Java8新特性——lambda表达式.（案例：完全数分类）

需求:输入一个数,判断其类型(完全数,过剩数,不足数)

完全数:自身之外所有因数和==自身

过剩数:自身之外所有因数和>自身

不足数:自身之外所有因数和<自身

package cn._3.numberclassifier;

//BEGIN number_classifier_java8
import java.util.List;
import java.util.stream.IntStream;

import static java.lang.Math.sqrt;
import static java.util.stream.Collectors.toList;
import static java.util.stream.IntStream.range;

public class NumberClassifier {

    // BEGIN java8_filter
    public static IntStream factorsOf(int number) {
        return range(1, number + 1).filter(potential -> number % potential == 0);
    }
    // END java8_filter
    public static int aliquotSum(int number) {
        return factorsOf(number).sum() - number;
    }
    public static boolean isPerfect(int number) {
        return aliquotSum(number) == number;
    }
    public static boolean isAbundant(int number) {
        return aliquotSum(number) > number;
    }
    public static boolean isDeficient(int number) {
        return aliquotSum(number) < number;
    }
    // END number_classifier_java8

    // BEGIN java8_filter_fast
    public static List<Integer> fastFactorsOf(int number) {
        List<Integer> factors = range(1, (int) (sqrt(number) + 1))
                .filter(potential -> number % potential == 0).boxed()// 返回stream<Integer>
                .collect(toList());// 折叠/化约,reduce
        List<Integer> factorsAboveSqrt = factors.stream().map(e -> number / e).collect(toList());
        factors.addAll(factorsAboveSqrt);
        //将stream变为ArrayList
        return factors.stream().distinct().collect(toList());
    }
    // END java8_filter_fast
}

测试类:

package cn._3.numberclassifier;

public class Mmain {

    public static void main(String[] args) {
        Integer number = 666;
        System.out.println(NumberClassifier.isPerfect(number)+","+NumberClassifier.isAbundant(number)+","+NumberClassifier.isDeficient(number));
    }
}