Accessing nested HashMaps in Java

Question

Hi,

I have a HashMap in Java, the contents of which (as you all probably know) can be accessed by

HashMap.get("keyname");

If a have a HashMap inside another HashMap i.e. a nested HashMap, how would i access the contents? Can i do this like this, inline:

HashMap.get("keyname").get("nestedkeyname");

Thank you.

Answer 1

You can do it like you assumed. But your HashMap has to be templated:

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

Otherwise you have to do a cast to Map after you retrieve the second map from the first.

Map map = new HashMap();
((Map)map.get( "keyname" )).get( "nestedkeyname" );

Answer 2

Yes, if you use the proper generic type signature for the outer hashmap.

HashMap<String, HashMap<String, Foo>> hm = new HashMap<String, HashMap<String, Foobar>>();
// populate the map
hm.get("keyname").get("nestedkeyname");

If you're not using generics, you'd have to do a cast to convert the object retrieved from the outer hash map to a HashMap (or at least a Map) before you could call its get() method. But you should be using generics ;-)

Answer 3

Yes.

See:

public static void main(String args[]) {

    HashMap<String, HashMap<String, Object>> map = new HashMap<String, HashMap<String,Object>>();
    map.put("key", new HashMap<String, Object>());
    map.get("key").put("key2", "val2");

    System.out.println(map.get("key").get("key2"));
}

Answer 4

As others have said you can do this but you should define the map with generics like so:

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

However, if you just blindly run the following:

map.get("keyname").get("nestedkeyname");

you will get a null pointer exception whenever keyname is not in the map and your program will crash. You really should add the following check:

String valueFromMap = null;
if(map.containsKey("keyname")){
  valueFromMap = map.get("keyname").get("nestedkeyname");
}