Java 简明教程
Java - Teeing Collectors
Java Collectors.teeing() Method
Java 12 引入了一个 new static 方法 Collectors interface，它可以在 collection 上执行两个不同的操作，然后合并结果。
Java 12 introduced a new static method to Collectors interface which can perform two different operations on collection and then merge the result.
Syntax
teeing 方法的语法如下 −
Following is the syntax of teeing method −
public static Collector<T, ?, R> teeing(
Collector<? super T, ?, R1> downstream1,
Collector<? super T, ?, R2> downstream2,
BiFunction<? super R1, ? super R2, R> merger
)此处，传递给 teeing 收集器的集合的每个元素都由 downstream1 和 downstream2 收集器处理，一旦这两个收集器完成处理，结果就会传递给 BiFunction 收集器，以合并结果或相应处理。它类似于对某个集合调用两个函数，然后调用第三个函数来处理前两个函数的结果。
Here each element of the collection passed to the teeing collector is processed by downstream1 and downstream2 collectors, once the processing is completed by both the collectors, the results are passed to the BiFunction collector to merge the result or process accordingly. It is similar to calling two functions on a collection and then calling the third function to process the results of first two functions.
此处，我们对某个集合执行不同的函数，然后使用合并器 BiFunction 合并结果。
Here we are performing different functions on a collection and then merge the result using merger BiFunction.
Example - Using teeing collectors to get mean of n numbers
在这个示例中，我们获取 downstream1 收集器中的数字总和、downstream2 收集器中的数字计数，然后在合并函数中计算平均值。当我们获得一大串数字并且流大小不可用时，这很有用。
In this example, we’re getting sum of numbers in downstream1 collector, count of numbers in downstream2 collector and then computing mean in the merger function. This is useful when we’re getting a stream of numbers and size of stream is not available.
package com.tutorialspoint;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class Tester {
public static void main(String[] args) {
double mean
= Stream.of(1, 2, 3, 4, 5, 6, 7)
.collect(Collectors.teeing(
Collectors.summingDouble(i -> i), Collectors.counting(),
(sum, n) -> sum / n));
System.out.println(mean);
}
}Example - Using teeing collectors to get lowest and highest marks of student objects
在此示例中，我们在 downstream1 收集器中获取学生最低分数，在 downstream2 收集器中获取学生最高分数。然后，使用合并函数，我们创建一个哈希表，其条目包含最低和最高分数的学生。
In this example, we’re getting lowest marks of students in downstream1 collector and highest marks of students in downstream2 collector. Then using merger function, we’re creating a hashmap with entry of student having lowest and highest marks.
package com.tutorialspoint;
import java.util.Arrays;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.stream.Collectors;
public class Tester {
public static void main(String args[]) {
// list of students
List<Student> students = Arrays.asList(
new Student(1, "Robert", 390),
new Student(2, "Julie", 410),
new Student(3, "John", 440),
new Student(4, "Michael", 420));
// collect the result in hashmap
HashMap<String, Student> result = students.stream().collect(
// apply the teeing operator
Collectors.teeing(
// get the student having highest marks
Collectors.maxBy(Comparator.comparing(Student::getMarks)),
// get the student having lowest marks
Collectors.minBy(Comparator.comparing(Student::getMarks)),
// put both student entries in the map using merger
(s1, s2) -> {
HashMap<String, Student> map = new HashMap<>();
map.put("Highest", s1.get());
map.put("Lowest", s2.get());
return map;
}
));
System.out.println(result);
}
}
class Student {
int rollNo;
String name;
int marks;
public Student(int rollNo, String name, int marks) {
this.rollNo = rollNo;
this.name = name;
this.marks = marks;
}
@Override
public String toString() {
return "Student [RollNo=" + rollNo + ", Name=" + name + ", Marks=" + marks + "]";
}
public int getRollNo() {
return rollNo;
}
public void setRollNo(int rollNo) {
this.rollNo = rollNo;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getMarks() {
return marks;
}
public void setMarks(int marks) {
this.marks = marks;
}
}