Map implementation with duplicate keys

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.

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.

If you want iterate about a list of key-value-pairs (as you wrote in the comment), then a List or an array should be better. First combine your keys and values:

public class Pair
{
   public Class1 key;
   public Class2 value;

   public Pair(Class1 key, Class2 value)
   {
      this.key = key;
      this.value = value;
   }

}

Replace Class1 and Class2 with the types you want to use for keys and values.

Now you can put them into an array or a list and iterate over them:

Pair[] pairs = new Pair[10];
...
for (Pair pair : pairs)
{
   ...
}

No fancy libs required. Maps are defined by a unique key, so dont bend them, use a list. Streams are mighty.

import java.util.AbstractMap.SimpleImmutableEntry;

List<SimpleImmutableEntry<String, String>> nameToLocationMap = Arrays.asList(
    new SimpleImmutableEntry<>("A", "A1"),
    new SimpleImmutableEntry<>("A", "A2"),
    new SimpleImmutableEntry<>("B", "B1"),
    new SimpleImmutableEntry<>("B", "B1"),
);

And thats it. Usage examples:

List<String> allBsLocations = nameToLocationMap.stream()
        .filter(x -> x.getKey().equals("B"))
        .map(x -> x.getValue())
        .collect(Collectors.toList());

nameToLocationMap.stream().forEach(x -> 
do stuff with: x.getKey()...x.getValue()...

You can use a TreeMap with a custom Comparator in order to treat each key as unequal to the others. It would also preserve the insertion order in your map, just like a LinkedHashMap. So, the net result would be like a LinkedHashMap which allows duplicate keys!

This is a very simple implementation without the need of any third party dependencies or complications of MultiMaps.

import java.util.Map;
import java.util.TreeMap;

...
...

//Define a TreeMap with a custom Comparator
Map<Integer, String> map = new TreeMap<>((a, b) -> 1); // See notes 1 and 2

//Populate the map
map.put(1, "One");
map.put(3, "Three");
map.put(1, "One One");
map.put(7, "Seven");
map.put(2, "Two");
map.put(1, "One One One");
    
//Display the map entries:
map.entrySet().forEach(System.out::println);

//See note number 3 for the following:
Map<Integer, String> sortedTreeMap = map.entrySet().stream()
            .sorted(Map.Entry.comparingByKey())
            .collect(Collectors.toMap(
                Map.Entry::getKey, Map.Entry::getValue,
                (x, y) -> x, () -> new TreeMap<>((a, b) -> 1)
             ));
//Display the entries of this sorted TreeMap: 
sortedTreeMap.entrySet().forEach(System.out::println);

    
...

Notes:

1. You can also use any positive integer in place of 1 in the comparator's definition here.
2. If you use any negative integer instead, then it will reverse the insertion order in your map.
3. If you also want to sort this map based on the keys (which is the default behavior of a TreeMap), then you may do this operation on the current map.

 1, Map<String, List<String>> map = new HashMap<>();

this verbose solution has multiple drawbacks and is prone to errors. It implies that we need to instantiate a Collection for every value, check for its presence before adding or removing a value, delete it manually when no values are left, etcetera.

2, org.apache.commons.collections4.MultiMap interface
3, com.google.common.collect.Multimap interface 

java-map-duplicate-keys

what about such a MultiMap impl?

public class MultiMap<K, V> extends HashMap<K, Set<V>> {
  private static final long serialVersionUID = 1L;
  private Map<K, Set<V>> innerMap = new HashMap<>();

  public Set<V> put(K key, V value) {
    Set<V> valuesOld = this.innerMap.get(key);
    HashSet<V> valuesNewTotal = new HashSet<>();
    if (valuesOld != null) {
      valuesNewTotal.addAll(valuesOld);
    }
    valuesNewTotal.add(value);
    this.innerMap.put(key, valuesNewTotal);
    return valuesOld;
  }

  public void putAll(K key, Set<V> values) {
    for (V value : values) {
      put(key, value);
    }
  }

  @Override
  public Set<V> put(K key, Set<V> value) {
    Set<V> valuesOld = this.innerMap.get(key);
    putAll(key, value);
    return valuesOld;
  }

  @Override
  public void putAll(Map<? extends K, ? extends Set<V>> mapOfValues) {
    for (Map.Entry<? extends K, ? extends Set<V>> valueEntry : mapOfValues.entrySet()) {
      K key = valueEntry.getKey();
      Set<V> value = valueEntry.getValue();
      putAll(key, value);
    }
  }

  @Override
  public Set<V> putIfAbsent(K key, Set<V> value) {
    Set<V> valueOld = this.innerMap.get(key);
    if (valueOld == null) {
      putAll(key, value);
    }
    return valueOld;
  }

  @Override
  public Set<V> get(Object key) {
    return this.innerMap.get(key);
  }

  @Override
  etc. etc. override all public methods size(), clear() .....

}

Could you also explain the context for which you are trying to implement a map with duplicate keys? I am sure there could be a better solution. Maps are intended to keep unique keys for good reason. Though if you really wanted to do it; you can always extend the class write a simple custom map class which has a collision mitigation function and would enable you to keep multiple entries with same keys.

Note: You must implement collision mitigation function such that, colliding keys are converted to unique set "always". Something simple like, appending key with object hashcode or something?

just to be complete, Apache Commons Collections also has a MultiMap. The downside of course is that Apache Commons does not use Generics.

If there are duplicate keys then a key may correspond to more than one value. The obvious solution is to map the key to a list of these values.

For example in Python:

map = dict()
map["driver"] = list()
map["driver"].append("john")
map["driver"].append("mike")
print map["driver"]          # It shows john and mike
print map["driver"][0]       # It shows john
print map["driver"][1]       # It shows mike