ArrayList of ArrayList?

Hello JavaProgrammingForums users!

I come to you with this problem: I am attempting to create an ArrayList of ArrayLists for an assignment, but I'm hitting a roadblock when it comes to figuring out how to populate this construction. So far it seems like I'm able to make an arraylist and get it to print out a sample where all of the values are null, but I cannot figure out how to go further to changing those data values, add another row of data to the arraylist, etc.

Here's what I've gotten so far:

Code Java:

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package test;
 
import java.util.ArrayList;
 
 
class SampleTableClass<Type> {
 
    private Type type;
    ArrayList<ArrayList<Type>> array;
 
 
    public void initializeArray()
    {
        array = new ArrayList<ArrayList<Type>>();
    }
 
    public void fillArray( int rows, int columns)
    {
        for (int r=0; r<rows; r++){
            array.add(r, new ArrayList<Type>());
            for (int c=0; c<columns; c++)         
                array.get(r).add(c, type);                
        }
    }
 
    public void print5x5Array(int rows, int columns)
    {
        fillArray(rows, columns);
        Object[] arrayObjects = array.toArray();
        for (int r=0; r<rows; r++){
            for (int c=0; c<columns; c++){
                System.out.println(arrayObjects[r]);
            }
        }
    }
}
 
public class Main {
       public static void main(String[] args){
       SampleTableClass array = new SampleTableClass();
       array.initializeArray();
       array.print5x5Array(1,5);
       }
}

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I feel like I've probably made a simple mistake or failed to grasp one of the main concepts entirely, so feel free to tell me I've done everything wrong if I have, if that's what's necessary to fix it. :-B

Thanks!

Here's some code I wrote up in a couple seconds. I used Strings to show you an example without doing 100% of your homework. It's quite chivalrous of me to give you this much, but I'm assuming by your current knowledge that you're here to learn. so please do. :)

Code java:

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import java.util.ArrayList;
 
public class Main {
 
    private static ArrayList<ArrayList<String>> parent;
 
    public static void main(String[] args) 
    {  
        parent = new ArrayList<ArrayList<String>>();
    }
 
    public static void addChild(String[] childText)
    {
        ArrayList<String> child = new ArrayList<String>();
        for(String text : childText)
        {
            child.add(text);
        }
 
        parent.add(child);
    }
 
    public static void removeChild(int index)
    {
        if(index >= 0 && index < parent.size())
        {
            parent.remove(index);
        }
    }
 
}

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You could always take a look at this: http://www.javaprogrammingforums.com...t-example.html.

It has samples of how to implement many common features of multi-dimensional ArrayLists (in that case, 2D).

Or I can get sniped by a mod. :) I should have used the search bar to look for that post.

Okay, that helps quite alot, I had already re-examined my code and had seen that I needed to change the structure of how I had everything set up, and this helps some of the pieces fall into place.

One other quick question though- I'm going to be implementing those functions with Object instead of String so that they can be filled up with whatever object that a user would want to do, but because of that I'm unsure how to put the syntax for that in my main called function, whenever I try to code a line such as

Code Java:

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parent.addChild(/*This is where I'm having trouble knowing what to put in here to correctly add a child*/);

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Any tips how to word this?

Quote:

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Originally Posted by ben456123

One other quick question though- I'm going to be implementing those functions with Object instead of String so that they can be filled up with whatever object that a user would want to do, but because of that I'm unsure how to put the syntax for that in my main called function, whenever I try to code a line such as

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You should almost definitely be using Generics instead of simply dealing with Objects- that way the user can still populate your data structure with anything, but you still have some semblance of type safety.

Quote:

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Originally Posted by ben456123

Code Java:

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parent.addChild(/*This is where I'm having trouble knowing what to put in here to correctly add a child*/);

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Any tips how to word this?

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Well, what do you want that method to take as a parameter? Another List? An array to be converted into a List? A single Object? Something else?