Arraylist retrieved from populated hashmap keeps returning an empty arraylist

Question

I have a hashmap and I am inserting an arraylist as the value in a while loop. During an iteration, if the hashmap already contains a key, I want to retreive the arraylist already stored, append new data to it and put it back into the hashmap.

The issue I have now is when I check if a hashmap contains a value and it does, the arraylist being returned is of size 0 as though I never put in anything before.

final HashMap<String, ArrayList<String>> map = new HashMap<String, ArrayList<String>>();
String status = "";
String channel = "";

while (daily.next()) {
    while (avrg.next())
        if (avrg.getString(1).equals(daily.getString(1)) && avrg.getString(2).equals(daily.getString(2))) {
            final Object pstmnt = null;
            final Object pstmnt2 = null;
            final float thresholdValue = calcThreshold(pstmnt, pstmnt2, daily.getString(1));
            channel = daily.getString("client_name");
            if (daily.getFloat(3) > avrg.getFloat(3) + thresholdValue * avrg.getFloat(3)) {
                status = "HIGHER";
            }
            else if (daily.getFloat(3) < avrg.getFloat(3) - thresholdValue * avrg.getFloat(3)) {
                status = "LOWER";
            }
            else {
                status = "Normal";
            }
            final PrintStream out;
            out.println(channel);
            out.println(thresholdValue);
            out.println(status);
            if (map.containsKey(channel)) {
                out.println("map contained key");
                final ArrayList<String> temp = new ArrayList<String>();
                temp.addAll(map.get(channel));
                out.println("previous size: " + temp.size());

                temp.add(daily.getString("event"));
                temp.add(Float.toString(daily.getFloat(3)));
                temp.add(Float.toString(avrg.getFloat(3)));
                temp.add(Float.toString(thresholdValue * 100));
                temp.add(status);
                out.println("current size: " + temp.size());
                map.put(channel, temp);
                out.println("array added to map");
                temp.clear();
                System.out.println("done");

            }

            else {
                final ArrayList<String> temp = new ArrayList<String>();
                out.println("new key created");
                temp.add(daily.getString("event"));
                temp.add(Float.toString(daily.getFloat(3)));
                temp.add(Float.toString(avrg.getFloat(3)));
                temp.add(Float.toString(thresholdValue * 100));
                temp.add(status);

                System.out.println(temp.size());
                map.put(channel, temp);
                System.out.println("array added to map");
            }
        }
    avrg.beforeFirst();
}

I am really not sure why this is happening. Any help would be appreciated!

What do you see when you try to debug your program in a debugger? — Peter Lawrey
– Peter Lawrey, Commented Mar 5, 2012 at 16:59

NPE · Accepted Answer · 2012-03-05 17:00:09Z

5

The problem is right here:

map.put(channel, temp);
out.println("array added to map");
temp.clear();

temp.clear() clears the same list that you've just added to the map. Since temp is about to go out of scope, that statement serves no useful purpose and can be removed.

answered Mar 5, 2012 at 17:00

NPE

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3 Comments

user1192724 Over a year ago

Thanks!!!!! That fixed it!!! I want to know though, why did temp.clear() affect the arraylist I inserted? Afterall the clear statement was called after I had inserted the arraylist into the hashmap

NPE Over a year ago

@user1192724: map.put() doesn't copy the data; it inserts the same reference as temp. So if you change the contents of the temp list post-insertion, the changes get reflected in the map.

user1192724 Over a year ago

Oh I see...That's very useful information. Thank you very much!

Robin · Accepted Answer · 2012-03-05 17:46:01Z

1

Let me start by saying that @aix has the correct answer, but you should rewrite you code to get rid of all the duplication. The if block starting from
if (map.containsKey(channel))

can be simplified to the following.

ArrayList<String> temp = map.get(channel);

if (temp == null) {
   out.println("new key created");
   ArrayList<String> temp = new ArrayList<String>();
   map.put(channel, temp);
}

temp.add(daily.getString("event"));
temp.add(Float.toString(daily.getFloat(3)));
temp.add(Float.toString(avrg.getFloat(3)));
temp.add(Float.toString(thresholdValue * 100));
temp.add(status);

There is no need to create a new list and copy every time you add to it.

answered Mar 5, 2012 at 17:46

Robin

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