源码中的案例
/**
* Implementations of {@link Collector} that implement various useful reduction
* operations, such as accumulating elements into collections, summarizing
* elements according to various criteria, etc.
*
* <p>The following are examples of using the predefined collectors to perform
* common mutable reduction tasks:
*
* <pre>{@code
* // Accumulate names into a List
* List<String> list = people.stream().map(Person::getName).collect(Collectors.toList());
*
* // Accumulate names into a TreeSet
* Set<String> set = people.stream().map(Person::getName).collect(Collectors.toCollection(TreeSet::new));
*
* // Convert elements to strings and concatenate them, separated by commas
* String joined = things.stream()
* .map(Object::toString)
* .collect(Collectors.joining(", "));
*
* // Compute sum of salaries of employee
* int total = employees.stream()
* .collect(Collectors.summingInt(Employee::getSalary)));
*
* // Group employees by department
* Map<Department, List<Employee>> byDept
* = employees.stream()
* .collect(Collectors.groupingBy(Employee::getDepartment));
*
* // Compute sum of salaries by department
* Map<Department, Integer> totalByDept
* = employees.stream()
* .collect(Collectors.groupingBy(Employee::getDepartment,
* Collectors.summingInt(Employee::getSalary)));
*
* // Partition students into passing and failing
* Map<Boolean, List<Student>> passingFailing =
* students.stream()
* .collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));
*
* }</pre>
*
* @since 1.8
*/
源码中的代码案例解析
package com.lambda.lambdademo.test_la_01;
import com.lambda.lambdademo.domain.Employee;
import com.lambda.lambdademo.domain.People;
import com.lambda.lambdademo.domain.Student;
import org.junit.jupiter.api.Test;
import java.util.*;
import java.util.stream.Collectors;
/**
* @date ：2019-12-13 16:31
* @description：
*/
public class CollectorTest {
private static final Integer PASS_THRESHOLD = 6;
private List<People> getPeopleList(){
List<People> pList = new ArrayList<>();
pList.add(new People(1, "lisi"));
pList.add(new People(2, "wangwu"));
pList.add(new People(3, "maliu"));
pList.add(new People(4, "maliu"));
return pList;
}
private List<Employee> getEmployeeList(){
List<Employee> eList = new ArrayList<>();
eList.add(new Employee(1, "lisi", "test"));
eList.add(new Employee(2, "wangwu", "dev"));
eList.add(new Employee(3, "maliu", "ceo"));
eList.add(new Employee(3, "maliu", "ceo"));
return eList;
}
private List<Student> getStudentList(){
List<Student> sList = new ArrayList<>();
sList.add(new Student(1, "lisi", 5));
sList.add(new Student(2, "wangwu", 7));
sList.add(new Student(3, "maliu", 9));
return sList;
}
/**
* 官方测试案例
* @see Collectors
*/
@Test
public void test_all(){
List<People> peopleList = getPeopleList();
/**
* new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
* (left, right) -> { left.addAll(right); return left; },
* CH_ID)
* TODO 底层实现转换的是ArrayList
*/
List<String> collect = peopleList.stream().map(People::getName).peek(System.out::println).collect(Collectors.toList());
collect.forEach(s -> System.out.println("ArrayList : " + s));
/**
* new CollectorImpl<>((Supplier<Set<T>>) HashSet::new, Set::add,
* (left, right) -> { left.addAll(right); return left; },
* CH_UNORDERED_ID)
* TODO 底层实现转换的是HashSet
*/
Set<String> collect1 = peopleList.stream().map(People::getName).peek(System.out::println).collect(Collectors.toSet());
collect1.forEach(s -> System.out.println("HashSet : " + s));
/**
* TODO 这个是 TreeSet
*/
TreeSet<String> collect2 = peopleList.stream().map(People::getName).peek(System.out::println).collect(Collectors.toCollection(TreeSet::new));
collect2.forEach(s -> System.out.println("TreeSet : " + s));
/**
* 将集合转换为逗号分隔的元素
* 以下者两个方法都可以
*/
String join = String.join(",", collect);
System.out.println("join : " + join);// lisi,wangwu,maliu,maliu
/**
* new CollectorImpl<CharSequence, StringBuilder, String>(
* StringBuilder::new, StringBuilder::append,
* (r1, r2) -> { r1.append(r2); return r1; },
* StringBuilder::toString, CH_NOID)
*/
String collect3 = collect.stream().map(String::toString).collect(Collectors.joining(","));
System.out.println("collect3 : " + collect3);// lisi,wangwu,maliu,maliu
/**
* joining(delimiter, "", "")
* TODO 从源码调的是下面的这个方法
*/
String collect4 = collect.stream().map(String::toString).collect(Collectors.joining());
System.out.println("collect4 : " + collect4); // lisiwangwumaliumaliu
/**
* new CollectorImpl<>(
* () -> new StringJoiner(delimiter, prefix, suffix),
* StringJoiner::add, StringJoiner::merge,
* StringJoiner::toString, CH_NOID);
*/
String collect5 = collect.stream().map(String::toString).collect(Collectors.joining(",", "", ""));
System.out.println("collect5 : " + collect5); // lisi,wangwu,maliu,maliu
/**
* 计算所有员工的工资总合
*/
List<Employee> employeeList = getEmployeeList();
/**
* 方法一
* new CollectorImpl<>(
* () -> new int[1],
* (a, t) -> { a[0] += mapper.applyAsInt(t); },
* (a, b) -> { a[0] += b[0]; return a; },
* a -> a[0], CH_NOID);
* TODO 返回一个Collector，有元素就是所有整数元素的和
* 如果没有任何元素则返回0
*/
int collect6 = employeeList.stream().collect(Collectors.summingInt(Employee::getSalary));
// 方法二： 对方法一的优化
int collect6_ = employeeList.stream().mapToInt(Employee::getSalary).sum();
System.out.println("方法一： " + collect6);
System.out.println("方法二： " + collect6_);
/**
* 根据部门名称进行分配，key 为部门 value 为该部门的员工
*/
Map<String, List<Employee>> map = employeeList.stream().collect(Collectors.groupingBy(Employee::getDepartment));
for (String s : map.keySet()) {
System.out.println("====== key: " + s + " =====value: " + map.get(s));
}
/**
* 根据部门名称进行分配，key 为部门 value 为该部门的员工薪资的总和
*/
Map<String, Integer> map1 = employeeList.stream().collect(Collectors.groupingBy(Employee::getDepartment, Collectors.summingInt(Employee::getSalary)));
for (String s : map1.keySet()) {
System.out.println("====== key: " + s + " =====value: " + map1.get(s));
}
/**
* 根据部门名称进行分配，key 为部门 value 为该部门的员工的个数
*/
Map<String, Long> map1_ = employeeList.stream().collect(Collectors.groupingBy(Employee::getDepartment, Collectors.counting()));
for (String s : map1_.keySet()) {
System.out.println("====== key: " + s + " =====value: " + map1_.get(s));
}
/**
* 根据年龄是否大于6进行分配
* 大于6 true
* 小于6 false
* key 是否大于6的布尔值 value 为学生集合
*/
List<Student> studentList = getStudentList();
Map<Boolean, List<Student>> listMap = studentList.stream().collect(Collectors.partitioningBy(student -> student.getAge() >= PASS_THRESHOLD));
for (Boolean aBoolean : listMap.keySet()) {
System.out.println("====== key: " + aBoolean + " =====value: " + listMap.get(aBoolean));
}
}
}
