java8 lambda表达式和stream

野性酷女 2022-04-11 09:54 213阅读 0赞

## java8 lambda表达式 ##

### 1. Lambda表达式的语法 ###

> 基本语法:  
> (parameters) -> expression  
> 或  
> (parameters) ->\{ statements; \}
> 
> // 1. 不需要参数,返回值为 5  
> () -> 5
> 
> // 2. 接收一个参数(数字类型),返回其2倍的值  
> x -> 2 \* x
> 
> // 3. 接受2个参数(数字),并返回他们的差值  
> (x, y) -> x – y
> 
> // 4. 接收2个int型整数,返回他们的和  
> (int x, int y) -> x + y
> 
> // 5. 接受一个 string 对象,并在控制台打印,不返回任何值(看起来像是返回void)  
> (String s) -> System.out.print(s)

### 2. Lambda表达式的基本实例 ###

*  lambda的循环遍历

String[] atp = {
                "Rafael Nadal", "Novak Djokovic",
                "Stanislas Wawrinka",
                "David Ferrer", "Roger Federer",
                "Andy Murray", "Tomas Berdych",
                "Juan Martin Del Potro"
        };
    
        @Test
        public void testForeach() {
            List<String> players = Arrays.asList(atp);
            //使用lambda表达对list进行迭代
            players.forEach(player -> System.out.println(player));
            System.out.println("--------------------");
            // 在 Java 8 中使用双冒号操作符(double colon operator)
            players.forEach(System.out::println);
        }

*  lambda字符串排序

/**
         * 测试lambda表达式在列表排序中的应用
         */
        @Test
        public void testSort() {
    
    
            //之前常用的排序算法
           /* Arrays.sort(atp, new Comparator<String>() {
                @Override
                public int compare(String o1, String o2) {
                    return o1.compareTo(o2);
                }
            });
    */
    
    //        Arrays.sort(atp, (String s1, String s2) -> (s1.compareTo(s2)));
    //        List<String> players = Arrays.asList(atp);
            System.out.println("-------");
    
            Comparator<String> sort = (String s1, String s2) -> (s1.compareTo(s2));
            Arrays.sort(atp, sort);
            List<String> players = Arrays.asList(atp);
            players.forEach(player -> System.out.println(player));
        }

### 3. Lambdas 和streams结合 ###

Stream是对集合的包装,通常和lambda一起使用。 使用lambdas可以支持许多操作,如 map, filter, limit, sorted, count, min, max, sum, collect 等等，Stream使用**懒运算**,他们并不会真正地读取所有数据,只在使用到相应数据才进行加载！

@Test
        public void lambdaAndStreamTest() {
            List<Person> javaProgrammers = new ArrayList<Person>() {
                {
                    add(new Person("Elsdon", "Jaycob", "Java programmer", "male", 43, 2000));
                    add(new Person("Tamsen", "Brittany", "Java programmer", "female", 23, 1500));
                    add(new Person("Floyd", "Donny", "Java programmer", "male", 33, 1800));
                    add(new Person("Sindy", "Jonie", "Java programmer", "female", 32, 1600));
                    add(new Person("Vere", "Hervey", "Java programmer", "male", 22, 1200));
                    add(new Person("Maude", "Jaimie", "Java programmer", "female", 27, 1900));
                    add(new Person("Shawn", "Randall", "Java programmer", "male", 30, 2300));
                    add(new Person("Jayden", "Corrina", "Java programmer", "female", 35, 1700));
                    add(new Person("Palmer", "Dene", "Java programmer", "male", 33, 2000));
                    add(new Person("Addison", "Pam", "Java programmer", "female", 34, 1300));
                }
            };
    
            List<Person> phpProgrammers = new ArrayList<Person>() {
                {
                    add(new Person("Jarrod", "Pace", "PHP programmer", "male", 34, 1550));
                    add(new Person("Clarette", "Cicely", "PHP programmer", "female", 23, 1200));
                    add(new Person("Victor", "Channing", "PHP programmer", "male", 32, 1600));
                    add(new Person("Tori", "Sheryl", "PHP programmer", "female", 21, 1000));
                    add(new Person("Osborne", "Shad", "PHP programmer", "male", 32, 1100));
                    add(new Person("Rosalind", "Layla", "PHP programmer", "female", 25, 1300));
                    add(new Person("Fraser", "Hewie", "PHP programmer", "male", 36, 1100));
                    add(new Person("Quinn", "Tamara", "PHP programmer", "female", 21, 1000));
                    add(new Person("Alvin", "Lance", "PHP programmer", "male", 38, 1600));
                    add(new Person("Evonne", "Shari", "PHP programmer", "female", 40, 1800));
                }
            };
    
            System.out.println("输出所有程序员的姓名：");
            javaProgrammers.forEach((person) -> System.out.printf("%s %s;", person.getFirstName(), person.getLastNmae()));
            System.out.println();
            phpProgrammers.forEach((person) -> System.out.printf("%s %s;", person.getFirstName(), person.getLastNmae()));
    
            System.out.println("给程序员加薪5%");
            Consumer<Person> getRais = e -> e.setSalary(e.getSalary() / 100 * 5 + e.getSalary());
    
            javaProgrammers.forEach(getRais);
            phpProgrammers.forEach(getRais);
    
            javaProgrammers.forEach((person) -> System.out.println(person.getFirstName() + " " + person.getLastNmae()  +" " + person.getSalary()));
            System.out.println();
            phpProgrammers.forEach((person) -> System.out.println(person.getFirstName() + " " + person.getLastNmae() + " " + person.getSalary()));
    
            // stream 中的filter来过滤数据
            System.out.println("显示月薪超过1400美元的php程序员");
            phpProgrammers.stream().filter((person -> (person.getSalary() > 1400))).forEach((person -> System.out.printf("%s %s，", person.getFirstName(), person.getLastNmae())));
    
     //使用limit方法，来限制结果的个数
            System.out.println("输出女java程序员的前两个");
    
            javaProgrammers.stream().filter(genderFilter).limit(2).forEach(person -> System.out.println(person.getFirstName() + " " + person.getLastNmae()));
    
            //根据 name 排序,并显示前5个 Java programmers
            List<Person> sortedJavaProgrammers = javaProgrammers.stream().sorted((p1, p2) -> (p1.getFirstName()).compareTo(p2.getFirstName())).limit(2).collect(Collectors.toList());
            System.out.println(sortedJavaProgrammers);
            List<Person> sortedJavaBySalary = javaProgrammers.stream().sorted((p1, p2) -> (p1.getSalary() - p2.getSalary())).limit(2).collect(Collectors.toList());
            System.out.println("按照工资排序的前两个" + sortedJavaBySalary);
    
            //如果我们只对最低和最高的薪水感兴趣,比排序后选择第一个/最后一个 更快的是min和max方法:
            System.out.println("工资最低的java programmer");
            Person person = javaProgrammers.stream().min((p1, p2) -> (p1.getSalary() - p2.getSalary())).get();
            System.out.println("工资 最低的java程序员是：" + person);
    
            System.out.println("工资最好的java程序员是：");
            Person person1 = javaProgrammers.stream().max((p1, p2) -> (p1.getSalary() - p2.getSalary())).get();
            System.out.println(person1);
        }

结合 map 方法,我们可以使用 collect 方法来将我们的结果集放到一个字符串,一个 Set 或一个TreeSet中.以及在streams中并行的计算数据

@Test
        public void sortTest() {
            System.out.println("将PHP programmers 的first name 拼接成字符串：");
            String phpDevlopers = phpProgrammers.stream().map(Person::getFirstName).collect(Collectors.joining(";"));
            System.out.println(phpDevlopers);
            System.out.println("将PHP programmers 的first name 放入set：");
            Set<String> javaDevloperFirstName = javaProgrammers.stream().map(Person::getFirstName).collect(Collectors.toSet());
            System.out.println(javaDevloperFirstName);
    
            System.out.println("将PHP programmers 的first name 放入Treeset：");
            TreeSet<String> javaProgram =
                    javaProgrammers.stream().map(Person::getFirstName).collect(Collectors.toCollection(TreeSet::new));
            System.out.println(javaProgram);
    
            //Streams 还可以是并行的(parallel)计算
            System.out.println("计算付给JavaProgrammers的所有money:");
            int totalsalary = javaProgrammers.parallelStream().mapToInt(p -> p.getSalary()).sum();
            System.out.println(totalsalary);
    }

我们可以使用summaryStatistics方法获得stream 中元素的各种汇总数据。 接下来,我们可以访问这些方法,比如getMax, getMin, getSum或getAverage:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
            IntSummaryStatistics stats = numbers.stream().mapToInt(x -> x).summaryStatistics();
            System.out.println("List中最大的数字：" + stats.getMax());
            System.out.println("list中最小的数字：" + stats.getMin());
            System.out.println("所有的数字总和:" + stats.getSum());
            System.out.println("所有数字平均值：" + stats.getAverage());

原文链接：[https://blog.csdn.net/renfufei/article/details/24600507][https_blog.csdn.net_renfufei_article_details_24600507]

[https_blog.csdn.net_renfufei_article_details_24600507]: https://blog.csdn.net/renfufei/article/details/24600507