How to Implement a HashMap with Multiple Values Per Key in Java?

Implementing a Map with Multiple Values Per Key

In certain scenarios, it may be necessary to store multiple values under the same key within a HashMap. While Java's standard HashMap does not natively support this behavior, there are several approaches to achieve it:

1. Map with List as the Value:

This approach involves creating a HashMap where the value is a list of the desired values. For example:

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

While this method provides flexibility regarding the number of values stored under the key, it lacks the guarantee of having exactly two values.

2. Using a Wrapper Class:

An alternative is to define a wrapper class that contains the desired values and use it as the value in the HashMap.

class Wrapper {
    private Person person1;
    private Person person2;

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

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

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

This method ensures the presence of exactly two values, but it requires the creation of additional boilerplate code for the wrapper class.

3. Using a Tuple:

A tuple class encapsulates multiple values into a single object. By using a tuple as the value in the HashMap, you can effectively achieve multiple values per key. For example:

import java.util.Tuple;

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

4. Multiple Maps Side-by-Side:

Finally, you can also use multiple HashMaps for different key types. For instance, you could create:

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

While this method allows for clear separation of the values, it introduces the potential for inconsistency if the two maps are not consistently updated.

The choice of approach depends on the specific requirements of the application. If strict enforcement of two values per key is crucial, using a wrapper class or tuple may be more suitable. Alternatively, if flexibility in the number of values is desired, using a map with a list as the value may be more appropriate.

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