Question

Nested HashMap performance.

HashMap I have the following:

HashMap <String, HashMap <String, HashMap <String,String>>> table = new HashMap <String, HashMap <String, HashMap <String, String> >>();

this is right?

Answer 1

HashMap <String, HashMap <String, HashMap <String,String>>> table1 = new HashMap <String, HashMap <String, HashMap <String, String> >>();

↑ perfectly fine.

In terms of nesting the HashMaps, a better option might be to concatenate the keys and use a non-nested HashMap if you’re typically interested in the end values and not the intermediate maps.

…in other words, define a Triple class with three strings, and define a hashcode and equals method, etc. Then use:

HashMap<Triple,String> table2 = new HashMap<Triple,String>();

…again, this assumes that your lookup operation takes three strings and returns one value.

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To do lookups on table1, you need to do:

String val = table1.get(key1).get(key2).get(key3);

To do lookups on table2, you need to do:

Triple keyt = new Triple(key1, key2, key3);
String val = table2.get(keyt);

(…FWIW, doing inserts on table2 is must easier).