Arrays of Objects

[Frank Hale]

Lets say I have a class foo defined like this:
class foo {
String a;
String b;
public foo(String a, String b)
{
this.a = a;
this.b = b;
}
}
Now I would like to make an array of class foo like this:
foo myFoo[][];
How do I initialize this array and store foo objects in this it?
I've tried everything I can think of but I keep getting a NullPointerException.
Thanks,
Frank

[Tony Alicea]

<PRE>
public class Test {
String a;
String b;
public void foo(String a, String b) {
this.a = a;
this.b = b;
}
public static void main (String[] args) {
Test t[] = new Test[5];
for (int i=0; i<t.length; i++) {
t[i] = new Test();
t[i].foo("A", "B");
}
}
}
</PRE>

[Frank Carver]

Surely Tony's code only builds and initializes a one-dimensional array, where the question calls for a two-dimensional array.
It looks as if the step you are missing is to call new for each "row" of the array. Here's my suggestion:

class foo
{
 String a;
 String b;
 
 public foo(String a, String b)
 {
  this.a = a;
  this.b = b;
 }
}
 
public class Test2
{
 private static int simpleInt(String s)
 {
  int ret = 0;
  try
  {
   ret = Integer.parseInt(s);
  }
  catch(NumberFormatException e) {}
  return ret;
 }
 
 private static String chars="abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz";
 /** 
  usage java Test2 <rows> <cols>
 */
 public static void main(String[] args)
 {
  int nrows = args.length > 0 ? simpleInt(args[0]) : 5;
  int ncols = args.length > 1 ? simpleInt(args[1]) : 5;
  foo[][] foos = new foo[nrows][];
 
  for (int row = 0; row < nrows; ++row)
  {
   foos[row] = new foo[ncols];
   for (int col = 0; col < ncols; ++col)
   {
    int index = (row * ncols) + col;
    String s = chars.substring(index, index+1);
    foos[row][col] = new foo("character", s);
System.out.println("setting foos[" + row + "][" + col + "] to foo(\"character\",\"" + s + "\")");
   }
  }
 } 
}}

[Randi Meilach]

Hi, I'm just starting out with java and played around with this problem. I have the following code which gives me an ArrayIndexOutOfBounds exception when I run it. Can anyone explain why?
class foo {
String a;
String b;
public static void main (String args[])
{
foo myFoo[][] = new foo [1][1];
myFoo[1][1] = new foo ("a", "b");
}
public foo(String a, String b)
{
this.a = a;
this.b = b;
}
}

[Frank Carver]

Your problem is that arrays start from 0 not 1, so any array with a size of 1 (created by new Something[1]) only has "slot" 0. To get a second "slot", numbered 1, you need to use new Something[2].

[Randi Meilach]

Frank,
Thanks, I realized that pretty much immediately after I posted. I am coming from a language where array indexing begins at 1 and I 'keep forgetting' this little fact.

[Dirk Schreckmann]

Duane,
Welcome to JavaRanch!
We ain't got many rules 'round these parts, but we do got one. Please change your display name to comply with The JavaRanch Naming Policy.
Thanks Pardner!

[Dirk Schreckmann]

I think someone could best help you figure this one out if you were to post an example of the code that you are trying to use?
Don't forget to surround your code with the [ code ] and [ /code ] ubb tags.
Good Luck.

[Duane Eddy]

Thanks, but I answered my own question.