Java8流List转Map
- 我们有一个用户信息类
通过JAVA8的流操作需要转换成userId为key， name为value的map。
public class User {
private Integer userId;
private String name;
private String email;
public User(Integer userId, String name, String email) {
this.userId = userId;
this.name = name;
this.email = email;
}
public Integer getUserId() {
return userId;
}
public void setUserId(Integer userId) {
this.userId = userId;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
@Override
public String toString() {
return "User{" +
"userId=" + userId +
", name='" + name + ''' +
", email='" + email + ''' +
'}';
}
}
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- 实现代码
public class ListToMap {
public static void main(String[] args) {
List users = new ArrayList<>();
users.add(new User(1, "user1", "[email protected]"));
users.add(new User(2, "user2", "[email protected]"));
users.add(new User(3, "user3", "[email protected]"));
users.add(new User(4, "user4", "[email protected]"));
Map userIdAndName = users.stream()
.collect(Collectors.toMap(User::getUserId, User::getName));
System.out.println(userIdAndName);
}
}
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输出结果
- userId为key，用户对象为value
public class ListToMap {
public static void main(String[] args) {
List users = new ArrayList<>();
users.add(new User(1, "user1", "[email protected]"));
users.add(new User(2, "user2", "[email protected]"));
users.add(new User(3, "user3", "[email protected]"));
users.add(new User(4, "user4", "[email protected]"));
Map userIdAndUser = users.stream()
.collect(Collectors.toMap(User::getUserId, user-> user));
System.out.println(userIdAndUser);
}
}
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输出
- 重复的key处理
在转换过程中，如果出现相同的key，那么会抛出重复key的异常。
我们将userId为3的修改为2，和第二个用户冲突
Collectors.toMap支持第三个参数，对重复值进行选取
public class ListToMap {
public static void main(String[] args) {
List users = new ArrayList<>();
users.add(new User(1, "user1", "[email protected]"));
users.add(new User(2, "user2", "[email protected]"));
users.add(new User(2, "user3", "[email protected]"));
users.add(new User(4, "user4", "[email protected]"));
Map userIdAndUser = users.stream()
.collect(Collectors.toMap(User::getUserId, user-> user, (oldValue, newValue) -> newValue));
System.out.println(userIdAndUser);
}
}
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选取新的值覆盖旧的值，得到的结果
只会出现user3，而user2被覆盖掉了。
- 将ID和用户列表进行映射
刚才上面出现重复的ID，是根据值进行覆盖，如果在某些情况下需要映射成列表。
即：List -> Map
这需要使用到groupingBy
public class ListToMap {
public static void main(String[] args) {
List users = new ArrayList<>();
users.add(new User(1, "user1", "[email protected]"));
users.add(new User(2, "user2", "[email protected]"));
users.add(new User(2, "user3", "[email protected]"));
users.add(new User(4, "user4", "[email protected]"));
Map> userMap = users.stream()
.collect(Collectors.groupingBy(User::getUserId));
System.out.println(userMap);
}
}
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ID为2的出现多条数据。
- 指定Map类型
Collectors.toMap支持第四个参数，对map类型进行实例化
public class ListToMap {
public static void main(String[] args) {
List users = new ArrayList<>();
users.add(new User(1, "user1", "[email protected]"));
users.add(new User(2, "user2", "[email protected]"));
users.add(new User(3, "user3", "[email protected]"));
users.add(new User(4, "user4", "[email protected]"));
Map userIdAndUser = users.stream()
.collect(Collectors.toMap(User::getUserId, user-> user, (oldValue, newValue) -> newValue, ConcurrentHashMap::new));
System.out.println(userIdAndUser);
System.out.println(userIdAndUser.getClass());
}
}
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输出
userIdAndUser类的类型是ConcurrentHashMap
