Java8中Stream的归约与收集

Java8中Stream的归约与收集

归约

reduce(T identity, BinaryOperator) / reduce(BinaryOperator) --可以将流中元素反复结合起来，得到一个值

public class Employee {

    private String name;
    private Integer age;
    private Double salary;
    private Status status;

    public Employee() {
        super();
    }

    public  Employee(Integer age){
        this.age = age;
    }

    public Employee(String name, Integer age, Double salary) {
        super();
        this.name = name;
        this.age = age;
        this.salary = salary;
    }

    public Employee(String name, Integer age, Double salary, Status status) {
        this.name = name;
        this.age = age;
        this.salary = salary;
        this.status = status;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(Double salary) {
        this.salary = salary;
    }

    public Status getStatus() {
        return status;
    }

    public void setStatus(Status status) {
        this.status = status;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "name=‘" + name + ‘\‘‘ +
                ", age=" + age +
                ", salary=" + salary +
                ", status=" + status +
                ‘}‘;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Employee employee = (Employee) o;
        return age == employee.age &&
                Double.compare(employee.salary, salary) == 0 &&
                Objects.equals(name, employee.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age, salary);
    }

    public enum Status{
        FREE,
        BUSY,
        VOCATION;
    }
}

//归约
    @Test
    public void test1(){
        List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

        Integer sum = list.stream()
                .reduce(0, (x, y) -> x + y);
        System.out.println(sum);//首先把0作为x，然后从流中取出一个元素作为y，然后把运算结果作为x，再取出下一个元素作为y

        System.out.println("--------------------------------------");

        Optional<Double> op = employees.stream()
                .map(Employee::getSalary)
                .reduce(Double::sum);
        System.out.println(op.get());
    }

收集

collect---将流转换为其他形式。接收一个Collector接口的实现，用于给Stream中元素做汇总的方法

//收集
    @Test
    public void test2(){
        List<String> list = employees.stream()
                .map(Employee::getName)
                .collect(Collectors.toList());

        list.forEach(System.out::println);

        System.out.println("--------------------------------");

        Set<String> set = employees.stream()
                .map(Employee::getName)
                .collect(Collectors.toSet());

        set.forEach(System.out::println);

        System.out.println("---------------------------------");

        HashSet<String> hs = employees.stream()
                .map(Employee::getName)
                .collect(Collectors.toCollection(HashSet::new));

        hs.forEach(System.out::println);
    }

    @Test
    public void test3(){
        //总数
        Long count = employees.stream()
                .collect(Collectors.counting());
        System.out.println(count);

        System.out.println("-------------------------------");

        //平均值
        Double avg = employees.stream()
                .collect(Collectors.averagingDouble(Employee::getSalary));
        System.out.println(avg);

        System.out.println("--------------------------------");
        //总和
        Double sum = employees.stream()
                .collect(Collectors.summingDouble(Employee::getSalary));
        System.out.println(sum);

        System.out.println("---------------------------------");
        //最大值
        Optional<Employee> max = employees.stream()
                .collect(Collectors.maxBy((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary())));
        System.out.println(max.get());

        System.out.println("---------------------------------");
        //最小值
        Optional<Double> min = employees.stream()
                .map(Employee::getSalary)
                .collect(Collectors.minBy(Double::compare));
        System.out.println(min.get());
    }

    //分组
    @Test
    public void test4(){
        Map<Employee.Status, List<Employee>> map = employees.stream()
                .collect(Collectors.groupingBy(Employee::getStatus));
        System.out.println(map);
    }

    //多级分组
    @Test
    public void test5(){
        Map<Employee.Status, Map<String, List<Employee>>> map = employees.stream()
                .collect(Collectors.groupingBy(Employee::getStatus, Collectors.groupingBy((e) -> {
                    if (((Employee) e).getAge() <= 35) {
                        return "青年";
                    } else if (((Employee) e).getAge() <= 50) {
                        return "中年";
                    } else {
                        return "老年";
                    }
                })));
        System.out.println(map);
    }

    //分区
    @Test
    public void test6(){
        Map<Boolean, List<Employee>> map = employees.stream()
                .collect(Collectors.partitioningBy((e) -> e.getSalary() > 8000));
        System.out.println(map);
    }

    @Test
    public void test7(){
        DoubleSummaryStatistics dss = employees.stream()
                .collect(Collectors.summarizingDouble(Employee::getSalary));
        System.out.println(dss.getSum());
        System.out.println(dss.getAverage());
        System.out.println(dss.getMax());
    }

    @Test
    public void test8(){
        String str = employees.stream()
                .map(Employee::getName)
                .collect(Collectors.joining(",", "开始", "结束"));
        System.out.println(str);
    }

原文地址：https://blog.51cto.com/pengge666/2390291