2)
	.put("C", 3)
	.build();

// Accumulator function with its internal state
Function<Integer, Integer> f = new Function<Integer, Integer>() {
	private int acc = 0;

	@Override
	public Integer apply(Integer integer) {
		acc += integer;
		return acc;
	}
};

// The result
Map<String, Integer> after = before.entrySet()
	.stream()
	.collect(toMap(Map.Entry::getKey, entry -> f.apply(entry.getValue())));