JAVA8函数式编程-简化编程代码方法介绍

一.常用的流

1.1Collect

将流转换为List、Set、Map等

public class Test2 {
    public static void main(String[] args) {
        
        //转换成list
        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);


        //转换成Map  注：存在key相同的情况需要设置策略 如下代码key相同保留最后一次value值
        Map<String, Student> map = Stream.of(new Student("路飞", 22, 175), new Student("路飞", 25, 195),
                new Student("红发", 40, 180),
                new Student("白胡子", 50, 185)).collect(Collectors.toMap(Student::getName, Function.identity(),(oldValue,newValue)->newValue));
        System.out.println(map);
        
    }
}

打印结果：

[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}]

{白胡子=Student{name='白胡子', age=50, stature=185, specialities=null}, 红发=Student{name='红发', age=40, stature=180, specialities=null}, 路飞=Student{name='路飞', age=25, stature=195, specialities=null}}

1.2Filter

顾名思义，起过滤筛选的作用。内部就是Predicate接口。惰性求值。

        //筛选出集合中对象的年龄小于等于40的
        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(scu -> scu.getAge() <= 40)
                .collect(Collectors.toList());
        System.out.println(list);

输出结果：

[Student{name='路飞', age=22, stature=175, specialities=null}, Student{name='红发', age=40, stature=180, specialities=null}]

1.3Map

转换功能，内部就是Function接口。惰性求值

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]

1.4FlatMap

将多个Stream合并为一个Stream。惰性求值

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}]

1.5Max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。

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='红发', age=40, stature=180, specialities=null}
Student{name='路飞', age=22, stature=175, specialities=null}

1.6Count

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

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

1.7Reduce

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。

 Integer reduce = Stream.of(1, 2, 3, 4).reduce(0, (acc, x) -> acc+ x);
 System.out.println(reduce);

输出结果：

10