Java Hashmap: How to get key from value?

Coping with many-to-one mappings

In Java, it's generally smooth sailing with HashMaps adhering to a one-to-many mapping. However, a storm comes brewing 🌩️ when you want to find all keys linked to a certain value, a many-to-one affair:

List<String> allKeys = map.entrySet().stream() // Unleash inner detective 🕵️‍♂️
                       .filter(e -> Objects.equals(e.getValue(), valueToFind)) // Matching clues?
                       .map(Map.Entry::getKey) // Keep them
                       .collect(Collectors.toList()); // Hand collection of clues
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Rapid key retrieval with a reverse map

For those speed thrills 😎, have a reverse map ready alongside the original:

HashMap<String, Integer> valueToKeyMap = new HashMap<>(); // It's like having a cheat-sheet for a test 📄✍️
map.forEach((key, value) -> valueToKeyMap.put(value, key));  // Put your cheat-sheet to good use
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Enjoying the perks of bidirectional maps

In the land where keys and values are both unique, the hero saving the day is Guava's BiMap:

BiMap<String, Integer> biMap = HashBiMap.create(); // Now you're the superhero 🦸‍♂️
biMap.put("a", 1);
Integer key = biMap.inverse().get(valueToFind); // Swift and clean. Just like a superhero!
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Exploring bidirectional maps

For keys and values having a 1:1 relationship and needing to flip regularly, bidi maps come to the rescue:

BidiMap<String, String> bidiMap = new DualHashBidiMap<>(); 
bidiMap.put("John", "Developer");
String profession = bidiMap.getKey("Developer"); // So, John does what for a living? 💻
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Making alternate routes and library pitstops

• Apache Commons' BidiMap offers getKey and ensures a one-to-one relationship between keys and values because java loves commitment too! 💍
• Guava's ImmutableBiMap is super handy when it comes to creating immutable, concurrent-friendly bidirectional maps. This one ain't changing, not even for a cup of coffee! ☕
• For handling large datasets, consider Java 8 Streams or a double-entry map to save navigation headaches.

The power of generics and comparison

Featuring generics in map creation bolsters type safety and diminishes casting errors:

BiMap<String, Integer> userIdToAge = HashBiMap.create(); // Strong type-checking.. just like bouncers at a club! 💪🚪
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Objects.equals paves the way for a safer comparison, particularly when dealing with possible nulls. Because precaution is better than NullPointerException! 😉