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What is a MultiValuedMap in Apache Commons Collections?

A MultiValuedMap<K,V> lets a single key be associated with more than one value, storing the values for each key in a backing Collection instead of a single slot.

MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
map.put("fruits", "apple");
map.put("fruits", "banana");

Collection<String> values = map.get("fruits");  // [apple, banana]

Implementations differ in how they store the values per key: ArrayListValuedHashMap keeps them in an ArrayList (ordered, duplicates allowed), while HashSetValuedHashMap keeps them in a HashSet (unordered, duplicates collapsed).

Unlike a plain Map<K, List<V>> you build yourself, MultiValuedMap handles the null-checking and list creation for the first insert automatically.

Take quiz

Calling put() twice on a MultiValuedMap with the same key: overwrites the previous value

✗ Try again if not.

adds the new value alongside the existing one(s)

✓ Correct! Well done.

throws an exception

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Which implementation collapses duplicate values per key? ArrayListValuedHashMap

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HashSetValuedHashMap

✓ Correct! Well done.

DualHashBidiMap

✗ Try again if not.

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