Java version of the unionWith algorithm – scala version here https://gist.github.com/3412529

Maps.java

import java.util.Map;
import java.util.Map.Entry;

import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableMap.Builder;
import com.google.common.collect.MapDifference;
import com.google.common.collect.MapDifference.ValueDifference;
import com.google.common.collect.Maps;

public final class Maps {
  public interface Combiner<K, A> {
    A combine(K k, A a, A b);
  }

  public interface Semigroup<A> {
    A append(A a, A b);
  }

  static <K, V> Semigroup<Map<K, V>> unionWithKey(final Combiner<K, V> semi) {
    return new Semigroup<Map<K, V>>() {
      public Map<K, V> append(Map<K, V> left, Map<K, V> right) {
        Builder<K, V> builder = ImmutableMap.builder();
        MapDifference<K, V> diff = Maps.difference(left, right);
        builder.putAll(diff.entriesOnlyOnLeft());
        builder.putAll(diff.entriesOnlyOnRight());
        for (Entry<K, V> e : diff.entriesInCommon().entrySet()) {
          K key = e.getKey();
          V value = e.getValue();
          builder.put(key, semi.combine(key, value, value));
        }
        for (Entry<K, ValueDifference<V>> e : diff.entriesDiffering().entrySet()) {
          K key = e.getKey();
          ValueDifference<V> value = e.getValue();
          builder.put(key, semi.combine(key, value.leftValue(), value.rightValue()));
        }
        return builder.build();
      }
    };
  }

  static <K, V> Semigroup<Map<K, V>> unionWith(final Semigroup<V> semi) {
    return unionWithKey(new Combiner<K, V>() {
      public V combine(K k, V a, V b) {
        return semi.append(a, b);
      };
    });
  }
}