Java invert map

Question

I need create inverse map - select unique values and for them find keys. Seems that only way is to iterate all key/value pairs, because entrySet returns set of so value not unique? Thanks.

Answer 1

You have to assume that values may be identical, since the Map contract allows it.

In my opinion the best solution lies in using a wrapper. It will contain the original value, and add an id. Its hashCode() function will rely on the id, and you provide a Getter for the original value. Code would be something like this:

public class MapKey
{
    /**
     * A new ID to differentiate equal values 
     */
    private int _id;
    /**
     * The original value now used as key
     */
    private String _originalValue;

    public MapKey(String originalValue)
    {
        _originalValue = originalValue;
       //assuming some method for generating ids...
        _id = getNextId();
    }

    public String getOriginalValue()
    {
        return _originalValue;
    }

    @Override
    public int hashCode()
    {
        final int prime = 31;
        int result = 1;
        result = prime * result + _id;
        return result;
    }

    @Override
    public boolean equals(Object obj)
    {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        MapKey other = (MapKey) obj;
        if (_id != other._id)
            return false;
        return true;
    }

    @Override
    public String toString()
    {
        StringBuilder sb = new StringBuilder();
        sb.append("MapKey value is ");
        sb.append(_originalValue);
        sb.append(" with ID number ");
        sb.append(_id);
        return sb.toString();
    }

Inverting the map would be something like this:

public Map <MapKey, Integer> invertMap(Map <Integer, String> map)
{

     Map <MapKey, Integer> invertedMap = new HashMap <MapKey, Integer>();

   Iterator<Entry<Integer, String>> it = map.entrySet().iterator();

   while(it.hasNext())
   {
       //getting the old values (to be reversed)
       Entry<Integer, String> entry = it.next();
       Integer oldKey = entry.getKey();
       String oldValue = entry.getValue();

       //creating the new MapKey
       MapKey newMapKey = new MapKey(oldValue);
       invertedMap.put(newMapKey, oldKey);
   }

   return invertedMap;
}

Printing the values something like this:

for(MapKey key : invertedMap.keySet())
       {
           System.out.println(key.toString() + " has a new value of " +  invertedMap.get(key));

       }

None of this code is tested, but I believe it's the best solution since it makes use of OO inheritance design instead of "c" style checks and allows you to display all the original keys and values.

Answer 2

With Guava

Multimaps.transformValues(Multimaps.index(map.entrySet(), Map.Entry::getValue),
        Map.Entry::getKey)

You'll get a multimap (basically a map of lists) in return.