How Can I Store Multiple Values per Key in a HashMap?

HashMap to Store Multiple Values per Key

HashMap is a widely used data structure that maps keys to values. However, it is typically designed to store a single value for each key. This limitation may not always align with real-world requirements, where the need arises to store multiple values under the same key.

Approaches to Implement Multiple Values in a HashMap

If the requirement is to store exactly two values per key, several approaches can be considered:

1. Map with List as Value: This involves using a Map> data structure. Each key in the map corresponds to a list of values, allowing multiple values to be associated with a single key.
2. Wrapper Class: A wrapper class can be created to encapsulate the two values. The map then stores instances of this wrapper class as values, providing a way to retrieve both values simultaneously.
3. Tuple Class: A tuple class can be employed, which is a built-in or external library that represents a collection of elements. The map can use a tuple as a value, allowing it to store exactly two values in a single object.

Example Implementations

Using List as Value:

// Initialize the HashMap
Map<String, List<Person>> peopleByForename = new HashMap<>();

// Populate the HashMap
List<Person> people = new ArrayList<>();
people.add(new Person("Bob Smith"));
people.add(new Person("Bob Jones"));

peopleByForename.put("Bob", people);

// Retrieve values
List<Person> bobs = peopleByForename.get("Bob");
Person bob1 = bobs.get(0);
Person bob2 = bobs.get(1);

Using Wrapper Class:

// Define the wrapper class
class Wrapper {
    private Person person1;
    private Person person2;

    public Wrapper(Person person1, Person person2) {
        this.person1 = person1;
        this.person2 = person2;
    }

    public Person getPerson1() { return person1; }
    public Person getPerson2() { return person2; }
}

// Initialize the HashMap
Map<String, Wrapper> peopleByForename = new HashMap<>();

// Populate the HashMap
peopleByForename.put("Bob", new Wrapper(new Person("Bob Smith"), new Person("Bob Jones")));

// Retrieve values
Wrapper bobs = peopleByForename.get("Bob");
Person bob1 = bobs.getPerson1();
Person bob2 = bobs.getPerson2();

Using Tuple Class:

// Initialize the HashMap
Map<String, Tuple2<Person, Person>> peopleByForename = new HashMap<>();

// Populate the HashMap
peopleByForename.put("Bob", new Tuple2<>(new Person("Bob Smith"), new Person("Bob Jones")));

// Retrieve values
Tuple2<Person, Person> bobs = peopleByForename.get("Bob");
Person bob1 = bobs.Item1;
Person bob2 = bobs.Item2;

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