HashMap with multiple values under the same key

Question

Is it possible for us to implement a HashMap with one key and two values. Just as HashMap?

Please do help me, also by telling (if there is no way) any other way to

Answer 1

No, not just as a HashMap. You'd basically need a HashMap from a key to a collection of values.

If you're happy to use external libraries, Guava has exactly this concept in Multimap with implementations such as ArrayListMultimap and HashMultimap.

Answer 2

The easiest way would be to use a google collection library:

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

public class Test {

    public static void main(final String[] args) {

        // multimap can handle one key with a list of values
        final Multimap<String, String> cars = ArrayListMultimap.create();
        cars.put("Nissan", "Qashqai");
        cars.put("Nissan", "Juke");
        cars.put("Bmw", "M3");
        cars.put("Bmw", "330E");
        cars.put("Bmw", "X6");
        cars.put("Bmw", "X5");

        cars.get("Bmw").forEach(System.out::println);

        // It will print the:
        // M3
        // 330E
        // X6
        // X5
    }

}

maven link: https://mvnrepository.com/artifact/com.google.collections/google-collections/1.0-rc2

more on this: http://tomjefferys.blogspot.be/2011/09/multimaps-google-guava.html

Answer 3

Yes, this is frequently called a multimap.

See: http://google-collections.googlecode.com/svn/trunk/javadoc/index.html?com/google/common/collect/Multimap.html

Answer 4

Another nice choice is to use MultiValuedMap from Apache Commons. Take a look at the All Known Implementing Classes at the top of the page for specialized implementations.

Example:

HashMap<K, ArrayList<String>> map = new HashMap<K, ArrayList<String>>()

could be replaced with

MultiValuedMap<K, String> map = new MultiValuedHashMap<K, String>();

So,

map.put(key, "A");
map.put(key, "B");
map.put(key, "C");

Collection<String> coll = map.get(key);

would result in collection coll containing "A", "B", and "C".

Answer 5

We can create a class to have multiple keys or values and the object of this class can be used as a parameter in map. You can refer to https://stackoverflow.com/a/44181931/8065321

Answer 6

Apache Commons collection classes can implement multiple values under same key.

    MultiMap multiMapDemo = new MultiValueMap();

    multiMapDemo .put("fruit", "Mango");
    multiMapDemo .put("fruit", "Orange");
    multiMapDemo.put("fruit", "Blueberry");

    System.out.println(multiMap.get("fruit"));

Maven Dependency

    <!-- https://mvnrepository.com/artifact/org.apache.commons/commons-collections4 -- 
     >
     <dependency>
         <groupId>org.apache.commons</groupId>
         <artifactId>commons-collections4</artifactId>
         <version>4.4</version>
    </dependency>