HashMap vs MultivaluedMap

Question

I'm recently into Rest API in java and came across MultivaluedMap. I don't find the advantage MultivaluedMap over HashMap or other classes like TreeMap or LinkedHashMap.Please explain the scenario when MultivaluedMap is necessary.

Answer 1

A MultivaluedMap is:

A map of key-values pairs. Each key can have zero or more values.

A Map is:

An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

So in a MultivaluedMap you can insert 0, 1, 2, 3 or more objects related to the same key.

In a Map you can insert exactly 1 object related to a key.

This is the difference, it can be useful if you need to store many values related to a single key, if you have only one value they are similar.

---

A possibile scenario is a Dictionary where you save all the words related starting with a letter. The keys are A B C... Z and the values are the words.

A -> Acid
     Apple 
     August

B -> Banana
     Boat

C -> Car
     Chain

...

Z -> Zebra

That can be coded with:

MultivaluedMap<String, String> multiMap = new MultivaluedHashMap<>();
multiMap.add("A", "Acid");
multiMap.add("A", "Apple");
multiMap.add("A", "August");
multiMap.add("B", "Banana");
multiMap.add("B", "Boat");
multiMap.add("C", "Car");
multiMap.add("C", "Chain");
multiMap.add("Z", "Zebra");

Another scenario is a Map holding all products buyed by a user. The keys are the user id and values are the products.

USER1 -> Pizza Margherita
         Pizza Pepperoni

USER2 -> Pizza Margherita

USER3 -> Pizza Margherita
         Pizza Pepperoni
         Pizza Four Cheese