Map implementation with duplicate keys

Question

You are searching for a multimap, and indeed both commons-collections and Guava have several implementations for that. Multimaps allow for multiple keys by maintaining a collection of values per key, i.e. you can put a single object into the map, but you retrieve a collection.

If you can use Java 5, I would prefer Guava's Multimap as it is generics-aware.

We don't need to depend on the Google Collections external library. You can simply implement the following Map:

Map<String, ArrayList<String>> hashMap = new HashMap<String, ArrayList>();

public static void main(String... arg) {
   // Add data with duplicate keys
   addValues("A", "a1");
   addValues("A", "a2");
   addValues("B", "b");
   // View data.
   Iterator it = hashMap.keySet().iterator();
   ArrayList tempList = null;

   while (it.hasNext()) {
      String key = it.next().toString();             
      tempList = hashMap.get(key);
      if (tempList != null) {
         for (String value: tempList) {
            System.out.println("Key : "+key+ " , Value : "+value);
         }
      }
   }
}

private void addValues(String key, String value) {
   ArrayList tempList = null;
   if (hashMap.containsKey(key)) {
      tempList = hashMap.get(key);
      if(tempList == null)
         tempList = new ArrayList();
      tempList.add(value);  
   } else {
      tempList = new ArrayList();
      tempList.add(value);               
   }
   hashMap.put(key,tempList);
}

Please make sure to fine tune the code.

Multimap<Integer, String> multimap = ArrayListMultimap.create();

multimap.put(1, "A");
multimap.put(1, "B");
multimap.put(1, "C");
multimap.put(1, "A");

multimap.put(2, "A");
multimap.put(2, "B");
multimap.put(2, "C");

multimap.put(3, "A");

System.out.println(multimap.get(1));
System.out.println(multimap.get(2));       
System.out.println(multimap.get(3));

Output is:

[A,B,C,A]
[A,B,C]
[A]

Note: we need to import library files.

http://www.java2s.com/Code/Jar/g/Downloadgooglecollectionsjar.htm

import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.Multimap;

or https://commons.apache.org/proper/commons-collections/download_collections.cgi

import org.apache.commons.collections.MultiMap;
import org.apache.commons.collections.map.MultiValueMap;

You could simply pass an array of values for the value in a regular HashMap, thus simulating duplicate keys, and it would be up to you to decide what data to use.

You may also just use a MultiMap, although I do not like the idea of duplicate keys myself.