Hash table of form - <key, hash table> in Java

Question

I just want to know if it is possible to create a hashtable in java of the form <key, hash table>.

Essentially the first key leads me to a new hash table; then I search that table using another key.

Answer 1

Sure it is:

Map<K1, Map<K2, V>> themap = new HashMap<K1, Map<K2, V>>();

where K1 is the key of the "hash table", and K2 and V are the key and value type of the inner "hash table".

edit: as @AndreiBodnarescu rightly points out, you must also choose your Map implementation carefully (Map is an interface). Ask yourself the following questions:

• is multithreading access required on the outer/inner map? If yes, consider Hashtable or Collections.synchronizedMap(...);
• does insertion order matter? If yes, consider LinkedHashMap;
• do you want keys to be sorted? If yes, consider TreeMap.

Choose your implementation carefully!

Answer 2

you can use

Hashtable<KeyType,Hashtable<InnerKeyType,InnerValueType>> ht = new Hashtable<>();

where obviouslly the InnerValueType can still be a Hashtable

If your data structure is not accessed by multiple threads, you can repalce Hashtable with HashMap which has all the behaviour of a hashtable structure but without the synchronisation.

Answer 3

Ofcourse that's possible. You should use HashMap, not Hashtable (because Hashtable is a legacy collection class that has been replaced by HashMap since Java 1.2).

Example:

Map<String, Map<String, Object>> mapOfMaps = new HashMap<String, Map<String, Object>>();
mapOfMaps.put("one", new HashMap<String, Object>());
mapOfMaps.put("two", new HashMap<String, Object>());

Answer 4

try

Hashtable<Integer, Hashtable> hashTable = new Hashtable<Integer, Hashtable>():