A faster way to find out the key for a given value from the map?

Question

I want to find out the key for a given value from HashMap, currently I need to go through all the keys and check its value on the map, is there a faster way?

Answer 1

No, there is no faster way (without introducing other data structures). If you need to do this often, rethink your design. Perhaps you need another one HashMapwhose keys are the values ​​of the other HashMap?

Answer 2

An alternative data structure for this would be BiMapfrom the google collections API.

The API document is here .

Answer 3

private Object getKey(LinkedHashMap lm, int val){

    Object[] j = lm.keySet().toArray();
    return j[val];
}

Answer 4

stackoverflow:

• Java HashMap ?
• Java?

BidiMap:

:

package com.discursive.jccook.collections.bidi;
import org.apache.commons.collections.BidiMap;
import org.apache.commons.collections.bidimap.DualHashBidiMap;
public class BidiMapExample {
    private BidiMap countryCodes = new DualHashBidiMap( );
    public static void main(String[] args) {
        BidiMapExample example = new BidiMapExample( );
        example.start( );
    }

    private void start( ) {
        populateCountryCodes( );

        String countryName = (String) countryCodes.get( "tr" );
        System.out.println( "Country Name for code 'tr': " + countryName );
        String countryCode = 
            (String) countryCodes.inverseBidiMap( ).get("Uruguay");
        System.out.println( "Country Code for name 'Uruguay': " + countryCode );

        countryCode = (String) countryCodes.getKey("Ukraine");
        System.out.println( "Country Code for name 'Ukraine': " + countryCode );
    }

    private void populateCountryCodes( ) {
        countryCodes.put("to","Tonga");
        countryCodes.put("tr","Turkey");
        countryCodes.put("tv","Tuvalu");
        countryCodes.put("tz","Tanzania");
        countryCodes.put("ua","Ukraine");
        countryCodes.put("ug","Uganda");
        countryCodes.put("uk","United Kingdom");
        countryCodes.put("um","USA Minor Outlying Islands");
        countryCodes.put("us","United States");
        countryCodes.put("uy","Uruguay");
    }
}

Answer 5

HashMap.get(key),

= getEntry ();

getEntry () private?? . , ...

HashMap

/**
 * Returns the entry associated with the specified key in the
 * HashMap.  Returns null if the HashMap contains no mapping
 * for the key.
 */
final Entry<K,V> getEntry(Object key) {
    int hash = (key == null) ? 0 : hash(key);
    for (Entry<K,V> e = table[indexFor(hash, table.length)];
         e != null;
         e = e.next) {
        Object k;
        if (e.hash == hash &&
            ((k = e.key) == key || (key != null && key.equals(k))))
            return e;
    }
    return null;
}

Answer 6

, ​​ TreeMap, .