数值流

import java.util.Arrays;
import java.util.List;
import java.util.OptionalInt;
import java.util.stream.IntStream;
import java.util.stream.Stream;


/**
 * 数值流
 */
public class MapDemo3 {
    public static void main(String[] args){
        List<Apple> appleList = Arrays.asList(new Apple("red",12),new Apple("yellow",15),new Apple("green",10),new Apple("red",10));
        //映射到数值流
        //IntStream还支持其他的方便方法，如max、min、average等。
        int sumWeight = appleList.stream()
                .mapToInt(Apple::getWeight)
                .sum();
        System.out.println(sumWeight);
        //原始流转换成一般流
        IntStream intStream = appleList.stream()
                .mapToInt(Apple::getWeight);//将 Stream 转 换为数值流
        Stream<Integer> boxed = intStream.boxed();//将数值流转 换为Stream
        //默认值OptionalInt
        OptionalInt optionalInt = appleList.stream()
                .mapToInt(Apple::getWeight)
                .max();
        int max = optionalInt.orElse(-1);
        System.out.println(max);//如果没有最大值的话，显式提供一个默认最大值

        //数值范围
        //Java 8引入了两个可以用于IntStream和LongStream的静态方法，帮助生成这种范围: range和rangeClosed。这两个方法都是第一个参数接受起始值，第二个参数接受结束值。但 range是不包含结束值的，而rangeClosed则包含结束值
        IntStream intStream1 = IntStream.rangeClosed(1, 100)
                .filter(n -> n % 2 == 0);
        System.out.println(intStream1.count());//从1到100 偶数个数


    }
}

import java.util.stream.IntStream;
import java.util.stream.Stream;

/**
 * 勾股数
 */
public class MapDemo4 {
    public static void main(String[] args) {
        Stream<double[]> pythagoreanTriples = IntStream.rangeClosed(1, 100)//从rangeClosed返回的IntStream生成一个IntStream
                .boxed()//生成Stream<Integer>
                .flatMap(a ->  //flatMap把所有生成的三元数流扁平化成一个流
                        IntStream.rangeClosed(a, 100)
                                .mapToObj(b -> new double[]{a, b, Math.sqrt(a * a + b * b)})//返回一个对象值流
                                .filter(t -> t[2] % 1 == 0));//元组中的第三个元素必须是整数

        pythagoreanTriples.limit(5) //明确限定从生成的流 中要返回多少组勾股数
                .forEach(t->System.out.println(t[0] + ", " + t[1] + ", " + t[2]));

        //3.0, 4.0, 5.0
        //5.0, 12.0, 13.0
        //6.0, 8.0, 10.0
        //7.0, 24.0, 25.0
        //8.0, 15.0, 17.0
    }
}