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

An alternate solution would be to use Commons MultiKey for avoiding map of maps. See details at http://commons.apache.org/collections/apidocs/ 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 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));
        }             
    }
}