How can I store and retrieve multiple values associated with a single key in a Java HashMap?

Retrieving Multiple Values for a Key in a HashMap

HashMap is a commonly used Java collection that maps keys to a single value. However, in certain scenarios, you may find yourself desiring the capability to store multiple values for a given key. This raises the question: Is it possible to access multiple values associated with a particular key in a HashMap?

While HashMap natively supports only one value per key, there are techniques to achieve this functionality. One straightforward approach is to utilize a Map of List structure.

Creating a Map of List

To store multiple values for each key, we can create a HashMap that maps objects to ArrayLists of objects, as demonstrated in the code snippet below:

<code class="java">Map&lt;Object, ArrayList&lt;Object&gt;&gt; multiMap = new HashMap&lt;&gt;();</code>

This approach allows us to add multiple values to a single key by appending them to the corresponding ArrayList. To retrieve the third value for a given key, we can simply access the ArrayList and retrieve the third element.

Example Usage

Assuming we have a HashMap with multiple values for certain keys, we can access the third value for a key as follows:

<code class="java">List&lt;Object&gt; values = multiMap.get(key);
Object thirdValue = values.get(2);</code>

Here, values is the list of values associated with the key, and thirdValue represents the third value in the list.

By utilizing a Map of List, you can effectively store and access multiple values for each key in a HashMap. This technique provides greater flexibility and allows you to handle more complex data structures effortlessly.

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