How to create a 2D ArrayList?

Question

I'm trying to use Java to create a 2-dimensional array. The size of rows is known, while the size of columns is unknown. Here is my code and it doesn't work. Could anyone give me some idea?

ArrayList<Integer> paths[];
paths = new ArrayList[2]；// 2 paths
for (int i=0; i<2; ++i)
    paths[i].add(1); // add an element to each path

gaborsch · Accepted Answer · 2013-07-17T01:08:08.290

1

Initialize the array element before adding to it. Put the initialization into the for loop:

@SuppressWarnings("unchecked")
ArrayList<Integer>[] paths = new ArrayList[2];

for (int i=0; i<2; ++i) {
    paths[i] = new ArrayList<Integer>();
    paths[i].add(1);
}

This way you can avoid the NullPointerException.

edited Jul 17 '13 at 01:08

answered Jul 17 '13 at 00:44

gaborsch

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I got a error saying "cannot create a generic array of ArrayList" for the 1st line. – user2570387 Jul 17 '13 at 00:54
What editor do you use? What java version? In my Eclipse it is only a warning, and compiles and runs perfectly (I use 1.7 compiler with 1.6 source code level). I also set `@SuppressWarnings("unchecked")`, it does not even warn. – gaborsch Jul 17 '13 at 00:58
I use eclipse and the JDK7u10 – user2570387 Jul 17 '13 at 01:04
Thanks GaborSch, I got it! But how to set the warning to be "unchecked"? Why ArrayList[] paths = new ArrayList[2] not ArrayList[] paths = new ArrayList[2]? – user2570387 Jul 17 '13 at 01:07
Updated the answer. Press `Ctrl-1` in Eclipse and select from the options. The other "magic" is `Ctrl-Space`. Use them everywhere, there are million suggestions! – gaborsch Jul 17 '13 at 01:10
2

@user2570387 Lazier way: Put your mouse on the warned tokens and the options will show. – johnchen902 Jul 17 '13 at 01:12
Just found it. Thanks! – user2570387 Jul 17 '13 at 01:12
If you put ``, you will get `Cannot create a generic array of ArrayList` error. – gaborsch Jul 17 '13 at 01:16
@johnchen902 I'm a keyboard-fan, but it's good to know. I simply can't wait until the popup shows :D – gaborsch Jul 17 '13 at 01:17

0x6C38 · Answer 2 · 2013-07-17T18:48:45.580

1

This is a "2d" ArrayList:

ArrayList<ArrayList<Integer>> paths = new ArrayList<>();

And here is the non-diamond operator version for Java < 1.7:

ArrayList<ArrayList<Integer>> paths = new ArrayList<ArrayList<Integer>>();

edited Jul 17 '13 at 18:48

answered Jul 17 '13 at 00:50

0x6C38

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Type mismatch: cannot convert from ArrayList> to ArrayList> – gaborsch Jul 17 '13 at 00:52
Type mismatch: cannot convert from ArrayList> to ArrayList>[] – user2570387 Jul 17 '13 at 00:58
@user2570387 the line I posted doesn't try to convert to `ArrayList>[] `, it creates an `ArrayList` of `ArrayLists` of `Integer` – 0x6C38 Jul 17 '13 at 00:59
"'<>' operator is not allowed for source level below 1.7" - most likely OP has something lower that 1.7, so probably this won't work. – gaborsch Jul 17 '13 at 01:03

score 1 · Answer 3 · answered Jul 17 '13 at 01:18

1

I would recomend this

    ArrayList<ArrayList<Integer>> paths = new ArrayList<ArrayList<Integer>>();
        for (int i=0; i<2; ++i)
            paths.add(new ArrayList<Integer>());

answered Jul 17 '13 at 01:18

pablobu

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How to create a 2D ArrayList?

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