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Thread: ArrayList of ArrayList?

  1. #1
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    Default 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:
    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);
           }
    }

    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.

    Thanks!


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    Default Re: ArrayList of ArrayList?

    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.

    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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    Default Re: ArrayList of ArrayList?

    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).

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    Default Re: ArrayList of ArrayList?

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

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    Default Re: ArrayList of ArrayList?

    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

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

    Any tips how to word this?

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    Default Re: ArrayList of ArrayList?

    Quote Originally Posted by ben456123 View Post
    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
    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 Originally Posted by ben456123 View Post
    parent.addChild(/*This is where I'm having trouble knowing what to put in here to correctly add a child*/);

    Any tips how to word this?
    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?
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