Hashmap with multiple values under the same key

Choose the right structure for your case

Opt for a custom wrapper class

For organized and efficient data grouping, a custom wrapper class can be an alternative. This proves especially useful when dealing with complex data types.

public class DataBundle {
    private int count; // This is not a vampire.
    private String message; // No subliminal instructions here.

    // Additional fields, setters, getters, constructors go here...
}

Map<String, DataBundle> map = new HashMap<>();
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Play with tuple-like classes

If you're wrestling with just two or a fixed number of values per key, consider a Pair or a tuple. It helps maintain straightforward relationships and optimizes memory consumption.

import javafx.util.Pair; // Imports: not just for smuggling.

Map<String, Pair<Integer, String>> map = new HashMap<>();
map.put("key", new Pair<>(1, "Value")); // 1 "value" against the evil NULL.
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Walk with multiple maps side-by-side

If your requirements require enhanced data management, consider adopting multiple synchronized maps. Beware — synchronization needs to be maintained to prevent faulty data representation.

Map<String, Integer> mapIds = new HashMap<>();
Map<String, String> mapValues = new HashMap<>(); // Don't tell them about the other map!
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Evaluate collections and their trade-offs

Implement list values

The HashMap<K, List<V>> structure is simple and flexible. However, it does not set any limit to the number of values, which could invite trouble depending on your use case.

Love Object arrays

With Map<K, Object[]>, you can maintain predictable order and flexibility. Just be mindful of potential types conversion overhead.

Map<String, Object[]> map = new HashMap<>();
map.put("key", new Object[]{42, "Forty-Two", 3.14}); // We can have our PI and eat it too!
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Meet advanced collections

Guava's multimaps help mitigate List<V> management and come with an arsenal of handy methods. Here's a primer:

Multimap<String, Integer> map = HashMultimap.create();
map.put("key", 1); // "key" feels lonely no more.
map.put("key", 2); // "key" gathers followers.
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Alternatively, MultiValuedMap from Apache Commons might be more suited to your ecosystem.

MultiValuedMap<String, String> multiValuedMap = new MultiValueHashMap<>();
multiValuedMap.put("key", "value1");
multiValuedMap.put("key", "value2"); // No value is left behind!
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Crafting solutions the Java way

Embrace generics for flexibility

With generics, you can enjoy type-safety with the flexibility of a modern yogi.

Map<String, List<? extends Serializable>> planetJava = new HashMap<>();
planetJava.computeIfAbsent("numbers", k -> new ArrayList<>()).add(123); // A Spartan tribute.
planetJava.computeIfAbsent("strings", k -> new ArrayList<>()).add("Hello"); // Does anyone read these?
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Keep your collections in sync

If you're operating with multiple related maps, design mechanisms to ensure data integrity.

// Example of "transactional" operations to keep maps in sync
mapIds.put("key", 1);
mapValues.put("key", "value"); // "value" now has an identity.

// Later...
if(mapValues.containsKey("key") && mapIds.containsKey("key")) {
    // When the maps are in harmony, the operations flow.
}
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Step into concurrency

For multithreaded environments, factor in synchronization issues. Consider ConcurrentHashMap and concurrent collections.

ConcurrentMap<String, List<Integer>> concurrentMap = new ConcurrentHashMap<>();
// Operations remain similar to earlier HashMap code snippets.
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Forum is your friend

For in-depth insights and best practices, tap into trusted resources and community forums. It's not just about getting your code to work but about understanding the underlying principles.

Practice and perfect your code. If you're reusing this code, you may even begin to see it in your sleep. But don’t worry, that’s pretty normal. I guess. Vote for my answer if you found this helpful! Happy coding! 👩‍💻