Java8新特性——lambda函数式编程

一、遍历循环

/**
 * @author jiaqing.xu@hand-china.com
 * @version 1.0
 * @name
 * @description 循环遍历
 * @date 2018/7/13
 */
public class test1 {
    public static void main(String[] args) {
        String[] atp = {
                "b",
                "a",
                "c",
                "d",
                "e",
                "f",
                "g",
                "h"};
        List<String> players = Arrays.asList(atp);

        // 直接对list进行循环输出
        players.forEach((p) -> System.out.print(p + "; "));
        System.out.println("\n");
        //循环输出带换行符，方法引用使用双冒号
        players.forEach(System.out::println);

    }
}

二、数据过滤

/**
 * @author jiaqing.xu@hand-china.com
 * @version 1.0
 * @name
 * @description 数据过滤
 * @date 2018/7/13
 */
public class test2 {
    public static void main(String args[]) {
        List languages = Arrays.asList("Java", "Scala", "C++", "Haskell", "Lisp");
        //打印以J开头的字符串
        System.out.println("Print all languages :");
        filter1(languages, (str) -> str.toString().startsWith("J"));
        //条件恒为假
        System.out.println("Print no language : ");
        filter(languages, (str) -> false);
        //混合过滤条件
        System.out.println("...");
        filter3(languages);
    }

    //传统过滤
    public static void filter(List<String> names, Predicate condition) {
        for (String name : names) {
            if (condition.test(name)) {
                System.out.println(name + " ");
            }
        }
    }

    //以lambda方式过滤
    public static void filter1(List names, Predicate condition) {
        names.stream().filter((name) -> (condition.test(name))).forEach((name) -> System.out.println(name + " "));
    }

    public static void filter3(List names) {
        //第一个规则是以J开头
        Predicate<String> startsWithJ = (n) -> n.startsWith("J");
        //第二个规则是长度为4
        Predicate<String> fourLetterLong = (n) -> n.length() == 4;
        names.stream().filter(startsWithJ.and(fourLetterLong)).forEach((n) -> System.out.println("The result is:" + n));
    }
}

三、Map和Reduce函数计算

/**
 * @author jiaqing.xu@hand-china.com
 * @version 1.0
 * @name
 * @description map和reduce 函数计算
 * @date 2018/7/13
 */
public class test3 {
    public static void main(String[] args) {
        // With Lambda expression:
        //Map用于函数计算，为集合中的每个元素增加一定的数值
        List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500);
        costBeforeTax.stream().map((cost) -> (Integer) cost + .12 * (Integer) cost).forEach(System.out::println);

        //reduce 类似sql中的sum avg count
        List costBeforeTax2 = Arrays.asList(100, 200, 300, 400, 500);
        Object bill = costBeforeTax2.stream()
                .map((cost) -> (Integer) cost + .12 * (Integer) cost)
                .reduce((sum, cost) -> (Double) sum + (Double) cost)
                .get();
        System.out.println("Total : " + bill);

        //应用函数将字符串转换为大写形式并用逗号拼接
        List<String> G7 = Arrays.asList("USA", "Japan", "France", "Germany", "Italy", "U.K.", "Canada");
        String G7Countries = G7.stream()
                .map(x -> x.toUpperCase())
                .collect(Collectors.joining(", "));
        System.out.println(G7Countries);


        //计算list中最大值、最小值和平均值
        List<Integer> primes = Arrays.asList(2, 3, 5, 7, 11, 13, 17, 19, 23, 29);
        IntSummaryStatistics stats = primes.stream().mapToInt((x) -> x)
                .summaryStatistics();
        System.out.println("Highest prime number in List : " + stats.getMax());
        System.out.println("Lowest prime number in List : " + stats.getMin());
        System.out.println("Sum of all prime numbers : " + stats.getSum());
        System.out.println("Average of all prime numbers : " + stats.getAverage());
    }
}