How to initialize an arraylist of Hashmap?

Question

I'm really new to these arraylist of hashmap. I want to create an arraylist of hashmap and will change the element of the arraylist in a loop dynamically. My code is as follows:

static List<HashMap<Character, Integer>> column = new ArrayList<HashMap<Character, Integer>>(9);
static HashMap<Character, Integer> columns = new HashMap<Character, Integer>();
for(int i=0;i<9;i++)//initialize?
    column.add(i, columns);
for(int i=0;i<9;i++) 
    column.get(i).put(b[xxx][xxx],(int)(b[xxx][xxx]));

At first I didn't use the for loop here, I thought I initialize the arraylist size in first line with (9) is enough, but when I tried to get element with column.get(5).put(something), it gave me exception with IndexOutOfBound.

Then I tried to use the column.add(i, null);, but it gave me the exception of NullPointerException. So I changed to this column.add(i, columns);

But now the problem is, when I tried to edit one of the hashmap in column arraylist, every other hashmap in the arraylist will be changed as well, I guess it's because they are set to be the same columns hashmap at first?

So my question is how to initialize the arraylist of hashmaps so that I can change every single one of them each time without affecting others?

Paul Boddington · Accepted Answer · 2015-03-08 22:57:49Z

6

You don't want to write

column.add(i, columns);

This keeps adding a reference to the same HashMap. column will then have 9 references to the same object, so when one changes, they all change.

Instead you want

column.add(i, new HashMap<Character, Integer>());

Then you add a new instance each time.

answered Mar 8, 2015 at 22:57

Paul Boddington

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Paul Boddington Over a year ago

@Zip Glad I could help. Also note that you don't need to pass the index i unless you are inserting at index i. If you are just adding to the end of a list you should write column.add(new HashMap<Character, Integer>());

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