How do I build up in a map with one Key with many Values in Java 7-CodePudding

I want to build up a map based on 2 arrays where 1 key has many objects inside it.

Key: "Letter A" Value: "Albatross" Value: "Alligator"

Key: "Letter B" Value: "Badger" Value: "Bandicoot"

The structure must show the key 1 time, without repetitions

CodePudding user response：

Hope the code is self explanatory.

Java 7:

public Map<String, List<String>> group(String[] input) {
        Map<String, List<String>> result = new HashMap<>();

        for (String str : input) {
            String key = "Letter "   str.charAt(0);
            if (result.containsKey(key)) {
                result.get(key).add(str);//if Key already exists, just add this word to existing list.
            } else {
                List<String> list = new ArrayList<>();
                list.add(str);
                result.put(key, list); //Otherwise, create a new list and add the new word into the list
            }
        }
        return result;
    }

Java 8:

public static Map<String, List<String>> group(String[] input) {
    return Arrays.stream(input)
            .collect(Collectors.groupingBy(k -> "Letter "   k.charAt(0)));    
            //Provide the key for how you want to group. In your case it is first character of string.
}

CodePudding user response：

You can use Guava's Mutlimap implementation, however that may not be Java 7 compatible. https://guava.dev/releases/23.0/api/docs/com/google/common/collect/Multimap.html

You can get the same effect by using a List for the values in your map like so:

Map<String, List<String>> map = new HashMap<>();

Then, let's say for each entry you want to add to the map you have the key in key and value in val, add it like so:

List<String> list = map.get(key);
if (list == null) {
  list = new ArrayList<>();
  map.put(key, list);
}
list.add(val);