Java: Issue with Nested Hashmap

Question

I have a nested LinkedHashMap that looks like this:

LinkedHashMap<String,LinkedHashMap<String,LinkedList<Long>>> map = new...

The issue is that only 1 inner map is added per outer map, whereas I am expecting 2. I think the problem is how I'm constructing my map, and over-writing the first inner map with the second. (To briefly summarize my program, I am mapping each hand onto each finger. The mapping structure needs to be Finger={Right_Hand=[],Left_Hand=[], not vice versa.)

The constructor:

Set<String> handNames = new HashSet<String>(Arrays.asList("Left","Right");
Set<String> fingerNames = new HashSet<String>(Arrays.asList("Pinky","Ring","Middle","Index","Thumb");
LinkedHashMap<String, LinkedHashMap<String,LinkedList<Long>>> fingerHandMap = new LinkedHashMap<String, LinkedHashMap<String,LinkedList<Long>>>();

createNestedMap() {
    for (String finger : fingerNames)   
        for (String hand : handNames) {
            LinkedHashMap<String, LinkedList<Long>> handMap = new LinkedHashMap<String, LinkedList<Long>>();
            handMap.put(hand, new LinkedList<Long>());
            fingerHandMap.put(finger, handMap);
        }
}

When I print out the map, though, it looks like this:
{Ring={Left=[]}, Pinky={Left=[]}, Thumb={Left=[]}, Middle={Left=[]}, Index={Left=[]}}

How would I go about creating unique LinkedLists, to allow the map to look like:
{Ring={Right=[], Left=[]}, Pinky={Right=[], Left=[]}, Thumb={Right=[], Left=[]}, Middle={Right=[], Left=[]}, Index={Right=[], Left=[]}}

Thanks!

Paul Bellora · Accepted Answer · 2013-04-26 22:24:16Z

1

I'm going to write what you're currently doing in psuedocode, so hopefully you can see what you're doing wrong:

create a new finger hand map
for each finger:
    for each hand:
        create a new hand map
        put an entry mapping the hand to an empty list in the hand map
        put an entry mapping the finger to the hand map in the finger hand map

Keep in mind that when you put a key-value entry in a map it replaces any existing entry with the same key.

Let me know if you need further clarification.

edited Apr 26, 2013 at 22:24

answered Apr 26, 2013 at 22:17

Paul Bellora

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5 Comments

Adam_G Over a year ago

I still don't see it. I feel dumb, but I've probably been staring at this program for too long today. Any clues?

Paul Bellora Over a year ago

@Adam_G See my edit just now. Basically you're overwriting each "right" map with a "left" map. You really want to look up the same map again and add to it.

Adam_G Over a year ago

Why am I over-writing it, though, if one of the keys is "right" and one is "left"? (I know you're right; I'm just not seeing it.)

Paul Bellora Over a year ago

@Adam_G Yeah. If you debug, you'll see it. It's the finger keys that are being overwritten.

Adam_G Over a year ago

This is why I should've just stopped coding and gone home for the day. Thank you so much for your help. I can keep some of my sanity now.

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