List.set replacing all indexes inside TreeMap

Question

I have a TreeMap<String, List<Double> that I would like to pull out the List at a certain key, add a value at a certain index and then put it back into the TreeMap.

At the time of the below code, the TreeMap contains:

{specificKey=[0,0,0], specificKey2=[0,0,0], specificKey3=[0,0,0]}

I want to add the value to specificKey at index 0, so {specificKey=[777,0,0], specificKey2=[0,0,0], specificKey3=[0,0,0]}

Here is the offending code...

if (myMap.containsKey(specificKey){
    List<Double> doubleList = myMap.get(specificKey);
    doubleList.set(0, someValue);
    myMap.put(specificKey, doubleList);
}

Instead, what happens is: {specificKey=[777,0,0], specificKey2=[777,0,0], specificKey3=[777,0,0]}

Why is this happening when I am pulling out the exact List with myMap.get(specificKey)? And any ideas on how to accomplish what I need?

Answer 1

You are doing everything right. Moreover, you can remove myMap.put(specificKey, doubleList), because the list is already there.

The reason why this happens in your scenario is that all three keys refer to the same instance of List<Double> that you created when populating your TreeMap. Change the code to insert new lists for each key to fix this problem:

myMap.put(specificKey1, new ArrayList<Double>(Collections.nCopies(3, Double.valueOf(0))));
myMap.put(specificKey2, new ArrayList<Double>(Collections.nCopies(3, Double.valueOf(0))));
myMap.put(specificKey3, new ArrayList<Double>(Collections.nCopies(3, Double.valueOf(0))));
...
if (myMap.containsKey(specificKey1){
    myMap.get(specificKey1).set(0, someValue);
}