Program to Convert List to Map in Java

The List is a child interface of Collection. It is an ordered collection of objects in which duplicate values can be stored. Since List preserves the insertion order, it allows positional access and insertion of elements. List Interface is implemented by ArrayList, LinkedList, Vector and Stack classes.

The java.util.Map interface represents a mapping between a key and a value. The Map interface is not a subtype of the Collection interface. Therefore it behaves a bit different from the rest of the collection types.

Example:

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}

Below are various ways to convert List to Map in Java. For this, it is assumed that each element of the List has an identifier which will be used as a key in the resulting Map.

1. Using by object of list:

   Approach:

   1. Get the List to be converted into Map
   2. Create an empty Map
   3. Iterate through the items in the list and add each of them into the Map.
   4. Return the formed 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);
       }
   }
   Output:

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

2. Using Collectors.toMap() method: This method includes creation of a list of the student objects, and uses Collectors.toMap() to convert it into a Map.
   Approach:
   1. Get the List to be converted into Map
   2. Convert the List into stream using List.stream() method
   3. Create map with the help of Collectors.toMap() method
   4. Collect the formed Map using stream.collect() method
   5. Return the formed 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));
       }
   }
   Output:

   1=Geeks
   2=For
   3=Geeks
   

3. Creating MultiMap using Collectors.groupingBy():

   Approach:

   1. Get the List to be converted into Map
   2. Convert the List into stream using List.stream() method
   3. Create map with the help of Collectors.groupingBy() method
   4. Collect the formed Map using stream.collect() method
   5. Return the formed 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);
       }
   }
   Output:

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