Map inside map in java

Question

What is wrong with this instantiation :

Map<String, String, HashMap<String,String>> map = new HashMap<String, String, HashMap<String,String>>();

Answer 1

A Map<K,V> is a mapping from keys of type K to values of type V. There are only 2 type parameters to a map.

You attempted to define a map with 3 type parameters; this is not possible, and has nothing to do with the fact that you're putting a Map inside a Map.

A Map<K1,Map<K2,V2>> works just fine.

A Map<X,Y,Z> does not.

It's possible that you need something like Map< Pair<L,R>, Map<K,V> >. Java does not have generic Pair<L,R> type, but see related questions below for solutions.

Related questions

On pairs/tuples:

• What is the equivalent of the C++ Pair<L,R> in Java?
• Java generics Pair<String, String> stored in HashMap not retrieving key->value properly
• A Java collection of value pairs? (tuples?)
• Does Java need tuples?
• How to return multiple objects from a Java method?

On nested maps:

• Java: Spring Framework: Declaring Nested Maps
• Java: How do you declare nested map in spring framework?
• Map of Maps data structure

Answer 2

Maps only have 2 type parameters, you have 3 (in your "outer" Map).