duplicate key value pairs

Question

Is there a native data structure in java that accepts key value pairs and allows duplicates? I am creating a checklist of characters in a string but some characters occur more than once.

ex

j -> false
a -> false
v -> false
a -> false

Answer 1

You can simulate multiple key-value (KV) pairs by saving a list of values for each in a map. This is a standard implementation approach for "multivalue" maps.

So, if the key is a Character object and the value is Boolean, you can do

Map<Character, List<Boolean>> multimap = new HashMap<Character, List<Boolean>>();

and every time you want to add a new value to an existing KV pair in the map just call

multimap.get(key).add(value);

where key is the Character and value its corresponding Boolean value.

The Guava library by Google (free download) has a Multimap interface implemented in various ways, so essentially you can instantiate a MultiMap<Character, Boolean> map and use it accordingly. Similarly, you can get the Apache Commons Collections library and use its MultiValueMap class. You may also want to check the answers to a similar StackOverflow question, or another one.

If you only want to store one of each value per key, then a Set should be used in the place of the List.

Answer 2

Use a List of Pairs:

public class Pair<T, U> {
    public final T key;
    public final U value;

    public Pair(T key, U value) {
        this.key = key;
        this.value = value;
    }
}

public class YourApp {
    public static void main(String[] args) {
        List<Pair<Character, Boolean>> charList = new ArrayList<Pair<Character, Boolean>>();
        charList.add(new Pair('j', false));
        charList.add(new Pair('a', false));
        charList.add(new Pair('v', false));
        charList.add(new Pair('a', false));

        for (Pair<Character, Boolean> pair : charList) {
            System.out.println(pair.key + " -> " + pair.value);
        }
    }
}

With the selfwritten generic Pair class you can hold a key and a value of any type you want. If you're adding pairs to a List, you can even hold duplicates of pair entries.

Answer 3

You can use MultiMap<Character,Boolean> bcoz it allows duplicate key which exist in org.apache.commons.collections package.

or

You can use ArrayList and add the objects of the Class that contain attribute as char & boolean pair.

Answer 4

I do not know of a build in solution.

A quick alternative would be to use a simple ArrayList, and create an object that is a char/boolean pair that you can add to it.

Answer 5

commons.apache.org have MultiHashMap class. Try this one...!!!

MultiHashMap mp = new MultiHashMap();
mp.put("a", "1");
mp.put("b", "4");
mp.put("c", "2");
mp.put("a", "6");
List list = null;
Set set = mp.entrySet();
Iterator i = set.iterator();
while (i.hasNext()) {
        Map.Entry<String, List<String>> me = (Map.Entry) i.next();
        for(int j = 0 ; j< me.getValue().size(); j++ ){
        System.out.println(me.getKey() +" : " +me.getValue().get(j));
        }
    }
}