How can you store and retrieve multiple values associated with the same key in a HashMap?

HashMap: Managing Multiple Values for a Single Key

Question:

Is it possible to store and retrieve multiple values associated with the same key in a HashMap? If so, how can I access the third value for the first key?

Answer:

While a standard HashMap cannot store duplicate keys, it is possible to implement a HashMap-like structure that allows multiple values for a single key using a Map of List. Here's a plain Java solution:

<code class="java">import java.util.HashMap;
import java.util.ArrayList;

public class MultiValueMapDemo {

    public static void main(String[] args) {
        // Create a Map of List to store key-value pairs
        Map<Object, ArrayList<Object>> multiMap = new HashMap<>();

        // Add key-value pairs
        multiMap.computeIfAbsent("keyA", k -> new ArrayList<>()).add("value1");
        multiMap.computeIfAbsent("keyA", k -> new ArrayList<>()).add("value2");
        multiMap.computeIfAbsent("keyA", k -> new ArrayList<>()).add("value3");

        // Get the third value for the first key
        Object thirdValue = multiMap.get("keyA").get(2);

        System.out.println("Third value for keyA: " + thirdValue);
    }
}</code>

This approach creates a HashMap where the values associated with a key are stored in a List. To access a specific value, you can use the get() method on the List object retrieved from the HashMap. In this example, the third value for the key "keyA" is accessed using multiMap.get("keyA").get(2).

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