CODEWITHSUNDEEP
javahashmap
cataschok
cataschok

Reputation: 339

a hashmap with multiple keys to value?

I have a question about hashmaps with multiple keys to value. Let's say I have (key / value )

1/a, 1/b, 1/3, 2/aa, 2/bb, 2/cc.

Would this work?

If it does, could I have a way to loop through it and display all values for only either key 1 or 2?

Upvotes: 1

Views: 5332

Answers (4)

Pintobean77
Pintobean77

Reputation: 1415

Maps will handle multiple keys to one value since only the keys need be unique: Map(key, value)

However one key to multiple values requires s multimap of a map strict of : Map(key, list(values))

Also, whatever you use as a key really should implement a good hadhCode() function if you decide to use a HashMap and/or HashSet

Edit: had to use() instead of <> because my mobile or sof's mobile site editor clobbered the <> symbols....odd

Upvotes: 0

Jim Garrison
Jim Garrison

Reputation: 86744

A simple MultiMap would look something like this skeleton:

public class MultiMap<K,V>  
{
    private Map<K,List<V>> map = new HashMap<K,List<V>>();
    public MultiMap() 
    { 
        // Define constructors
    }

    public void put(K key, V value)
    {
        List<V> list = map.get(key);
        if (list == null)
        {
            list = new ArrayList<V>();
            map.put(key, list);
        }
        list.add(value);
    }

    public List<V> get(K key)
    {
        return map.get(key);
    }

    public int getCount(K key)
    {
        return map.containsKey(key) ? map.get(key).size() : 0;
    }
}

It cannot directly implement Map<K,V> because put can't return the replaced element (you never replace). A full elaboration would define an interface MultiMap<K,V> and an implementation class, I've omitted that for brevity, as well as other methods you might want, such as V remove(K key) and V get(K key, int index)... and anything else you can think of that might be useful :-)

Upvotes: 3

TotoroTotoro
TotoroTotoro

Reputation: 17612

You can use a map with lists as values, e.g.:

HashMap<Integer, List<String>> myMap = new HashMap<Integer, List<String>>();

Upvotes: 6

Amir Afghani
Amir Afghani

Reputation: 38511

java.util.HashMap does not allow you to map multiple values to a single key. You want to use one of Guava's Multimap's. Read through the interface to determine which implemented version is suitable for you.

Upvotes: 3

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