1. Introduction

A Multimap is a type of map in Java that can map a single key to multiple values. It’s not a part of the standard Java Collections Framework but is available through third-party libraries like Google’s Guava. Converting a List to a Multimap can be quite handy when we have a list of objects, and we want to group them by a specific property or field. In this example, we’ll be using Google Guava’s Multimap to achieve this.

2. Program Steps

1. Add the Guava library to your project.

2. Define a simple class to demonstrate the conversion.

3. Create and initialize a List with objects of the class.

4. Convert the List to a Multimap.

5. Display the contents of the Multimap.

3. Code Program

// Step 1: Add Guava library to your project.
// You can add it using build tools like Maven or Gradle.
// Here is a Maven dependency example:
// <dependency>
//     <groupId>com.google.guava</groupId>
//     <artifactId>guava</artifactId>
//     <version>30.1-jre</version> <!-- Replace with the version you are using -->
// </dependency>

import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.Multimap;

import java.util.Arrays;
import java.util.List;

// Step 2: Define a simple class to demonstrate the conversion
class Student {
    String name;
    String grade;

    Student(String name, String grade) {
        this.name = name;
        this.grade = grade;
    }
}

public class ListToMultimap {

    public static void main(String[] args) {
        // Step 3: Create and initialize a List with objects of the class
        List<Student> studentList = Arrays.asList(
                new Student("John", "A"),
                new Student("Jane", "B"),
                new Student("Mike", "A"),
                new Student("Sara", "C"),
                new Student("Paul", "B")
        );

        // Step 4: Convert the List to a Multimap
        Multimap<String, Student> studentsMultimap = ArrayListMultimap.create();
        for (Student student : studentList) {
            studentsMultimap.put(student.grade, student);
        }

        // Step 5: Display the contents of the Multimap
        System.out.println("Multimap: ");
        studentsMultimap.asMap().forEach((grade, studentsInGrade) ->
                System.out.println("Grade: " + grade + " - Students: " + studentsInGrade)
        );
    }
}

Output:

Multimap:
Grade: A - Students: [John, Mike]
Grade: B - Students: [Jane, Paul]
Grade: C - Students: [Sara]

4. Step By Step Explanation

Step 1: Include the Google Guava library in your project using a build tool like Maven or Gradle.

Step 2: Define a simple Student class that has name and grade as attributes.

Step 3: Create and initialize a List of Student objects.

Step 4: Instantiate a Multimap and populate it by iterating through the list of students, using the grade as the key and the Student object as the value.

Step 5: Display the contents of the Multimap. Here, each grade is mapped to a list of Student objects associated with that grade.