how to get key and how to add more values?

Question

public static void main(String[] args) {
// write your code here
    LinkedHashMap<String,String>category1=new LinkedHashMap();

    category1.put("action","die hard");
    Scanner s=new Scanner(System.in);
    String answer=s.nextLine();
    if (category1.containsKey(answer))
        System.out.println(category1.get("action"));
    if (category1.containsValue(answer))
        System.out.println(category1.keySet());

How to get the key when the user answer with it's specific value, and how to add more values to one key?

"How to get the key when the user answer with it's specific value" and "how to get the key when the user answer with it's specific value" are two separate questions. Please [ask one question per post](https://meta.stackexchange.com/q/222735). — Pshemo, Dec 24 '18 at 16:00
Anyway it looks like you need two maps: (1) `category -> [collection,of,movies]` (2) `movie->category`. — Pshemo, Dec 24 '18 at 16:02

Daniel B. · Accepted Answer · 2018-12-24T16:01:41.650

2

1. The map collection, does not support multiple values under the same key, it will override whatever was stored there before.

2. However, you can change it from <String,String> to <String,List<String>>, thus gaining the ability to accumulate the answers from the client into the list. The key will refer to only one object, the list of Strings, but the list itself can hold many values.

3. In order to add more Strings to the list, you will need to retrieve the list by the desired key, and then add your new String to it.

Here is some code that implements the idea:

private void test(){
        Map<String, List<String>> categories = new HashMap<>();
        String answerFromClient = "Some text";
        List<String> actionAnswers = categories.get("action");
        if (actionAnswers == null){
            actionAnswers = new ArrayList<>();
            actionAnswers.add(answerFromClient);
            categories.put("action",actionAnswers);
        }
        else{
            actionAnswers.add(answerFromClient);
        }
    }

edited Dec 24 '18 at 16:01

answered Dec 24 '18 at 15:53

Daniel B.

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Have you actually tried that? I don't think simply changing the second type parameter to `List` will magically make it accept multiple strings without changing the code. – Rudy Velthuis Dec 24 '18 at 15:56
Of course you will need to change the code to support the list of strings, but I was just giving a general idea. – Daniel B. Dec 24 '18 at 15:58
then that should be said in the answer. Simply changing the parameter will not suffice. – Rudy Velthuis Dec 24 '18 at 16:00
I edited the answer,and provided an example. – Daniel B. Dec 24 '18 at 16:02
you could also use a multivaluemap implementation. https://commons.apache.org/proper/commons-collections/javadocs/api-4.2/org/apache/commons/collections4/MultiValuedMap.html – stacktraceyo Dec 24 '18 at 16:42
Sure, for simplicity, however I believe that under the hood, their implementation is similar. – Daniel B. Dec 24 '18 at 16:54
@DanielB.: yes, that will probably work. But I would think that `Map<,>` is simply not the correct vehicle for multiple values per key. – Rudy Velthuis Dec 24 '18 at 17:55
Do you have a better data structure in mind? – Daniel B. Dec 24 '18 at 17:56

how to get key and how to add more values?

1 Answers1