> Collectors.toCollection(): Collect using a custom data structure that implements Collection
Stage 1: Preparation work, the entity class implements the Comparable interface
1. collect() method
-
[ Terminal operation ] used to aggregate data in the stream
-
The parameter accepted by the collect method is a Collector
2. Two overloading methods
-
In the Stream interface
2.1 Method 1
<R> R collect(Supplier<R> supplier,
BiConsumer<R, ? super T> accumulator,
BiConsumer<R, R> combiner);2.2 Method 2 (commonly used)
<R, A> R collect(Collector<? super T, A, R> collector);3. The role of Collector
- It is a collector and an interface. Its tool class Collectors provides many factory methods.
4. The role of Collectors
-
Utility class , providing many common collector implementations
> Collectors.toList()
-
ArrayList::new , creates an ArrayList as an accumulator
-
List::add, the operation on the elements in the stream is to add them directly to the accumulator.
-
The reduce operation addsAll to the subtask aggregation results, and all the results of the latter subtask are directly added to the results of the previous subtask.
-
CH_ID is an unmodifiableSet collection
> Collectors.toMap()
> Collectors.toSet()
> Collectors.toCollection(): Collect using a custom data structure that implements Collection
-
Collectors.toCollection (LinkedList::new)
-
Collectors.toCollection (CopyOnWriteArrayList::new)
-
Collectors.toCollection (TreeSet::new)
5. Actual combat
Stage 1: Preparation work, the entity class implements the Comparable interface
-
The characteristics of TreeSet are that it is sortable and non-repeating, that is, the objects stored in TreeSet must be sortable. If the objects cannot be sorted, this exception will be thrown (cannot be cast to java.lang.Comparable).
package com.learning.javalearning.lambda.chapter5;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
@NoArgsConstructor
@AllArgsConstructor
@Data
public class User implements Comparable<User> {
private int id;
private String name;
private String pwd;
private int age;
@Override
public int compareTo(User o) {
int cmp = age - o.age;
return cmp != 0 ? cmp : name.compareTo(o.name);
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", name='" + name + '\'' +
", pwd='" + pwd + '\'' +
", age=" + age +
'}';
}
public User(String name, int age) {
this.name = name;
this.age = age;
}
public User(int id, String name, String pwd) {
this.id = id;
this.name = name;
this.pwd = pwd;
}
public User(int id, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}
}
Stage 2: Example
import com.learning.javalearning.lambda.chapter5.User;
import java.util.*;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.stream.Collectors;
public class CollectDemo {
public static void main(String[] args) {
List<User> list = Arrays.asList(new User(1, "陆小凤", 19),
new User(2, "西门吹雪", 20),
new User(3, "西门吹雪", 20),//思考为什么返回TreeSet这个User会删除
new User(4, "叶孤城", 20));
List<User> result = list.stream().collect(Collectors.toList());
System.out.println("【不常用】Collectors.toCollection()返回自定义类型：如 Linkedlist");
List<User> linkedList = list.stream().collect(Collectors.toCollection(LinkedList::new));
List<User> copyOnWriteArrayList = list.stream().collect(Collectors.toCollection(CopyOnWriteArrayList::new));
Set<User> treeSet = list.stream().collect(Collectors.toCollection(TreeSet::new));
System.out.println("LinkedList:" + linkedList);
System.out.println("CopyOnWriteArrayList:" + copyOnWriteArrayList);
System.out.println("TreeSet:" + treeSet);
System.out.println("【常用】直接返回：Collectors.toXXX()");
List<User> list1 = list.stream().collect(Collectors.toList());
Set<User> set = list.stream().collect(Collectors.toSet());
Map<Integer, String> map = list.stream().collect(Collectors.toMap(User::getId, User::getName));
System.out.println("toList:" + list1);
System.out.println("toSet():" + set);
System.out.println("toMap():" + map);
}
}
Results of the
6. Reference
How to solve the problem of cannot be cast to java.lang.Comparable? _msg1_'s blog-CSDN blog