感受 lambda 之美！

1.1 java8自带的常用函数式接口。

public class Test {
    public static void main(String[] args) {
        Predicate<Integer> predicate = x -> x > 185;
        Student student = new Student("9龙", 23, 175);
        System.out.println(
            "9龙的身高高于185吗？：" + predicate.test(student.getStature()));

        Consumer<String> consumer = System.out::println;
        consumer.accept("命运由我不由天");

        Function<Student, String> function = Student::getName;
        String name = function.apply(student);
        System.out.println(name);

        Supplier<Integer> supplier = 
            () -> Integer.valueOf(BigDecimal.TEN.toString());
        System.out.println(supplier.get());

        UnaryOperator<Boolean> unaryOperator = uglily -> !uglily;
        Boolean apply2 = unaryOperator.apply(true);
        System.out.println(apply2);

        BinaryOperator<Integer> operator = (x, y) -> x * y;
        Integer integer = operator.apply(2, 3);
        System.out.println(integer);

        test(() -> "我是一个演示的函数式接口");
    }

    /**
     * 演示自定义函数式接口使用
     *
     * @param worker
     */
    public static void test(Worker worker) {
        String work = worker.work();
        System.out.println(work);
    }

    public interface Worker {
        String work();
    }
}
//9龙的身高高于185吗？：false
//命运由我不由天
//9龙
//10
//false
//6
//我是一个演示的函数式接口

2、常用的流

2.1 collect(Collectors.toList())

将流转换为list。还有toSet()，toMap()等。及早求值。

public class TestCase {
    public static void main(String[] args) {
        List<Student> studentList = Stream.of(new Student("路飞", 22, 175),
                new Student("红发", 40, 180),
                new Student("白胡子", 50, 185)).collect(Collectors.toList());
        System.out.println(studentList);
    }
}
//输出结果
//[Student{name='路飞', age=22, stature=175, specialities=null}, 
//Student{name='红发', age=40, stature=180, specialities=null}, 
//Student{name='白胡子', age=50, stature=185, specialities=null}]

2.2 filter

public class TestCase {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>(3);
        students.add(new Student("路飞", 22, 175));
        students.add(new Student("红发", 40, 180));
        students.add(new Student("白胡子", 50, 185));

        List<Student> list = students.stream()
            .filter(stu -> stu.getStature() < 180)
            .collect(Collectors.toList());
        System.out.println(list);
    }
}
//输出结果
//[Student{name='路飞', age=22, stature=175, specialities=null}]

2.3 map

public class TestCase {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>(3);
        students.add(new Student("路飞", 22, 175));
        students.add(new Student("红发", 40, 180));
        students.add(new Student("白胡子", 50, 185));

        List<String> names = students.stream().map(student -> student.getName())
                .collect(Collectors.toList());
        System.out.println(names);
    }
}
//输出结果
//[路飞, 红发, 白胡子]

2.4 flatMap

public class TestCase {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>(3);
        students.add(new Student("路飞", 22, 175));
        students.add(new Student("红发", 40, 180));
        students.add(new Student("白胡子", 50, 185));

        List<Student> studentList = Stream.of(students,
                asList(new Student("艾斯", 25, 183),
                        new Student("雷利", 48, 176)))
                .flatMap(students1 -> students1.stream()).collect(Collectors.toList());
        System.out.println(studentList);
    }
}
//输出结果
//[Student{name='路飞', age=22, stature=175, specialities=null}, 
//Student{name='红发', age=40, stature=180, specialities=null}, 
//Student{name='白胡子', age=50, stature=185, specialities=null}, 
//Student{name='艾斯', age=25, stature=183, specialities=null},
//Student{name='雷利', age=48, stature=176, specialities=null}]

2.5 max和min

我们经常会在集合中求最大或最小值，使用流就很方便。及早求值。

public class TestCase {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>(3);
        students.add(new Student("路飞", 22, 175));
        students.add(new Student("红发", 40, 180));
        students.add(new Student("白胡子", 50, 185));

        Optional<Student> max = students.stream()
            .max(Comparator.comparing(stu -> stu.getAge()));
        Optional<Student> min = students.stream()
            .min(Comparator.comparing(stu -> stu.getAge()));
        //判断是否有值
        if (max.isPresent()) {
            System.out.println(max.get());
        }
        if (min.isPresent()) {
            System.out.println(min.get());
        }
    }
}
//输出结果
//Student{name='白胡子', age=50, stature=185, specialities=null}
//Student{name='路飞', age=22, stature=175, specialities=null}

2.6 count

统计功能，一般都是结合filter使用，因为先筛选出我们需要的再统计即可。及早求值

public class TestCase {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>(3);
        students.add(new Student("路飞", 22, 175));
        students.add(new Student("红发", 40, 180));
        students.add(new Student("白胡子", 50, 185));

        long count = students.stream().filter(s1 -> s1.getAge() < 45).count();
        System.out.println("年龄小于45岁的人数是：" + count);
    }
}
//输出结果
//年龄小于45岁的人数是：2

2.7 reduce

reduce 操作可以实现从一组值中生成一个值。在上述例子中用到的 count 、 min 和 max 方法，因为常用而被纳入标准库中。事实上，这些方法都是 reduce 操作。及早求值。

public class TestCase {
    public static void main(String[] args) {
        Integer reduce = Stream.of(1, 2, 3, 4).reduce(0, (acc, x) -> acc+ x);
        System.out.println(reduce);
    }
}
//输出结果
//10

三、高级集合类及收集器

3.1 转换成值

收集器，一种通用的、从流生成复杂值的结构。只要将它传给 collect 方法，所有的流就都可以使用它了。标准类库已经提供了一些有用的收集器，以下示例代码中的收集器都是从 java.util.stream.Collectors 类中静态导入的。

public class CollectorsTest {
    public static void main(String[] args) {
        List<Student> students1 = new ArrayList<>(3);
        students1.add(new Student("路飞", 23, 175));
        students1.add(new Student("红发", 40, 180));
        students1.add(new Student("白胡子", 50, 185));

        OutstandingClass ostClass1 = new OutstandingClass("一班", students1);
        //复制students1，并移除一个学生
        List<Student> students2 = new ArrayList<>(students1);
        students2.remove(1);
        OutstandingClass ostClass2 = new OutstandingClass("二班", students2);
        //将ostClass1、ostClass2转换为Stream
        Stream<OutstandingClass> classStream = Stream.of(ostClass1, ostClass2);
        OutstandingClass outstandingClass = biggestGroup(classStream);
        System.out.println("人数最多的班级是：" + outstandingClass.getName());

        System.out.println("一班平均年龄是：" + averageNumberOfStudent(students1));
    }

    /**
     * 获取人数最多的班级
     */
    private static OutstandingClass biggestGroup(Stream<OutstandingClass> outstandingClasses) {
        return outstandingClasses.collect(
                maxBy(comparing(ostClass -> ostClass.getStudents().size())))
                .orElseGet(OutstandingClass::new);
    }

    /**
     * 计算平均年龄
     */
    private static double averageNumberOfStudent(List<Student> students) {
        return students.stream().collect(averagingInt(Student::getAge));
    }
}
//输出结果
//人数最多的班级是：一班
//一班平均年龄是：37.666666666666664

maxBy或者minBy就是求最大值与最小值。

3.2 转换成块

常用的流操作是将其分解成两个集合，Collectors.partitioningBy帮我们实现了，接收一个Predicate函数式接口。

将示例学生分为会唱歌与不会唱歌的两个集合。

public class PartitioningByTest {
    public static void main(String[] args) {
        //省略List<student> students的初始化
        Map<Boolean, List<Student>> listMap = students.stream().collect(
            Collectors.partitioningBy(student -> student.getSpecialities().
                                      contains(SpecialityEnum.SING)));
    }
}

3.3 数据分组

数据分组是一种更自然的分割数据操作，与将数据分成 ture 和 false 两部分不同，可以使用任意值对数据分组。Collectors.groupingBy接收一个Function做转换。

例子：根据学生第一个特长进行分组

public class GroupingByTest {
    public static void main(String[] args) {
        //省略List<student> students的初始化
         Map<SpecialityEnum, List<Student>> listMap = 
             students.stream().collect(
             Collectors.groupingBy(student -> student.getSpecialities().get(0)));
    }
}

Collectors.groupingBy与SQL 中的 group by 操作是一样的。

3.4 字符串拼接

如果将所有学生的名字拼接起来，怎么做呢？通常只能创建一个StringBuilder，循环拼接。使用Stream，使用Collectors.joining()简单容易。

public class JoiningTest {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>(3);
        students.add(new Student("路飞", 22, 175));
        students.add(new Student("红发", 40, 180));
        students.add(new Student("白胡子", 50, 185));

         String names = students.stream()
             .map(Student::getName).collect(Collectors.joining(",","[","]"));
        System.out.println(names);
    }
}
//输出结果
//[路飞,红发,白胡子]

joining接收三个参数，第一个是分界符，第二个是前缀符，第三个是结束符。也可以不传入参数Collectors.joining()，这样就是直接拼接。

https://mp.weixin.qq.com/s/O3uiIJ6vRNWth0RLwEo_0w