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Program to Convert List to Map in Java
  • Difficulty Level : Medium
  • Last Updated : 19 Jun, 2019
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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.
mapinterface

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]}
    

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