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
    
      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
     
       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
    
      list = employees.stream()                .map(Employee::getName)                .collect(Collectors.toList());        list.forEach(System.out::println);        System.out.println("--------------------------------");        Set
     
       set = employees.stream()                .map(Employee::getName)                .collect(Collectors.toSet());        set.forEach(System.out::println);        System.out.println("---------------------------------");        HashSet
      
        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
       
         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
        
          min = employees.stream()                .map(Employee::getSalary)                .collect(Collectors.minBy(Double::compare));        System.out.println(min.get());    }    //分组    @Test    public void test4(){        Map
         
          > map = employees.stream() .collect(Collectors.groupingBy(Employee::getStatus)); System.out.println(map); } //多级分组 @Test public void test5(){ Map
          
           >> 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
           
            > 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); }