lambda 10大常见用法

这次的总结是之前在java1.8发布之后从各大论坛和博客中总结的10种写法，基本囊括了所有开发中必备的技能，本文是为自己的知识储备做的总结，现在发布出来，希望可以帮到您

public class Main {

    public static void main(String[] args) {
        String[] players = {"Rafael Nadal", "Novak Djokovic",
                "Stanislas Wawrinka", "David Ferrer",
                "Roger Federer", "Andy Murray",
                "Tomas Berdych", "Juan Martin Del Potro",
                "Richard Gasquet", "John Isner"};
        Arrays.sort(players, new Comparator() {
            @Override
            public int compare(String s1, String s2) {
                return (s1.compareTo(s2));
            }
        });
    }

    /*public static void filter(List names, Predicate condition) {
        for(String name: names)  {
            if(condition.test(name)) {
                System.out.println(name + " ");
            }
        }
    }*/
    // 更好的办法
    public static void filter(List names, Predicate condition) {
        names.stream().filter((name) -> (condition.test(name))).forEach((name) -> {
            System.out.println(name + " ");
        });
    }
}

1.用lambda表达式实现Runnable

// Java 8之前：
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("Before Java8, too much code for too little to do");
            }
        }).start();
        //Java 8方式：
        new Thread( () -> System.out.println("In Java8, Lambda expression rocks !!") ).start();

2.使用Java 8 lambda表达式进行事件处理

// Java 8之前：
        JButton show =  new JButton("Show");
        show.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                System.out.println("Event handling without lambda expression is boring");
            }
        });
        // Java 8方式：
        show.addActionListener((e) -> {
            System.out.println("Light, Camera, Action !! Lambda expressions Rocks");
        });

3.使用lambda表达式对列表进行迭代

// Java 8之前：
        List features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API");
        for (String feature : features) {
            System.out.println(feature);
        }
        // Java 8之后：
        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
        features.forEach(System.out::println);

4.使用lambda表达式和函数式接口Predicate

List languages = Arrays.asList("Java", "Scala", "C++", "Haskell", "Lisp");

        System.out.println("Languages which starts with J :");
        filter(languages, (str)->str.startsWith("J"));

        System.out.println("Languages which ends with a ");
        filter(languages, (str)->str.endsWith("a"));

        System.out.println("Print all languages :");
        filter(languages, (str)->true);

        System.out.println("Print no language : ");
        filter(languages, (str)->false);

        System.out.println("Print language whose length greater than 4:");
        filter(languages, (str)->str.length() > 4);

5.如何在lambda表达式中加入Predicate

甚至可以用and()、or()和xor()逻辑函数来合并Predicate，

例如要找到所有以J开始，长度为四个字母的名字，你可以合并两个Predicate并传入

Predicate startsWithJ = (n) -> n.startsWith("J");
Predicate fourLetterLong = (n) -> n.length() == 4;
List names = new ArrayList();
names.stream()
    .filter(startsWithJ.and(fourLetterLong))
    .forEach((n) -> System.out.print("nName, which starts with 'J' and four letter long is : " + n));

6.Java 8中使用lambda表达式的Map和Reduce示例

// 不使用lambda表达式为每个订单加上12%的税
        List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500);
        for (Integer cost : costBeforeTax) {
            double price = cost + .12*cost;
            System.out.println(price);
        }
        // 使用lambda表达式
        costBeforeTax.stream().map((cost) -> cost + .12*cost).forEach(System.out::println);

6.2.Java 8中使用lambda表达式的Map和Reduce示例

// 为每个订单加上12%的税
// 老方法：
       double total = 0;
       for (Integer cost : costBeforeTax) {
           double price = cost + .12*cost;
           total = total + price;
       }
       System.out.println("Total : " + total);
       // 新方法：
       double bill = costBeforeTax.stream().map((cost) -> cost + .12*cost).reduce((sum, cost) -> sum + cost).get();
       System.out.println("Total : " + bill);

7.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 strList = Arrays.asList("abc","" ,"bcd","" , "defg", "jk");
       List filtered = strList.stream().filter(x -> x.length()> 2).collect(Collectors.toList());
       System.out.printf("Original List : %s, filtered list : %s %n", strList, filtered);

8.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 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);

9.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 numbers = Arrays.asList(9, 10, 3, 4, 7, 3, 4);
       List distinct = numbers.stream().map( i -> i*i).distinct().collect(Collectors.toList());
       System.out.printf("Original List : %s,  Square Without duplicates : %s %n", numbers, distinct);

10.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 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());