1、JDK1.8流方式
用户类
import lombok.Data;
@Data
public class User {
private String id;
private String name;
public User(String id, String name) {
this.id = id;
this.name = name;
}
}
1. 收集成key为id，value为name：
List<User> userList = new ArrayList<>();
User user1 = new User("1", "name1");
User user2 = new User("2", "name2");
User user3 = new User("3", "name3");
userList.add(user1);
userList.add(user2);
userList.add(user3);
Map<String, String> collect = userList.stream().collect(Collectors.toMap(User::getId, User::getName));
for (Map.Entry<String, String> next : collect.entrySet()) {
System.out.println("key: " + next.getKey() + " , " + "value: " + next.getValue());
}
2022-03-31_181249.png
2. 收集成实体本身
List<User> userList = new ArrayList<>();
User user1 = new User("1", "name1");
User user2 = new User("2", "name2");
User user3 = new User("3", "name3");
userList.add(user1);
userList.add(user2);
userList.add(user3);
Map<String, User> collect = userList.stream().collect(Collectors.toMap(User::getId, user -> user));
for (Map.Entry<String, User> next : collect.entrySet()) {
System.out.println("key: " + next.getKey() + " , " + "value: " + next.getValue());
}
222.png
user -> user是一个返回本身的lambda表达式，其实还可以使用Function接口中的一个默认方法代替，使整个方法更简洁优雅：
Map<String, User> collect1 = userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));
重复key的情况：
代码如下：
Map<String, User> collect2 = userList.stream().collect(Collectors.toMap(User::getName, Function.identity());
这个方法可能报错（java.lang.IllegalStateException: Duplicate key），因为name是有可能重复的。
333.png
toMap有个重载方法，可以传入一个合并的函数来解决key冲突问题：
List<User> userList = new ArrayList<>();
User user1 = new User("1", "name2");
User user2 = new User("2", "name2");
User user3 = new User("3", "name3");
userList.add(user1);
userList.add(user2);
userList.add(user3);
Map<String, User> collect = userList.stream().collect(Collectors.toMap(User::getName, Function.identity(), (key1, key2) -> key2));
for (Map.Entry<String, User> next : collect.entrySet()) {
System.out.println("key: " + next.getKey() + " , " + "value: " + next.getValue());
}
44444.png
这里只是简单的使用后者覆盖前者来解决key重复问题。还有一种分组的方法：
List<User> userList = new ArrayList<>();
User user1 = new User("1", "name2");
User user2 = new User("2", "name2");
User user3 = new User("3", "name3");
userList.add(user1);
userList.add(user2);
userList.add(user3);
Map<String, List<User>> collect3 = userList.stream().collect(Collectors.groupingBy(User::getName));
for (Map.Entry<String, List<User>> next : collect3.entrySet()) {
System.out.println("key: " + next.getKey() + " , " + "value: " + next.getValue());
}
555.png
3. 指定具体收集的map
toMap还有另一个重载方法，可以指定一个Map的具体实现，来收集数据：
List<User> userList = new ArrayList<>();
User user1 = new User("1", "name2");
User user2 = new User("2", "name2");
User user3 = new User("3", "name3");
userList.add(user1);
userList.add(user2);
userList.add(user3);
Map<String, User> collect3 = userList.stream().collect(Collectors.toMap(User::getName, Function.identity(), (key1, key2) -> key2, LinkedHashMap::new));
for (Map.Entry<String, User> next : collect3.entrySet()) {
System.out.println("key: " + next.getKey() + " , " + "value: " + next.getValue());
}
666.png
