Retrieval of Key through Values in a Map.

[dinesh Venkatesan]

Hi All,

I am doing a simple application in which I am using a LinkedHashMap implementation of Map to store <contactName> and <contactNumber> pairs.

and i am implementing two kinds of search searchByName() and searchByNumber();

while searching by Name i can easily retrieve the results where as while searching through the number there is no direct way to do that according to what i know so far. So i used the following approach.

 
    public int searchByNumber(Object contactNumber) throws Exception {
        int count = 0;
        Map originalMap = new ContactsLoader().loadContacts();
        Collection values = originalMap.values();
        Set keySet = originalMap.keySet();
        Iterator keyIterator = keySet.iterator();
        Set valuesSet = new LinkedHashSet(values);
        Iterator valuesIterator = valuesSet.iterator();
        while( valuesIterator.hasNext() ) {
            String keyString="";
            if(keyIterator.hasNext()) {
                keyString = (String)keyIterator.next();
            }
            String valueString = (String)valuesIterator.next();
            if ( valueString.startsWith(contactNumber.toString()) ) {
                resultMap.put(valueString,keyString);
                count++;
            }
         }
         return count;
    }

If anybody knows some other efficient way for doing the same please let me know.

Thanks in Advance!!!
dinesh.

[ Added code tags - Jim ]
[ January 01, 2007: Message edited by: Jim Yingst ]

[Ernest Friedman-Hill]

Use multiple maps, with keys appropriate to each search.

[Peter Chase]

Or, if you know that no key is ever the same as any value, then you can use just one map, with one forward and one backward mapping per key-value pair. In the case of telephone numbers and people's names, you're pretty safe; there's unlikely to be a "Mr 990-12345" or anyone whose number is "Jane Smith".

[Mr. C Lamont Gilbert]

That is interesting peter. I never thought of that. I like it.

To the OP, I don't think your technique even works. The valuset and the key set I do not think are guaranteed to be returned in the same order, nor are they guaranteed to iterate in the same order. I don't think.

Try using entry set.

There is another dirty approach like so;

 
    public int searchByNumber(Object contactNumber) throws Exception {
        int count = 0;
        Map originalMap = new ContactsLoader().loadContacts();
        Collection entries= originalMap.entrySet();
        for(Iterator it = entries.iterator();it.hasNext() ; ) {
            Map.Entry me = (Map.Entry)it.next();
            String valueString = (String)me.getValue();
            if ( valueString.startsWith(contactNumber.toString()) ) {
                resultMap.put(valueString,me.getKey());
                count++;
            }
         }
         return count;
    }

[ January 02, 2007: Message edited by: Mr. C Lamont Gilbert ]

[Ilja Preuss]

Take a look at http://jakarta.apache.org/commons/collections/api-3.1/org/apache/commons/collections/BidiMap.html