Allowing Duplicate Keys in Java Collections

Java Collections allows you to add one or more elements with the same key by using the MultiValueMap class, found in the Apache org.apache.commons.collections package (http://commons.apache.org/collections/):

package multihashmap.example;

import java.util.Iterator;
import java.util.List;  
import java.util.Map;
import java.util.Set;
import org.apache.commons.collections.map.MultiValueMap;

public class MultiHashMapExample {

    public static void main(String[] args) {

        List list;
        MultiValueMap map = new MultiValueMap();
        map.put("A", 4);
        map.put("A", 6);
        map.put("B", 7);
        map.put("C", 1);
        map.put("B", 9);
        map.put("A", 5);

        Set entrySet = map.entrySet();
        Iterator it = entrySet.iterator();
        System.out.println("  Object key  Object value");
        while (it.hasNext()) {
            Map.Entry mapEntry = (Map.Entry) it.next();
            list = (List) map.get(mapEntry.getKey());
            for (int j = 0; j < list.size(); j++) {
                System.out.println("\t" + mapEntry.getKey() + "\t  " + list.get(j));
            }
        }
    }
}

From http://e-blog-java.blogspot.com/2012/02/how-to-allow-duplicate-key-in-java.html