java8 Lambda 测试示例

import com.google.gson.Gson;
import org.junit.Test;

import java.util.Arrays;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.stream.Collectors;

/**
 * https://www.cnblogs.com/coprince/p/8692972.html
 */
public class LambdaTest {
    @Test
    public void testLambdaTest(){
        //用lambda表达式实现Runnable
        new Thread( () -> System.out.println("In Java8, Lambda expression rocks !!") ).start();
        /**
         * In Java8, Lambda expression rocks !!
         */

        List<String> features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API");
        //Comparator
//        List<String> featureSort  = features.stream().sorted((a,b) -> (a.compareTo(b))).collect(Collectors.toList());
        List<String> featureSort  = features.stream().sorted((a,b) -> (a.length() - b.length())).collect(Collectors.toList());
        System.out.println("featureSort=" + new Gson().toJson(featureSort));

        //使用lambda表达式对列表进行迭代

//        features.forEach(n -> System.out.println(n));
        // 使用Java 8的方法引用更方便，方法引用由::双冒号操作符标示，
        // 看起来像C++的作用域解析运算符  方法引用
        features.forEach(System.out::println);
        /**
         * Lambdas
         * Default Method
         * Stream API
         * Date and Time API
         */

        //使用lambda表达式和函数式接口Predicate  函数式编程  Predicate接口非常适用于做过滤。

        //Java 8中使用lambda表达式的Map和Reduce示例
        // 使用lambda表达式
        List<Integer> costBeforeTax = Arrays.asList(100, 200, 300, 400, 500);
        costBeforeTax.stream().map((cost) -> cost + 0.12*cost).forEach(System.out::println);
        /**
         * 112.0
         * 224.0
         * 336.0
         * 448.0
         * 560.0
         */

        //Java 8中使用lambda表达式的Map和Reduce示例
        // 新方法：
        List<Integer> costBeforeTax2 = Arrays.asList(100, 200, 300, 400, 500);
        double bill = costBeforeTax2.stream().map((cost) -> cost + 0.12*cost).reduce((sum, cost) -> sum + cost).get();
        System.out.println("Total : " + bill);
        /**
         * Total : 1680.0
         */

        //通过过滤创建一个String列表
        // 创建一个字符串列表，每个字符串长度大于2
        List<String> filtered = features.stream().filter(x -> x.length()> 2).collect(Collectors.toList());
        System.out.printf("Original List : %s, filtered list : %s %n", features, filtered);
        /**
         * Original List : [Lambdas, Default Method, Stream API, Date and Time API], filtered list : [Lambdas, Default Method, Stream API, Date and Time API]
         */

        //对列表的每个元素应用函数
        // 将字符串换成大写并用逗号链接起来
        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);
        /**
         * USA, JAPAN, FRANCE, GERMANY, ITALY, U.K., CANADA
         */

        //复制不同的值，创建一个子列表
        // 用所有不同的数字创建一个正方形列表
        List<Integer> numbers = Arrays.asList(9, 10, 3, 4, 7, 3, 4);
        List<Integer> distinct = numbers.stream().map( i -> i*i).distinct().collect(Collectors.toList());
        System.out.printf("Original List : %s,  Square Without duplicates : %s %n", numbers, distinct);
        /**
         * Original List : [9, 10, 3, 4, 7, 3, 4],  Square Without duplicates : [81, 100, 9, 16, 49]
         */

        //计算集合元素的最大值、最小值、总和以及平均值
        //获取数字的个数、最小值、最大值、总和以及平均值
        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());
        /**
         * Highest prime number in List : 29
         * Lowest prime number in List : 2
         * Sum of all prime numbers : 129
         * Average of all prime numbers : 12.9
         */
    }
}