java list stream avg_Java8中Stream的归约与收集

「爱情、让人受尽委屈。」 2022-11-03 11:17 290阅读 0赞

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);

java list stream avg_Java8中Stream的归约与收集

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