Question

In the below key/value list “val3” is repeating for different keys. Which map suits best for such type of list.

I want “val3” should be stored only once with multiple keys pointing to it.

key1—->val1
key2—->val3
key3—->val3
key5—->val5
key6—->val3
key7—->val6
key8—->val3
key9—->val3

Answer 1

Any Map implementation will do. Every Java collection only holds references, not actual objects, so if you put the same object several times in a map, all the values will point to the same object. In other words changes made to val3 through key2 lookup will be reflected when looking up by any other key pointing to val3.

Consider simplified example:

VeryLarge v = new VeryLarge();
Map<Integer, VeryLarge> map = new HashMap<>();
map.put(1, v);
map.put(2, v);
map.put(3, v);

Single VeryLarge instance is referenced by all 1, 2 and 3 keys.