HashMap with multiple values under the same key

1. Use a map that has a list as the value. Map<KeyType, List<ValueType>>.
2. Create a new wrapper class and place instances of this wrapper in the map. Map<KeyType, WrapperType>.
3. Use a tuple like class (saves creating lots of wrappers). Map<KeyType, Tuple<Value1Type, Value2Type>>.
4. Use mulitple maps side-by-side.

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Examples

1. Map with list as the value

// create our map
Map<string, List<Person>> peopleByForename = new HashMap<string, List<Person>>();    

// populate it
List<Person> people = new ArrayList<Person>();
people.Add(new Person("Bob Smith"));
people.Add(new Person("Bob Jones"));
peopleByForename.Add("Bob", people);

// read from it
List<Person> bobs = peopleByForename["Bob"];
Person bob1 = bobs[0];
Person bob2 = bobs[1];

2. Using wrapper class

// define our wrapper
class Wrapper {
    public Wrapper(Person person1, Person person2) {
       this.person1 = person1;
       this.person2 = person2;
    }

    public Person getPerson1 { return this.person1; }
    public Person getPerson2 { return this.person2; }

    private Person person1;
    private Person person2;
}

// create our map
Map<string, Wrapper> peopleByForename = new HashMap<string, Wrapper>();

// populate it
Wrapper people = new Wrapper()
peopleByForename.Add("Bob", new Wrapper(new Person("Bob Smith"),
                                        new Person("Bob Jones"));

// read from it
Wrapper bobs = peopleByForename["Bob"];
Person bob1 = bobs.Person1;
Person bob2 = bobs.Person2;

3. Using a tuple

// you'll have to write or download a Tuple class in Java, (.NET ships with one)

// create our map
Map<string, Tuple2<Person, Person> peopleByForename = new HashMap<string, Tuple2<Person, Person>>();

// populate it
peopleByForename.Add("Bob", new Tuple2(new Person("Bob Smith",
                                       new Person("Bob Jones"));

// read from it
Tuple<Person, Person> bobs = peopleByForename["Bob"];
Person bob1 = bobs.Item1;
Person bob2 = bobs.Item2;

4. Multiple maps

// create our maps
Map<string, Person> firstPersonByForename = new HashMap<string, Person>();
Map<string, Person> secondPersonByForename = new HashMap<string, Person>();

// populate them
firstPersonByForename.Add("Bob", new Person("Bob Smith"));
secondPersonByForename.Add("Bob", new Person("Bob Jones"));

// read from them
Person bob1 = firstPersonByForename["Bob"];
Person bob2 = secondPersonByForename["Bob"];

Java : MultiKeyMap Example

•  A Map implementation that uses multiple keys to map the value.
• This class is the most efficient way to uses multiple keys to map to a value. The best way to use this class is via the additional map-style methods. These provide get, containsKey, put and remove for individual keys which operate without extra object creation.
• The additional methods are the main interface of this map. As such, you will not normally hold this map in a variable of type Map.
• The normal map methods take in and return a MultiKey. If you try to use put() with any other object type a ClassCastException is thrown. If you try to use null as the key in put() a NullPointerException is thrown.
• This map is implemented as a decorator of a AbstractHashedMap which enables extra behaviour to be added easily.
•  MultiKeyMap.decorate(new LinkedMap()) creates an ordered map
•  MultiKeyMap.decorate(new LRUMap()) creates an least recently used map.
• MultiKeyMap.decorate(new ReferenceMap()) creates a garbage collector sensitive map. 
• Note that IdentityMap and ReferenceIdentityMap are unsuitable for use as the key comparison would work on the whole MultiKey, not the elements within.
• Note that MultiKeyMap is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization. This class may throw exceptions when accessed by concurrent threads without synchronization.
• Apache Commons Collections to create a MultiKeyMap that will store two keys with one corresponding value and then using MapIterator to walk through the map.

• MultiKeyMap keys are instances of MultiKey, which has a getKey(int index) method. There is also a getKeys() method which returns Object[].