Initializing a three dimensional array of arraylist in Java

Question

I have a multidimentional array, as:

private static ArrayList [] [] pVTable = new ArrayList [35] [12];

My first try to initialize it was:

for (ArrayList[] x : pVTable) {
    for (ArrayList y : x) {
        y = new ArrayList<TableValue>();
    }
}

which didn't work.

I ended up doing it more manually, as in:

for ( int i = 0; i < pVTable.length; i++) {
    for ( int j = 0; j < pVTable[0].length; j++) {
        pVTable [i] [j] =  new ArrayList<TableValue>();
    }
}

which works fine.

Although I have a solution, I was wondering why the first (more elegant) piece of code doesn't do the same job?

that's because you are overwriting the object reference with a new ArrayList. Actually, y was anyways a null reference so initialization that ways would not have worked either ways. On a side note, Arrays.fill may help you — aishwarya
– aishwarya, Commented Nov 28, 2011 at 15:59
I haven't seen .fill before, I'll try to remember that. If I read it correctly, though, it would assign the SAME reference to all the table entries, not create a new ArrayList for each. — shakeshuck
– shakeshuck, Commented Nov 28, 2011 at 16:28

Eli Acherkan · Accepted Answer · 2011-11-28 16:30:06Z

1

In the first snippet, if we strip away the syntactic sugar of the foreach operator (:), the code translates to:

for (int xIndex = 0; xIndex < pVTable.length; xIndex++) {
    ArrayList[] x = pVTable[xIndex];
    for (int yIndex = 0; yIndex < x.length; yIndex++) {
        ArrayList y = x[yIndex];
        y = new ArrayList<TableValue>();
    }
}

As you can see, nothing is ever assigned to the actual array – only to the temporary y variable.

edited Nov 28, 2011 at 16:30

answered Nov 28, 2011 at 15:56

Eli Acherkan

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1 Comment

shakeshuck Over a year ago

When broken down this way, I can see what's going on. Thanks, Eli!

Roman Byshko · Accepted Answer · 2011-11-28 15:44:23Z

1

In the first example your code although modifies y does not change x.

answered Nov 28, 2011 at 15:44

Roman Byshko

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1 Comment

shakeshuck Over a year ago

Thanks for the reply, but I needed a little more explanation, as per Eli's comment.

NimChimpsky · Accepted Answer · 2011-11-28 16:09:10Z

1

You are mixing ArrayList (part of collections api) with Arrays, which is rather confusing (for me anyway)

I would suggest something like this instead :

List<Point> myShape =  new ArrayList<Point>;

Where point contains two ints representing X and Y.

edited Nov 28, 2011 at 16:09

answered Nov 28, 2011 at 15:55

NimChimpsky

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3 Comments

shakeshuck Over a year ago

Mmmhhh. Creating objects just to contain references to other objects sounds even more complicated to me, LOL. I guess I'm an old programmer still trying to program the old way!

NimChimpsky Over a year ago

@shakeshuck An arraylist is an object, as is TableValue. So you are doing this anyway.

shakeshuck Over a year ago

Yes, but what I mean is that referencing row a, column b seems more straightforward than creating a new object to reference row a, column b. It feels like you're adding an extra element to do the same job? Like I said, maybe it's just an old brain's thinking...

Kane · Accepted Answer · 2011-11-28 15:51:15Z

0

The scope of the first is incorrect. y is just a placeholder variable. Changing that doesn't change the underlying object, just the object that y refers to. You can see the same problem in the following code snippet:

public static  int x = 2;

public static void foo(int y) {
    y = 3;//does nothing outside of foo
}

public static void main(String[] args) {
    System.out.println(x);//prints 2
    foo(x);
    System.out.println(x);//prints 2, x hasn't changed.
}

answered Nov 28, 2011 at 15:51

Kane

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