如何在 Java 中将一个流转换成一个地图
Java 8 中引入的流 API 用于处理对象的集合。流是支持各种方法的对象序列，这些方法可以通过流水线来产生所需的结果。
本文讨论了将一个流转换成一个图的方法。
方法一:使用 Collectors.toMap() 功能T5】
collectors . tomap()方法以两个参数作为输入:
- 关键点映射器:该功能用于从流值中提取映射的关键点。
- 值映射器:该函数用于提取给定键的映射值。
以下是将给定流转换为地图的 toMap 函数的示例:
-
Example 1: Here, we will convert a string into a Map with the keys as the words of the string and the value as the length of each word.
```java // 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)); } } ```
Output:
```java
```
在上例中， toMap 收集器采用两个λ函数作为参数:
- (值- >值):读取当前流值，作为地图的关键字返回。
- (value->value . length):它读取当前流值，找到其长度，并将该值返回给定键的映射。
- 示例 2: 现在，让我们使用 toMap 功能来执行更复杂的地图转换。在这里，我们将把用户列表转换成一个映射，其中用户标识是键，用户是值。
```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 {
// 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 =用户[用户标识= 1,名称=用户 1,城市=浦那],2 =用户[用户标识= 2,名称=用户 2,城市=孟买],3 =用户[用户标识= 3,名称=用户 3,城市=那格浦尔]}
方法二:使用采集器
分组 By 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。
-
Example 1: In this example, we will convert a user stream into a map whose key is the city and the value is the users living in that city.
```java // 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)); } } ```
Output:
【T 7] {Nagpur = [user [user ID = 3, name = user 3, city = Nagpur]], Pune = [user [user ID = 1, name = user 1, city = Pune], user [user ID = 4, name = user 4, city = Pune]]
-
示例 2: 如果我们需要比实际对象更多的信息，我们还可以通过为*组提供一个额外的收集器。在这个例子中，我们将看到如何获得属于每个城市的用户数。
```java // 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:
```java
```*
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