Java8中Stream的归约与收集-创新互联
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("---------------------------------");
//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);
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