Guava Multimap Example

Guava’s Multimap associates a key with multiple values.

If we have to do the traditional way it would be using Map<key, Collection<value>> where Collection can be an ArrayList, a HashSet, a TreeSet etc.

In this article, we will see an example of Multimap using ArrayListMultimap implementation. The main data structure is a HashMap that uses an ArrayList to store the values for a given key.

If we have to maintain Map<key, Collection<value>>, we will be doing something like below:

Collection collection = map.get(key);
if (collection == null) {
    collection = createCollection(key);
    map.put(key, collection);
}
collection.add(value));

We create using MultiMap using code>ArrayListMultimap.

Multimap<String,String> bag = ArrayListMultimap.create();

We can add multiple values against the same key. For example, below we add multiple colors against ‘Color’ key and multiple furniture items against key ‘Furniture’.

bag.put(COLOR, RED);
bag.put(COLOR, RED);
bag.put(COLOR, YELLOW);
bag.put(COLOR, GREEN);
bag.put(COLOR, GREEN);
bag.put(COLOR, GREEN);
       
bag.put(FURNITURE, TABLE);
bag.put(FURNITURE, CHAIR);
bag.put(FURNITURE, SOFA);

ArrayListMultimap allows duplicate key-value pairs.
We add ‘Red’ twice and ‘Green’ thrice.
We can get all the added items for a key using Multimap.get(key).
All the colors added can be obtained using the below:

bag.get(COLOR);

an the furniture items using:

bag.get(FURNITURE);

MultimapExample:

package com.javarticles.guava;

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


public class MultimapExample {
    private static final String COLOR = "Color";
    private static final String RED = "Red";
    private static final String YELLOW = "Yellow";
    private static final String GREEN = "Green";
    
    private static final String FURNITURE = "Furniture";
    private static final String TABLE = "Table";
    private static final String CHAIR = "Chair";
    private static final String SOFA = "Sofa";
    
    public static void main(String[] args) {
       Multimap<String,String> bag = ArrayListMultimap.create();
       bag.put(COLOR, RED);
       bag.put(COLOR, RED);
       bag.put(COLOR, YELLOW);
       bag.put(COLOR, GREEN);
       bag.put(COLOR, GREEN);
       bag.put(COLOR, GREEN);
       
       bag.put(FURNITURE, TABLE);
       bag.put(FURNITURE, CHAIR);
       bag.put(FURNITURE, SOFA);

       System.out.println("Total items in Bag: " + bag.size());
       System.out.println("Total colors in Bag: " + bag.get(COLOR).size());
       System.out.println("Colors in Bag: " + bag.get(COLOR));
       
       System.out.println("Total items in furniture: " + bag.get(FURNITURE).size());
       System.out.println("Furniture Items: " + bag.get(FURNITURE));
    }   
}

Total items in the bag include even the duplicate items.
Output:

Total items in Bag: 9
Total colors in Bag: 6
Colors in Bag: [Red, Red, Yellow, Green, Green, Green]
Total items in furniture: 3
Furniture Items: [Table, Chair, Sofa]

Download the source code

This was an example about Guava Multimap.