Hey all! I'm currently teaching myself Java and I stumbled upon some code involving ArrayLists in a DSA program. I came across this signature: `static void markRow(ArrayList<ArrayList> matrix, int n, int m, int i) { /* function body */ }` and I also found this snippet: `ArrayList<ArrayList> matrix = new ArrayList(); matrix.add(new ArrayList(Arrays.asList(1, 1, 1))); matrix.add(new ArrayList(Arrays.asList(1, 0, 1))); matrix.add(new ArrayList(Arrays.asList(1, 1, 1)));`. I get the basics of Java and arrays, but I've never dealt with `ArrayList<ArrayList>` before. I tried asking ChatGPT, but it didn't clear things up completely. Could someone break it down for me in simpler terms? I'm especially confused about the `new ArrayList(Arrays.asList(...))` part and the reason for using `ArrayList<ArrayList>`. Sorry if this sounds like a silly question, but I'm eager to grasp this! Thanks!
2 Answers
Your initial expression is defining a nested data structure. `ArrayList<ArrayList>` creates a list where each element is itself another list containing integers. So you got it right there! This allows you to have a dynamic 2D array-like structure in Java, which can grow or shrink as needed, unlike fixed-size arrays. It's a neat way to handle matrices or grids when the number of rows/columns might change. Just make sure you include types when declaring your ArrayLists to prevent any compiler errors!
What you're looking at is an ArrayList containing other ArrayLists. You're right to wonder why someone would use it over a regular array like `List` or `int[][]`. It's mainly because `ArrayList<ArrayList>` allows for flexibility with the sizes of the inner lists. For instance, if you need to change the size of those inner lists or utilize specific ArrayList methods, this approach is useful.
However, just a heads up: using `Arrays.asList()` creates a fixed-size list for the inner elements, so you can't modify it later. If you were to print the contents of an `int[]` directly, you’d see something like `"[I@7daf6ecc"`, which is its memory address, rather than the actual contents. On the other hand, printing an `ArrayList` directly will show you the elements as expected. You can try out some simple tests with both and see how they behave!
Thank you for the clarification!
Got it! So, each element in this main list is another list containing integers. Thanks for breaking it down!