HashMap question?

[watsonyaya]

In HashMap, we can get the value if we know the key:

HashMap hm = new HashMap( );
hm.put("java1","eclipse");
hm.put("java2","NetBean");

String value = (Object)hm.get("java1");

How to get the key if we know the mapping value(object) only?


thanks

[Ernest Friedman-Hill]

Hi,

Welcome to JavaRanch!

If you really need to do this, you can call iterator() to get an iteration over Map.Entry objects, and look at the value() in each Map.Entry until you find the value of interest, and then look at the key(). This is obviously quite inefficient if you need to do it multiple times; in that case, consider keeping two maps, one in each "direction."

[watsonyaya]

I can not figure it out, can you show me a sample?

HashMap hm = new HashMap();
hm.put("java1","NetBean");
hm.put("java2","Eclipse");
hm.put("java3","JBuilder");
hm.put("java4","JDeveloper");
Can you retrieve String "java4" if you know the mapping object "Jdeveloper" only?

thanks

[Chit Ming Chong]

There is no easy way to get it (by easy I meant by making a simple method call)

As the values in a hashmap are not distinct, one value may correspond to multiple keys.

You can code a linear search by accessing each elements corresponding to all keys and mark the key(s)

[David Harkness]

Here's semi-psuedocode for Ernest's suggestion. It returns the first key that it finds mapped to the given value.

Object findKey(Map map, Object value)
{
    Iterator pairs = map.entrySet().iterator();
    for each entry in pairs
        if entry's value equals value parameter
            return entry's key
    
    // value not found
    return null
}

Map.Entry is an inner class of Map and contains a single pairing (key and value). Note that some Maps allow null values.

[Ilja Preuss]

Or simply use a BidiMap implementation from http://jakarta.apache.org/commons/collections