java - replace key + values in hashmap with identical key + different values

Question

I have researched this question for ages and cannot get it right!

I have a populated hashmap and an identically formatted hashmap ( Map<Integer, ArrayList<String>> ) that I have been working on (it has key value/s equal to other keys in the populated hashmap such as 0,1,2 etc). When I use the .put command to update the populated hashmap the few/one I have been working on replaces everything in the populated hashmap - is this normal? Where am I going wrong? I was expecting it to simply replace the key in question + values....

Excuse the not supplying code but it would mean posting quite an amount to demonstrate, just wondering if anyone could help explain where this might be going wrong. I could throw something together to show if needed...

Much obliged!

Answer 1

This is how a code example might look like:

import java.util.*;

public class NumFormEx
{
    public static ArrayList <String> listIt (String... params) 
    {
        ArrayList <String> as = new ArrayList <String> ();
        for (String s: params)
            as.add (s);
        return as;
    }

    public static void main (String args[])
    {
        Map <Integer, ArrayList<String>> mils = new HashMap<Integer, ArrayList<String>> ();
        mils.put (1, listIt ("foo", "bar")); 
        mils.put (2, listIt ("zacka", "zacka")); 
        System.out.println ("mils:\t" + mils);
        mils.put (1, listIt ("foobar"));        
        System.out.println ("mils:\t" + mils);
    }
}

Testing:

java NumFormEx
mils:   {1=[foo, bar], 2=[zacka, zacka]}
mils:   {1=[foobar], 2=[zacka, zacka]}

I would say: as expected.

Answer 2

Since map doesn't allow duplicate values you can do :

myMap.put(2, new ArrayList<String>());

This will take element with key 2 and replace it's list with new ("blank") list.