CODEWITHSUNDEEP
javaoopobjecthashmapset
jitron
jitron

Reputation: 135

in Java, how can I map String to a Set<String> if I do not know how many keys in total

I need a HashMap, which the key is String and value is Set, like:

Key: "a", Value: {"a","b","c"....} Key: "b", Value: {"a,","d"....} ...

But I do not know how many keys in total, it depends on the result from other method.

So basically, here is the method looks like: (map could be field)

public void mapKeyValue(int numbersOfKey, HashMap map){
    //some code
}

So if I write the code like this:

public void mapKeyValue(int numbersOfKey, HashMap map){
    for (int i = 0; i < numbersOfKey; i++){
        Set<String> set = new HashSet<String>();
        set.add("some strings");// we can add some strings here
        map.put("OneString", set);
    }
}

After the method, I will get nothing because I will lose all the Set object created by the method, so I cannot get the Set by calling map.get("OneString"). So what should I do if I want to get that hashMap?

Upvotes: 0

Views: 6673

Answers (3)

Edwin Buck
Edwin Buck

Reputation: 70959

There are a number of issues with your code, but I suggest the following approach.

In your case, it looks like you have a Map<String, Set<String>> which is a map of String keys to a set of Strings.

If that's what you were after, I suggest that you

  1. check if the key has a value. If not add an empty set for the key to the surrounding map.
  2. Fetch the set from the map by it's key.
  3. add or remove any desired values from the set

Note that your code as it is written, always replaces the Set stored with the key "OneString" meaning that regardless of value "numbersOfKey" you are really just rebuilding the set at the single key "OneString" numbersOfKey times.

You probably want to do something like

public void addToSet(String setName, String value) {
  if (!sets.containsKey(setName)) {
    sets.put(setName, new HashSet<String>());
  }
  Set<String> values = sets.get(setName);
  values.add(value);
}

This block assumes you have somewhere in the class a member variable like

private Map<String, Set<String>> sets = new HashMap<>();

Note that this code is an idea, and not production code. In the real world, what you add probably should eventually be removed at some point in time. As such, you want to have a facility to remove specific values, or entire sets of values along with their keys at some future point of your program's execution.

Upvotes: 2

Brooks
Brooks

Reputation: 7410

I would suggest using the Apache Commons Collection MultiValueMap instead of creating a Set each time. Both work just fine, but there is a Map that does all of that for you and it's based on a HashMap, keeping your constant time access. Javadoc here:

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

Something like this...

public void someOtherMethod() {
    // Assuming the Map is created and used somewhere outside the mapKeyValue method.  Otherwise it should be instantiated inside the mapKeyValue method

    MultiValueMap<String, String> map = new MultiValueMap<>();

    //2 is an arbitrary, made up number that you select somehow
    mapKeyValue(2, map);

    //Access the values of the map dynamically without knowing how many keys there are
    for (String key : map.keySet()) {
        System.out.print(key + " : ");
        for (String value : map.getCollection(key)) {
            System.out.print(value + ", ");
        }
    }
}

public MultiValueMap<String, String> mapKeyValue(int numbersOfKey, MultiValueMap<String, String> map){
    for (int i = 0; i < numbersOfKey; i++){
        //We need to create a unique key here, so let's use 'i'
        //There are several ways to skin the cat and get the int to String
        //Also want to create unique values, but that's up to you, they're not required to be unique
        map.put(Integer.toString(i), Integer.toString(i) + "a");
        map.put(Integer.toString(i), Integer.toString(i) + "b");
        map.put(Integer.toString(i), Integer.toString(i) + "c");
    }
    //At this point, the map has in it the following key : value pairs
    //"0" : ["0a", "0b", "0c"]
    //"1" : ["1a", "1b", "1c"]
    //"2" : ["2a", "2b", "2c"]

   //Not technically required to return the map IFF the map is instantiated outside the method
   return map;
}

Upvotes: 0

Felipe Romani
Felipe Romani

Reputation: 1

you can not do that?

public HashMap<String, Set<String>> mapKeyValue(int numbersOfKey){
   HashMap<String, Set<String>> map = new HashMap<>();
   for (int i = 0; i < numbersOfKey; i++){
       Set<String> set = new HashSet<String>();
       set.add("some strings" + "" + i);// we can add some strings here
       map.put("OneString", set);
   }
   return map;
}

Upvotes: 0

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