1. Introduction

A Multimap is a special kind of Map in Java, which can map a single key to multiple values. It is part of third-party libraries like Google’s Guava. In this example, we will demonstrate how to convert a Set to a Multimap using Guava, which can be useful when you want to group the elements of a Set based on some properties.

2. Program Steps

1. Add the Guava library to your project.

2. Define a simple class to be stored in the Set.

3. Create and initialize a Set with objects of that class.

4. Convert the Set to a Multimap based on a property of the objects.

5. Print the Multimap to the console.

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.HashSet;
import java.util.Set;

// Step 2: Define a simple class to be stored in the Set
class Animal {
    String name;
    String type;

    Animal(String name, String type) {
        this.name = name;
        this.type = type;
    }

    @Override
    public String toString() {
        return name;
    }
}

public class SetToMultimap {

    public static void main(String[] args) {
        // Step 3: Create and initialize a Set with objects of that class
        Set<Animal> animalSet = new HashSet<>(Set.of(
                new Animal("Lion", "Mammal"),
                new Animal("Tiger", "Mammal"),
                new Animal("Snake", "Reptile"),
                new Animal("Crocodile", "Reptile")
        ));

        // Step 4: Convert the Set to a Multimap based on a property of the objects
        Multimap<String, Animal> animalMultimap = ArrayListMultimap.create();
        for (Animal animal : animalSet) {
            animalMultimap.put(animal.type, animal);
        }

        // Step 5: Print the Multimap to the console
        System.out.println("Multimap: ");
        animalMultimap.asMap().forEach((type, animalsOfType) ->
                System.out.println("Type: " + type + " - Animals: " + animalsOfType)
        );
    }
}

Output:

Multimap:
Type: Mammal - Animals: [Lion, Tiger]
Type: Reptile - Animals: [Snake, Crocodile]

4. Step By Step Explanation

Step 1: Include the Guava library in your project for utilizing the Multimap.

Step 2: Define a simple Animal class with name and type as attributes.

Step 3: Create and initialize a Set of Animal objects.

Step 4: Create an empty Multimap, then iterate through the Set of animals, and put each animal in the Multimap with the type as the key.

Step 5: Print the contents of the Multimap to the console, displaying the type of animal mapped to the set of animals of that type.