Map of maps - how to keep the inner maps as maps?

My goal is to create a map of maps so that I can retrieve info of the outer map by its key and then access its "inner" maps by their keys.

However, when I got each inner map, the map I created originally became an Object and I cannot use key to access its value as I do with the outer map.

To learn from you experts, I would like to know how to keep all the maps as maps. Or, is it possible at all?

here is my exercise program:

import java.util.HashMap; import java.util.Iterator; import java.util.Map; import java.util.Set; public class MapExample { public static void main(String[] args) { Map<Object,String> mp=new HashMap<Object, String>(); // adding or set elements in Map by put method key and value pair mp.put(new Integer(2), "Two"); mp.put(new Integer(1), "One"); mp.put(new Integer(3), "Three"); mp.put(new Integer(4), "Four"); Map<Object,String> mp2=new HashMap<Object, String>(); mp2.put(new Integer(2), "Two2"); mp2.put(new Integer(1), "One2"); mp2.put(new Integer(3), "Three2"); mp2.put(new Integer(4), "Four2"); Map<Object,String> mpMaps=new HashMap(); mpMaps.put("Map1",mp); mpMaps.put("Map2",mp2); System.out.println("This is a map of Maps: " + mpMaps); for (int i=0;i<mpMaps.size();i++){ ArrayList a = new ArrayList(mpMaps.keySet()); Object o=a.get(i); System.out.println("all together: " + mpMaps.size() + "each element is: " + o + " value: " + mpMaps.get(o)); } } } 

SOLUTIONS:

 Map<Object,Map<Object,String> Map<String, Object> mpMaps=new HashMap<String, Object>(); 

by ameer and sleske

6 Answers

Here is the updated code that seems to work, you need to type the map of maps as <String, Object> since mp isn't a string you can't do <Object, String>.

import java.util.HashMap; import java.util.Iterator; import java.util.Map; import java.util.Set; import java.util.ArrayList; public class MapExample { public static void main(String[] args) { Map<Object,String> mp=new HashMap<Object, String>(); // adding or set elements in Map by put method key and value pair mp.put(new Integer(2), "Two"); mp.put(new Integer(1), "One"); mp.put(new Integer(3), "Three"); mp.put(new Integer(4), "Four"); Map<Object,String> mp2=new HashMap<Object, String>(); mp2.put(new Integer(2), "Two2"); mp2.put(new Integer(1), "One2"); mp2.put(new Integer(3), "Three2"); mp2.put(new Integer(4), "Four2"); Map<String, Object> mpMaps=new HashMap<String, Object>(); mpMaps.put("Map1",mp); mpMaps.put("Map2",mp2); System.out.println("This is a map of Maps: " + mpMaps); for (int i=0;i<mpMaps.size();i++){ ArrayList<Object> a = new ArrayList<Object>(mpMaps.keySet()); Object o=a.get(i); System.out.println("all together: " + mpMaps.size() + "each element is: " + o + " value: " + mpMaps.get(o)); } } } 

An alternate solution would be to use Commons MultiKey for avoiding map of maps. See details at and org.apache.commons.collections.keyvalue.MultiKey

Your code does not compile.

One problem is this:

Map<Object,String> mpMaps=new HashMap(); mpMaps.put("Map1",mp); 

This won't work, as you put a Map (mp) into a Map whose values need to be Strings.

Use Map<Object,Map<Object,String> and you should be fine.

Here is a simple example using HashMap

 HashMap<String, HashMap<String, Object>> outerMap = new HashMap<String, HashMap<String, Object>>(); HashMap<String, Object> innerMap1 = new HashMap<String, Object>(); HashMap<String, Object> innerMap2 = new HashMap<String, Object>(); innerMap1.put("Name", "az"); innerMap1.put("Address", "7 lab"); innerMap1.put("Age", 40); innerMap2.put("Name", "sab"); innerMap2.put("Address", "bk3"); innerMap2.put("Age", 35); outerMap.put("1",innerMap1); outerMap.put("2", innerMap2); System.out.println("outerMap = " + outerMap); 

output:

outerMap = {1={Address=7 lab, Age=40, Name=az}, 2={Address=bk3, Age=35, Name=sab}} 

Map<Object,String> mpMaps=new HashMap(); mpMaps.put("Map1",mp); 

you'll get an exception with this statement: mp has type Map but you're treating it as a String.

If I understood your question right, you'll either need a reference to the object used as key or you'll need to type cast your key/values.

This is an example on creating a map of maps.