如何在Java中将 Stream 转换为 Map

在Java 8 中引入的 Stream API 用于处理对象的集合。流是支持各种方法的对象序列，这些方法可以流水线化以产生所需的结果。

在本文中，将讨论将流转换为映射的方法。

方法一：使用 Collectors.toMap()函数

Collectors.toMap()方法将两个参数作为输入：

KeyMapper：该函数用于从流值中提取 Map 的键。
ValueMapper：此函数用于提取给定键的映射值。

以下是toMap函数将给定流转换为地图的示例：

示例 1：这里，我们将一个字符串转换为一个 Map，其中键是字符串的单词，值是每个单词的长度。

// Program to convert
// the Stream to Map
  
import java.io.*;
import java.util.stream.*;
import java.util.Arrays;
import java.util.Map;
  
class GFG {
  
    // Function to convert the string
    // to the map
    public static Map toMap(String input)
    {
        Map lengthMap
            = Arrays.stream(input.split(" "))
                  .collect(Collectors.toMap(
                      value
                      -> value,
                      value -> value.length()));
  
        return lengthMap;
    }
    public static void main(String[] args)
    {
        String input = "Geeks for Geek";
  
        System.out.println(toMap(input));
    }
}

输出：

{Geek=4, for=3, Geeks=5}

在上面的示例中， toMap收集器将两个 lambda 函数作为参数：

(value -> value)：它读取当前流的值，并将其作为 Map 的键返回。
(value -> value.length)：它读取当前流值，找到它的长度并将值返回给给定键的 Map。

示例 2：现在，让我们使用toMap函数执行更复杂的地图转换。在这里，我们将用户列表转换为一个映射，其中 UserId 是键，用户是值。

// Program to convert User[] into
// Map
  
import java.util.Arrays;
import java.util.Map;
import java.util.stream.*;
  
// Implementing the User class
public class User {
  
    // Attributes of the user class
    private int userId;
    private String name;
    private String city;
  
    // Constructor
    public User(int userId, String name,
                String city)
    {
        this.userId = userId;
        this.name = name;
        this.city = city;
    }
  
    // Getters of the user class
    public int getUserId() { return userId; }
  
    public String getName() { return name; }
  
    public String getCity() { return city; }
  
    // Overriding the toString method
    // to return the custom string
    @Override
    public String toString()
    {
        return "User [userId = "
            + userId + ", name = "
            + name + ", city = "
            + city + "]";
    }
}
  
class GFG {
  
    // Function to convert the User
    // to the map
    public static Map toMap(User user1, User user2,
                            User user3)
    {
  
        Map userMap
            = Arrays.asList(user1, user2, user3)
                  .stream()
                  .collect(Collectors.toMap(
                      user
                      -> user.getUserId(),
                      user -> user));
  
        return userMap;
    }
  
    // Driver code
    public static void main(String[] args)
    {
  
        // Creating users
        User user1
            = new User(1, "User1", "Pune");
  
        User user2
            = new User(2, "User2", "Mumbai");
  
        User user3
            = new User(3, "User3", "Nagpur");
  
        System.out.println(toMap(user1, user2,
                                 user3));
    }
}

输出：

{1=User [userId = 1, name = User1, city = Pune], 2=User [userId = 2, name = User2, city = Mumbai], 3=User [userId = 3, name = User3, city = Nagpur]}

编程需要懂一点英语

方法二：使用收集器

groupingBy收集器将一个函数作为输入，并使用该函数创建一组流对象。以下是使用 groupingBy 收集器将流转换为地图的示例。

示例 1：在本示例中，我们将用户流转换为地图，其键是城市，值是居住在该城市的用户。

// Java program to convert the User[]
// into Map>
  
import java.util.Arrays;
import java.util.Map;
import java.util.List;
import java.util.stream.*;
  
// Implementing the User class
public class User {
  
    // Parameters of the user class
    private int userId;
    private String name;
    private String city;
  
    // Constructor of the User class
    public User(int userId, String name,
                String city)
    {
        this.userId = userId;
        this.name = name;
        this.city = city;
    }
  
    // Getter functions
    public int getUserId() { return userId; }
  
    public String getName() { return name; }
  
    public String getCity() { return city; }
  
    // Overriding the toString() method
    // to create a custom function
    @Override
    public String toString()
    {
        return "User [userId = "
            + userId + ", name = "
            + name + ", city = "
            + city + "]";
    }
}
  
class GFG {
  
    // Function to convert the user
    // object to the map
    public static Map toMap(User user1,
                            User user2,
                            User user3,
                            User user4,
                            User user5)
    {
        Map >
            cityUserListMap
            = Arrays.asList(user1, user2, user3,
                            user4, user5)
                  .stream()
                  .collect(Collectors.groupingBy(
                      User::getCity));
  
        return cityUserListMap;
    }
  
    // Driver code
    public static void main(String[] args)
    {
  
        // Creating new users
        User user1
            = new User(1, "User1", "Pune");
        User user2
            = new User(2, "User2", "Mumbai");
        User user3
            = new User(3, "User3", "Nagpur");
        User user4
            = new User(4, "User4", "Pune");
        User user5
            = new User(5, "User5", "Mumbai");
  
        System.out.println(toMap(user1, user2,
                                 user3, user4,
                                 user5));
    }
}

输出：

{Nagpur=[User [userId = 3, name = User3, city = Nagpur]], Pune=[User [userId = 1, name = User1, city = Pune], User [userId = 4, name = User4, city = Pune]], Mumbai=[User [userId = 2, name = User2, city = Mumbai], User [userId = 5, name = User5, city = Mumbai]]}

编程需要懂一点英语

示例 2：如果我们需要比实际对象更多的信息，我们还可以为groupingBy提供额外的收集器。在此示例中，我们将了解如何获取属于每个城市的用户数。

// Java program to convert User[]
// into Map
  
import java.util.Arrays;
import java.util.Map;
import java.util.stream.*;
  
// Implementing the user class
public class User {
  
    // Parameters of the user class
    private int userId;
    private String name;
    private String city;
  
    // Constructor
    public User(int userId, String name,
                String city)
    {
        this.userId = userId;
        this.name = name;
        this.city = city;
    }
  
    // Getter functions
    public int getUserId() { return userId; }
  
    public String getName() { return name; }
  
    public String getCity() { return city; }
  
    // Overriding the toString() method
    // to create a custom function
    @Override
    public String toString()
    {
        return "User [userId = "
            + userId + ", name = "
            + name + ", city = "
            + city + "]";
    }
}
  
class GFG {
  
    public static Map toMap(User user1,
                            User user2,
                            User user3,
                            User user4,
                            User user5)
    {
  
        Map
            cityUserCountMap
            = Arrays.asList(user1, user2, user3,
                            user4, user5)
                  .stream()
                  .collect(
                      Collectors.groupingBy(
                          User::getCity,
                          Collectors.counting()));
  
        return cityUserCountMap;
    }
  
    // Driver code
    public static void main(String[] args)
    {
  
        // Creating new users
        User user1
            = new User(1, "User1", "Pune");
        User user2
            = new User(2, "User2", "Mumbai");
        User user3
            = new User(3, "User3", "Nagpur");
        User user4
            = new User(4, "User4", "Pune");
        User user5
            = new User(5, "User5", "Mumbai");
  
        System.out.println(toMap(user1, user2,
                                 user3, user4,
                                 user5));
    }
}

输出：

{Nagpur=1, Pune=2, Mumbai=2}