Java8GroupBy一个字段,然后映射到多个字段
我有一个对象结构,比如Java8GroupBy一个字段,然后映射到多个字段,java,java-8,Java,Java 8,我有一个对象结构,比如 class Person{ String userId; String firstName; String lastName; Set<Courses> courses = new HashSet<Courses>(); } 从数据库将结果检索到列表resultSet 现在,我需要按userId分组,然后将课程映射到集合中,并创建一个列表对象 现在我可以按用户ID分组并将课程收集到集合中,但无法映射firstName和la
class Person{
String userId;
String firstName;
String lastName;
Set<Courses> courses = new HashSet<Courses>();
}
从数据库将结果检索到列表resultSet
现在,我需要按userId分组,然后将课程映射到集合中,并创建一个列表
对象
现在我可以按用户ID分组并将课程收集到集合中,但无法映射firstName和lastName
Map<Object, Set<Object>> userList = resultSet.stream().collect()
.Collectors.groupingBy( usr -> usr.get("user_id"),
Collectors.mapping( usr -> usr.get("courses"), Collectors.toSet()) ));
// Result {user1=[course1, course2, course3]}
Map userList=resultSet.stream().collect()
.Collectors.groupingBy(usr->usr.get(“用户id”),
Collectors.mapping(usr->usr.get(“课程”),Collectors.toSet());
//结果{user1=[course1,course2,course3]}
然后我创建Person对象
List<Person> = userList.entrySet.stream().
.map( usr -> new Person(usr.getKey().toString(),
(Set<Courses)(Set<?>)usr.getValue()))
.collect(Collectors.toList())
List=userList.entrySet.stream()。
.map(usr->newperson(usr.getKey().toString(),
(设置您可以使用收集器.toMap
而不是收集器.groupingBy
。您将通过传递到toMap
的合并函数实现分组
它将创建一些不必要的Person
实例,但最终结果将包含您想要的内容
Map<String,Person> persons =
resultSet.stream()
.collect(Collectors.toMap(usr -> usr.get("user_id"),
usr -> new Person(usr.get("user_id"),usr.get("first_name"),usr.get("last_name"),usr.get("course")),
(p1,p2)->{p1.addCourses(p2.getCourses()); return p1;}));
映射人员=
resultSet.stream()
.collect(Collectors.toMap(usr->usr.get(“用户id”),
usr->新人(usr.get(“用户id”)、usr.get(“名字”)、usr.get(“姓氏”)、usr.get(“课程”),
(p1,p2)->{p1.addCourses(p2.getCourses());返回p1;});
这是假设您在Word类中有相关的构造函数和方法.
您也可以尝试将您的自定义收集器定义为下游函数传递给<代码>收藏家.GuangPin By.()<代码> >考虑下面的例子:
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collector;
import java.util.stream.Collectors;
public class PersonGroupByExample {
public static void main(String[] args) {
final List<Map<String, Object>> input = Arrays.asList(
new HashMap<String, Object>(){{
put("userId", "user1");
put("firstName", "John");
put("lastName", "Smith");
put("courses", "course1");
}},
new HashMap<String, Object>(){{
put("userId", "user1");
put("firstName", "John");
put("lastName", "Smith");
put("courses", "course2");
}},
new HashMap<String, Object>(){{
put("userId", "user1");
put("firstName", "John");
put("lastName", "Smith");
put("courses", "course3");
}},
new HashMap<String, Object>(){{
put("userId", "user2");
put("firstName", "Jack");
put("lastName", "Smith");
put("courses", "course1");
}},
new HashMap<String, Object>(){{
put("userId", "user2");
put("firstName", "Jack");
put("lastName", "Smith");
put("courses", "course2");
}}
);
final Collection<Person> result = input.stream()
.parallel()
.collect(Collectors.groupingBy(it -> it.get("userId"), Collector.of(
// Start with an empty Person object
Person::new,
// Collect a list of map objects grouped by the same userId into a single Person object
(person, map) -> {
// Override common properties
person.setUserId(map.getOrDefault("userId", "").toString());
person.setFirstName(map.getOrDefault("firstName", "").toString());
person.setLastName(map.getOrDefault("lastName", "").toString());
// Add person's course to a courses set
person.getCourses().add(new Course(map.getOrDefault("courses", "").toString()));
},
// Combiner function that join partials results (for parallel execution)
(person, person2) -> {
person.getCourses().addAll(person2.getCourses());
return person;
}
))).values();
result.forEach(System.out::println);
}
static class Person {
String userId;
String firstName;
String lastName;
Set<Course> courses = new HashSet<>();
public Person() {}
public String getUserId() {
return userId;
}
public void setUserId(String userId) {
this.userId = userId;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public Set<Course> getCourses() {
return courses;
}
public void setCourses(Set<Course> courses) {
this.courses = courses;
}
@Override
public String toString() {
return "Person{" +
"userId='" + userId + '\'' +
", firstName='" + firstName + '\'' +
", lastName='" + lastName + '\'' +
", courses=" + courses +
'}';
}
}
static class Course {
String id;
public Course(String id) {
this.id = id;
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
@Override
public String toString() {
return "Course{" +
"id='" + id + '\'' +
'}';
}
}
}
你应该使用还原法
Map<String, Person> people = userList = resultSet.stream()
.map(v -> toPerson())
.collect(groupingBy(Person::getId, reducing(null, (p1, p2) -> {
p1.getCourses().addAll(p2.getCourses);
return p1;
})));
其思想是使用单个课程集的Person对象映射每一行,然后分组并缩小
Map<String, Person> people = userList = resultSet.stream()
.map(v -> toPerson())
.collect(groupingBy(Person::getId, reducing(null, (p1, p2) -> {
p1.getCourses().addAll(p2.getCourses);
return p1;
})));
Map people=userList=resultSet.stream()
.map(v->toPerson())
.collect(groupbyperson::getId,reduced(null,(p1,p2)->{
p1.getCourses().addAll(p2.getCourses);
返回p1;
})));
希望您能理解。结果集的类型是什么?结果集的类型是List是来自dbOh的查询结果,但是否可以像我试图学习的那样使用groupingbyit@RinsenS我不确定。它的性能要好一点(p1,p2)->{if(p1.getCourses().size()>p2.getCourses().size())){p1.getCourses().addAll(p2.getCourses());返回p1;}其他{p2.getCourses().addAll(p1.getCourses());返回p2;}
非常感谢您的回答,但我没有将输入用户列为列表。我将输入列为列表,这使得它必须获得课程和设置对象,您能在这方面提供帮助吗…@RinsenS我已经更新了示例以使用列表
作为输入。太棒了,我刚刚完成。也感谢您的解决方案
Map<String, Person> people = userList = resultSet.stream()
.map(v -> toPerson())
.collect(groupingBy(Person::getId, reducing(null, (p1, p2) -> {
p1.getCourses().addAll(p2.getCourses);
return p1;
})));