How Can I Implement a HashMap to Store Multiple Values for the Same Key?

Implementing HashMap with Multiple Values for the Same Key

A HashMap is a data structure that maps keys to values. By default, each key can only have one associated value. However, it is possible to implement a HashMap that allows one key to have multiple values.

Options for Implementing Multiple Values:

1. Map with List as the Value:
Map>

Use a map where the value is a list. This allows you to store multiple values under a single key. The disadvantage is that the list could contain more or less than two values.

2. Custom Wrapper Class:
Map

Create a wrapper class that contains the multiple values. This keeps the values bound to a single entity and provides encapsulation. However, it requires writing additional code to create and manage the wrapper class.

3. Tuple Class:
Map>

Use a tuple class (if available in your programming language) to store the multiple values. This provides a simple and type-safe way to represent them.

4. Multiple Maps:
Map
Map

Use multiple maps to store the different values. While convenient, it can lead to disconnected values and maintenance issues if the maps get out of sync.

Examples:

Using a Map with List as the Value:

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

List<Person> bobs = new ArrayList<>();
bobs.add(new Person("Bob Smith"));
bobs.add(new Person("Bob Jones"));
peopleByForename.put("Bob", bobs);

Using a Custom Wrapper Class:

class Wrapper {
    public Person person1;
    public Person person2;

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

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

peopleByForename.put("Bob", new Wrapper(new Person("Bob Smith"), new Person("Bob Jones")));

Using a Tuple Class:

// Assumes a Tuple2 class is available
Map<String, Tuple2<Person, Person>> peopleByForename = new HashMap<>();

peopleByForename.put("Bob", Tuple2.create(new Person("Bob Smith"), new Person("Bob Jones")));

Using Multiple Maps:

Map<String, Person> firstPersonByForename = new HashMap<>();
Map<String, Person> secondPersonByForename = new HashMap<>();

firstPersonByForename.put("Bob", new Person("Bob Smith"));
secondPersonByForename.put("Bob", new Person("Bob Jones"));

Consider the advantages and disadvantages of each approach before choosing the best option for your specific use case.

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