Entering Data to a HashMap Dynamically

Question

I was wondering how to properly (or even if you can) add data to a HashMap like an ArrayList?

So I have a function, which checks the entered string for commas and separates the values, adds them into an ArrayList and then stores the ArrayList keyed against the name of the field they were entering data to:

ArrayList<String> dataList = new ArrayList<>();
Map<String, ArrayList<String>> dataMap = new HashMap<String, ArrayList<String>>();
String[] parts;
public void CheckForMultipleEntries(String field, String data){
    dataList.clear();
    if(data.contains(",")){
        data.replace("", "");
        parts = data.split(",");
        for(String part : parts){
            dataList.add(part);
        }
        dataMap.put(field, dataList);
    }

    System.out.println(dataMap);
    System.out.println(dataMap.get("test"));
}

However say first for "Header" I enter "test1, test2, test3" - this will enter & output correctly (as: {Header=[test1, test2, test3]}) but if for the next value I then enter, say, "Body" "test4, test5, test6" then when I output the map I get {Header=[test4, test5, test6], Body=[test4, test5, test6]}.

Why is it overwriting the values, but not the keys? And how do I fix it?

Thanks for any help!

Read the accepted answer of the linked question. (Topic "Adding the Same Object") — Tom, Oct 30 '18 at 09:19
I know - in the actual code its .repalce(" ", ""); - don't know why it came across like that — AristotleTheAxolotl, Oct 30 '18 at 09:22
You need to re-read what `replace` does and think about the question "does it actually change the current string?". — Tom, Oct 30 '18 at 09:28
Yes, it does actually change the current string, in the way I expected and wanted (to get rid of whitespace), so what exactly do you mean it wont do anything? — AristotleTheAxolotl, Oct 30 '18 at 09:31
Sorry, but "Yes, it does actually change the current string" is a blantant lie. Strings are immutable and it ___can't___ change the String. The documentation even says so. — Tom, Oct 30 '18 at 09:35
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/182780/discussion-between-aristotletheaxolotl-and-tom). — AristotleTheAxolotl, Oct 30 '18 at 10:05

Deepak Gunasekaran · Accepted Answer · 2018-10-30T09:21:04.767

2

The Value is getting overidden because, you are using

dataList.clear();

you are putting the reference of arraylist dataList in the hashmap. so you have to re initialize the arraylist evertime you allot with new values

replacing dataList.clear(); with dataList=new ArrayList<>(); will solve your issue

edited Oct 30 '18 at 09:21

answered Oct 30 '18 at 09:17

Deepak Gunasekaran

747
5
13

Haha crap you're right. I feel so foolish for not spotting this. I forgot its storing the actual ArrayList and not just the values inside. How can I change this so I can do what I want? Edit: Just saw the edit you made to your answer. Thank you – AristotleTheAxolotl Oct 30 '18 at 09:21
replace dataList.clear(); with dataList=new ArrayList<>(); – Deepak Gunasekaran Oct 30 '18 at 09:21
Welcome @AristotleTheAxolotl – Deepak Gunasekaran Oct 30 '18 at 09:22

score 0 · Answer 2 · answered Oct 30 '18 at 09:24

0

Smth. like that. Do not forget, that each map key have it's own List:

public static void add(Map<String, List<String>> map, String key, String value) {
    map.computeIfAbsent(key, k -> new ArrayList<>()).add(value);
}

answered Oct 30 '18 at 09:24

Oleg Cherednik

17,377
4
21
35

Entering Data to a HashMap Dynamically

2 Answers2