using Guava

Hoge.java

package net.elliptium;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

import com.google.common.base.Function;
import com.google.common.base.Predicate;
import com.google.common.collect.Collections2;

public class Hoge {

	static class Fuga {
		final String color;
		final int weight;

		Fuga(String color, int weight) {
			this.color = color;
			this.weight = weight;
		}
	}

	private static class FugaColorFilter implements Predicate<Fuga> {

		String color;

		public FugaColorFilter(String color) {
			this.color = color;
		}

		public boolean apply(Fuga fuga) {
			return this.color.equals(fuga.color);
		}

		@Override
		public boolean equals(Object object) {
			if (!(object instanceof Fuga)) {
				return false;
			}

			Fuga that = (Fuga) object;
			return this.color.equals(that.color);
		}
	}

	private static class FugaWeightMapper implements Function<Fuga, Integer> {

		public Integer apply(Fuga input) {
			return (Integer) input.weight;
		}

		@Override
		public boolean equals(Object object) {
			return object instanceof FugaWeightMapper;
		}

	}

	private interface FoldFunction<T, F> {

		T apply(T acc, F object);

		boolean equals(Object object);

	}

	private static class Collections3 {

		static <T, F> T fold(Collection<F> unfolded, T initialValue, FoldFunction<T, F> foldFunction) {

			T result = initialValue;
			for (F element : unfolded) {
				result = foldFunction.apply(result, element);
			}
			return result;

		}
	}

	private static class IntegerAddition implements FoldFunction<Integer, Integer> {

		public Integer apply(Integer acc, Integer object) {
			return acc + object;
		}

		@Override
		public boolean equals(Object object) {
			return object instanceof IntegerAddition;
		}

	}

	public static void main(String... args) {
		List<Fuga> list = new ArrayList<Fuga>();
		list.add(new Fuga("blue", 10));
		list.add(new Fuga("red", 30));
		list.add(new Fuga("blue", 50));

		// Guava で書くとこう. 一部自作あり.
		int weight =
			Collections3.fold(
				Collections2.transform(
					Collections2.filter(list, new FugaColorFilter("blue")),
					new FugaWeightMapper()),
				0,
				new IntegerAddition());

		System.out.println(weight);
	}
}