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!

Answer 1

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.