Reduction.java

// List-based generic reduction with wildcard type - Page 136
package org.effectivejava.examples.chapter05.item28;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Reduction {
	// Wildcard type for parameter that serves as an E producer
	static <E> E reduce(List<? extends E> list, Function<E> f, E initVal) {
		List<E> snapshot;
		synchronized (list) {
			snapshot = new ArrayList<E>(list);
		}
		E result = initVal;
		for (E e : snapshot)
			result = f.apply(result, e);
		return result;
	}

	private static final Function<Number> MAX = new Function<Number>() {
		public Number apply(Number n1, Number n2) {
			return Double.compare(n1.doubleValue(), n2.doubleValue()) > 0 ? n1
					: n2;
		}
	};

	public static void main(String[] args) {
		// We can use a Number functionto reduce a list of Integer or Double
		List<Integer> intList = Arrays.asList(2, 7, 1, 8, 2, 8, 1, 8, 2, 8);
		System.out.println(reduce(intList, MAX, Integer.MIN_VALUE));

		List<Double> doubleList = Arrays.asList(2.718281828, 3.141592654,
				1.61803399);
		System.out.println(reduce(doubleList, MAX, Double.NEGATIVE_INFINITY));
	}
}