Creating a hashmap of array list not working as expected

Question

I'm pretty new to Java so sorry if this is super basic. Basically I have some data in a array list and it works fine but in a later part of my program I need to read the list and it takes a long time because the list becomes very large. So I thought I could take certain parts of the array list(thats needed later) into a hash map that I can extract that section of the array list later from(hopefully that makes sense).

I first create the array list with all the data I want(I have a loop and call each data thats created in a single loop data_cache_local_loop):

HashMap hashmap_of_values = new HashMap(); //hashmap of arraylist
for (int i = 0; i<100; i++) {
....program logic and more loops
Set uniqie_T_cache = new HashSet(T_cache_local_loop); //make values unique
hashmap_of_values.put(i, new ArrayList(uniqie_T_cache));
}

Then later on when I need the data I extract it like this:

List t = new ArrayList();
t.add(hashmap_of_values.get(i));

But my program fails and I'm not sure exactly why. When I print out the size of hashmap_of_values I get the number of i's I'm expecting but when I print out the number of T it only shows one(when I run the program it says T_cache_local_loop has hundreds of items).

Can anyone suggest to me what I'm doing wrong? I'm not 100% but I think I maybe looking at the array list I add as a single item but I'm unsure of what I'm doing wrong.

Honestly, one thing I've never understood is that I have the right to edit but I alone can not review/accept an edit. Doesn't make too much sense IMO, I should just do it myself... But then since an edit is pending, I can't do it myself. — alternative
– alternative, Commented Apr 3, 2012 at 16:16

Louis Wasserman · Accepted Answer · 2012-04-03 16:19:23Z

4

You should really be using generics -- it'd help you a lot, and make these issues much more obvious -- but for the moment your problem is that that line should be

List t = (List) hashmap_of_values.get(i);

or

List t = new ArrayList((List) hashmap_of_values.get(i));

to get a copy of the list in the map. What you were doing was creating a list of lists, because hashmap_of_values.get(i) was already a list.

edited Apr 3, 2012 at 16:19

answered Apr 3, 2012 at 16:14

Louis Wasserman

200k26 gold badges367 silver badges437 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Carl Manaster Over a year ago

Because basically the code from the OP is creating a list of lists of values instead of a list of values. (in case that wasn't clear).

Lostsoul Over a year ago

ahh your right. I thought I was doing that but wasn't sure how to stop it. Didn't realize I could simply cast the list when creating the t variable. Thank you so much.

Churk · Accepted Answer · 2012-04-03 16:15:17Z

1

trying t.addAll(hashmap_of_values.get(i));

answered Apr 3, 2012 at 16:15

Churk

4,6475 gold badges25 silver badges40 bronze badges

Comments

ulmangt · Accepted Answer · 2012-04-03 16:19:59Z

0

When you add your HashSet to your ArrayList, you're adding the entire set as a single object to the first index of the ArrayList. You're not adding each element of the HashSet to the ArrayList. Instead, you should do something like:

List newList= new ArrayList();
newList.addAll( unique_T_cache );
hashmap_of_values.put(i, unique_T_cache );

answered Apr 3, 2012 at 16:19

ulmangt

5,4233 gold badges25 silver badges37 bronze badges

Collectives™ on Stack Overflow

Creating a hashmap of array list not working as expected

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related