Java List转Map用法及代码示例

List 是 Collection 的子接口。它是一个有序的对象集合，其中可以存储重复的值。由于 List 保留插入顺序，因此它允许位置访问和插入元素。列表接口由 ArrayList 、 LinkedList 、 Vector 和 Stack 类实现。

Map接口表示键和值之间的映射。 Map 接口不是 Map 接口的子类型集合接口。因此它的行为与其他集合类型有点不同。

例子：

Input: List : [1="1", 2="2", 3="3"]
Output: Map : {1=1, 2=2, 3=3}

Input: List : [1="Geeks", 2="for", 3="Geeks"]
Output: Map : {1=Geeks, 2=for, 3=Geeks}

下面是在 Java 中将 List 转换为 Map 的多种方法。为此，假设 List 的每个元素都有一个标识符，该标识符将用作结果 Map 中的键。

1. 按列表对象使用：

   方法：

   1. 获取要转换为Map的List
   2. 创建一个空Map
   3. 迭代列表中的项目并将每个项目添加到 Map 中。
   4. 返回形成的Map

   
   // Java program for list convert in map 
   // with the help of Object method 
     
   import java.util.ArrayList; 
   import java.util.List; 
   import java.util.Map; 
   import java.util.HashMap; 
     
   // create a list 
   class Student { 
     
       // id will act as Key 
       private Integer id; 
     
       // name will act as value 
       private String name; 
     
       // create curstuctor for reference 
       public Student(Integer id, String name) 
       { 
     
           // assign the value of id and name 
           this.id = id; 
           this.name = name; 
       } 
     
       // return private variable id 
       public Integer getId() 
       { 
           return id; 
       } 
     
       // return private variable name 
       public String getName() 
       { 
           return name; 
       } 
   } 
     
   // main class and method 
   public class GFG { 
     
       // main Driver 
       public static void main(String[] args) 
       { 
     
           // create a list 
           List<Student> 
               lt = new ArrayList<Student>(); 
     
           // add the member of list 
           lt.add(new Student(1, "Geeks")); 
           lt.add(new Student(2, "For")); 
           lt.add(new Student(3, "Geeks")); 
     
           // create map with the help of 
           // Object (stu) method 
           // create object of Map class 
           Map<Integer, String> map = new HashMap<>(); 
     
           // put every value list to Map 
           for (Student stu : lt) { 
               map.put(stu.getId(), stu.getName()); 
           } 
     
           // print map 
           System.out.println("Map  : " + map); 
       } 
   } 

   输出：

   Map  : {1=Geeks, 2=For, 3=Geeks}
   

2. 使用 Collectors.toMap() 方法：此方法包括创建学生对象列表，并使用 Collectors.toMap() 将其转换为 Map。
   Approach:
   1. 获取要转换为Map的List
   2. 使用List.stream()方法将List转换为流
   3. 借助 Collectors.toMap() 方法创建Map
   4. 使用stream.collect()方法收集形成的Map
   5. 返回形成的Map

   
   // Java program for list convert  in map 
   // with the help of Collectors.toMap() method 
     
   import java.util.ArrayList; 
   import java.util.LinkedHashMap; 
   import java.util.List; 
   import java.util.stream.Collectors; 
     
   // create a list 
   class Student { 
     
       // id will act as Key 
       private Integer id; 
     
       // name will act as value 
       private String name; 
     
       // create curstuctor for reference 
       public Student(Integer id, String name) 
       { 
     
           // assign the value of id and name 
           this.id = id; 
           this.name = name; 
       } 
     
       // return private variable id 
       public Integer getId() 
       { 
           return id; 
       } 
     
       // return private variable name 
       public String getName() 
       { 
           return name; 
       } 
   } 
     
   // main class and method 
   public class GFG { 
     
       // main Driver 
       public static void main(String[] args) 
       { 
     
           // create a list 
           List<Student> lt = new ArrayList<>(); 
     
           // add the member of list 
           lt.add(new Student(1, "Geeks")); 
           lt.add(new Student(2, "For")); 
           lt.add(new Student(3, "Geeks")); 
     
           // create map with the help of 
           // Collectors.toMap() method 
           LinkedHashMap<Integer, String> 
               map = lt.stream() 
                         .collect( 
                             Collectors 
                                 .toMap( 
                                     Student::getId, 
                                     Student::getName, 
                                     (x, y) 
                                         -> x + ", " + y, 
                                     LinkedHashMap::new)); 
     
           // print map 
           map.forEach( 
               (x, y) -> System.out.println(x + "=" + y)); 
       } 
   } 

   输出：

   1=Geeks
   2=For
   3=Geeks
   

3. 使用Collector创建MultiMap。groupingBy()：

   方法：

   1. 获取要转换为Map的List
   2. 使用List.stream()方法将List转换为流
   3. 借助 Collectors.groupingBy() 方法创建Map
   4. 使用stream.collect()方法收集形成的Map
   5. 返回形成的Map

   
   // Java program for list convert  in map 
   // with the help of Collectors.groupingBy() method 
     
   import java.util.*; 
   import java.util.stream.Collectors; 
     
   // create a list 
   class Student { 
     
       // id will act as Key 
       private Integer id; 
     
       // name will act as value 
       private String name; 
     
       // create curstuctor for reference 
       public Student(Integer id, String name) 
       { 
     
           // assign the value of id and name 
           this.id = id; 
           this.name = name; 
       } 
     
       // return private variable id 
       public Integer getId() 
       { 
           return id; 
       } 
     
       // return private variable name 
       public String getName() 
       { 
           return name; 
       } 
   } 
     
   // main class and method 
   public class GFG { 
     
       // main Driver 
       public static void main(String[] args) 
       { 
     
           // create a list 
           List<Student> lt = new ArrayList<Student>(); 
     
           // add the member of list 
           lt.add(new Student(1, "Geeks")); 
           lt.add(new Student(1, "For")); 
           lt.add(new Student(2, "Geeks")); 
           lt.add(new Student(2, "GeeksForGeeks")); 
     
           // create map with the help of 
           // Object (stu) method 
           // create object of Multi Map class 
     
           // create multimap and store the value of list 
           Map<Integer, List<String> > 
               multimap = lt 
                              .stream() 
                              .collect( 
                                  Collectors 
                                      .groupingBy( 
                                          Student::getId, 
                                          Collectors 
                                              .mapping( 
                                                  Student::getName, 
                                                  Collectors 
                                                      .toList()))); 
     
           // print the multiMap 
           System.out.println("MultiMap = " + multimap); 
       } 
   } 

   输出：

   MultiMap = {1=[Geeks, For], 2=[Geeks, GeeksForGeeks]}