How to Store Multiple Values with the Same Key in a HashMap?

Creating HashMaps with Multiple Values per Key

In some scenarios, it may be necessary to store multiple values with the same key in a HashMap. Although Java's HashMap does not natively support this, there are several alternative approaches to achieve this functionality.

1. Map with List as Value

One option is to create a HashMap where the values are lists. This allows for multiple values to be associated with a single key. For instance:

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

2. Wrapper Class

An alternative is to define a wrapper class that holds the multiple values. This wrapper can then be used 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;
    }

    // Getter methods
}

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

3. Tuples

If your programming language supports tuples, you can utilize them as keys or values in a HashMap. For example:

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

4. Multiple Maps

Finally, another strategy is to use separate maps for each value type:

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

Examples

Considering the example scenario of a HashMap with a userId, clientID, and timeStamp:

Option 1: Map with List as Value

Map<Integer, List<Pair<String, Long>>> data = new HashMap<>();

data.put(1, Arrays.asList(new Pair<>("client-1", System.currentTimeMillis())));
data.put(1, Arrays.asList(new Pair<>("client-2", System.currentTimeMillis())));

Option 2: Wrapper Class

class Data {
    private Integer userId;
    private String clientID;
    private Long timeStamp;

    public Data(Integer userId, String clientID, Long timeStamp) {
        this.userId = userId;
        this.clientID = clientID;
        this.timeStamp = timeStamp;
    }
}

Map<Integer, Data> data = new HashMap<>();

data.put(1, new Data(1, "client-1", System.currentTimeMillis()));
data.put(1, new Data(1, "client-2", System.currentTimeMillis()));

The choice of approach depends on the specific requirements of your application and the programming language being used.

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