How to create a 2D ArrayList

Programming Questions

thiscould..

in Programming Questions • 8 months ago

Im trying to create a 2D ArrayList but is having some issues

Currently I have a single list like this

ArrayList aObject;
.
.
aObject = new ArrayList();
for (int i = 0; i< 5;i++)
    {
    aObject.add(new Entity());    //Entity is a Class
    }

However I would like to be able to have a 2D (or possible 3D) ArrayList

Any suggestions ?

Replies(6)

PhiLho

Re: How to create a 2D ArrayList

8 months ago

An array list can hold any kind of object, including other collections, for example an array list...
A 2D array uses the same idea: it is only an array of arrays.

thiscould..

Re: How to create a 2D ArrayList

8 months ago

Okay, how do I declare a multi-dimension arraylist ?

A normal multi-dimension array is simple enough, but I can't figure out how to do a multi-dimension arraylist

quarks

Re: How to create a 2D ArrayList

8 months ago

The code below creates a 2D ArrayList of String objects, simple replace String with the name of your class, replace the new String() constructor in line 11 with one for your class and you (if you want) override the toString method to create your own custom output string for your class.

// Declare the 2D array list
ArrayList<ArrayList<String>> aObject;
// Create the 2D array list
aObject = new ArrayList<ArrayList<String>>();
// Add an element to the first dimension
aObject.add(new ArrayList<String>());
// Place a string in position [0,0]
aObject.get(0).add(new String("Quarks"));
// Display the string in position [0,0]
println(aObject.get(0).get(0).toString());

rbrgbrtgrt

PhiLho

Re: How to create a 2D ArrayList

8 months ago

The way you did, you don't declare anything special: you just add() an array list to this array list.
But it is better to use typed array lists, so you don't need casts when getting stuff out of it, and can use the modern for each loop.
First, let's show how to use a typed array list:

class Stuff
{
int id;
String name;
Stuff(int i, String n)
{
id = i;
name = n;
}
public String toString()
{
return "Stuff: " + name + " (" + id + ")";
}
}
ArrayList<Stuff> lotOfStuff = new ArrayList<Stuff>();
void setup()
{
for (int i = 0; i < 5; i++)
{
lotOfStuff.add(new Stuff(i + 1, "Bla-" + (1 + i * 2)));
}
for (Stuff s : lotOfStuff)
{
println(s);
}
exit();
}

Now, let's make a 2D list out of that:

ArrayList<ArrayList<Stuff>> lotOfStuff = new ArrayList<ArrayList<Stuff>>();
void setup()
{
for (int j = 0; j < 3; j++)
{
ArrayList<Stuff> someStuff = new ArrayList<Stuff>();
lotOfStuff.add(someStuff);
for (int i = 0; i < 5; i++)
{
someStuff.add(new Stuff(j * 10 + i + 1, "Bla-" + (j + 1) + "-" + (1 + i * 2)));
}
}
for (ArrayList<Stuff> als : lotOfStuff)
{
println(als);
}
exit();
}

Slightly more complex, but if you read attentively, you will see it is quite logical...

thiscould..

Re: How to create a 2D ArrayList

8 months ago

Thanks
a bit reading to do :-)

GoToLoop

Re: How to create a 2D ArrayList

8 months ago

My! This 2D ArrayList stuff made me dizzy!
I will always try to stick to regular arrays anytime!

Here's a tweaked version from quark. While exploring it a bit more:

/**
 * 2D ArrayList (v2.1)
 * by Quarks (2013/Feb)
 * tweaked by GoToLoop
 *
 * http://forum.processing.org/topic/how-to-create-a-2d-arraylist
 */

// Declare & create a 2D array list
final ArrayList< ArrayList<String> > words2D = new ArrayList();

// Add an element (another ArrayList of strings)
// to the 1st dimension (we have row 0 now!)
words2D.add( new ArrayList() );

// Get current number of rows
// and last index number we can use
int numRows = words2D.size();
int row = numRows - 1;

// Add a string in position [row 0][col 0]
words2D.get(row).add("Quarks");

// Add a string in position [0][1]
words2D.get(row).add("PhiLho");

// Display the string in position [0][0] & [0][1]
println( words2D.get(row).get(0) );
println( words2D.get(row).get(1) );

// Add an element (another ArrayList of strings)
// to the 1st dimension (we now reach row 1 now!)
words2D.add( new ArrayList() );

// Get current number of rows
// and last index number we can use (again)
numRows = words2D.size();
row = numRows - 1;

// Add a string in position [row 1][col 0]
words2D.get(row).add("ThisCouldBeAnywhere");

// Add a string in position [1][1]
words2D.get(row).add("GoToLoop");

// Add a string in position [1][2]
words2D.get(row).add("This is 3rd column element from 2nd row");

// Print whole 2D ArrayList
row = 0;

for (ArrayList<String> lists: words2D) {
  print("\n" + row++ + " -> ");
  for (String elements: lists)    print(elements + "\t");
}

exit();

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