How to dynamically add multiple values to a single key in HashMap?

Question

I want to add multiple card values to a distinct single key in HashMap at run time if i give an input like this

s 2 
d 5 
s 3 
d 7 
s 4 

it should give an output like

s 2 3 4 
d 5 7

Answer 1

Hint: For this you need to use ArrayList as values and String as keys in HashMap . Here is an example:

HashMap<String, ArrayList<Integer>> myMap = new HashMap<>();
ArrayList<Integer> list = new ArrayList<>();
list.add(2);
list.add(3);
list.add(4);
myMap.put("s", list);

If there are n key, then to dynamically add values to each key, you can do following:

//1. Get the value (list here) for the key you want to add new values. 

ArrayList<Integer> myList =  myMap.get("s");

//2. Check if myList is null
if(myList==null){
    myList = new ArrayList<Integer>();
}

//3. Now add the values to this list.


myList.add(5);

Answer 2

if you don't wanna get your hands dirty and instead rely on bulletproof 3rd party library classes check the following classes, that should do the work:

https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/util/MultiValueMap.html

https://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/map/MultiValueMap.html

https://google.github.io/guava/releases/21.0/api/docs/com/google/common/collect/Multimap.html