array of arrays

[Bruno Sant Ana]

Hi guys,

I did a code that have two arrays, one I call array1 and the other array2, two of them two-dimensional. What's the difference between the 2 implementations bellow? Is there any difference? Because I've never seen this kind of implementation where you inicialize the array without passing the number of colums: byte[][] array12 = new byte[3][];
I saw this only in the book about certification Java.

package arrays;  
  
public class MultiArray2 {  
    public static void main(String[] args) {  
          
        System.out.println("Array1 \n");  
          
        byte[][] array1 = new byte[3][2];  
          
        array1[0][0] = 0;  
        array1[0][1] = 1;  
          
        array1[1][0] = 2;  
        array1[1][1] = 3;  
          
        array1[2][0] = 4;  
        array1[2][1] = 5;  
          
        for(int l=0; l < array1.length; l++){  
            for(int c=0; c < 2; c++){  
                System.out.print(array1[l][c] + " ");  
            }  
            System.out.println("\n");  
        }  
          
        System.out.println("Array2 \n");  
          
        byte[][] array2 = new byte[3][];  
  
        // Now allocate the columns  
        array2[0] = new byte[2];  
        array2[1] = new byte[2];  
        array2[2] = new byte[2];  
          
        array2[0][0] = 0;  
        array2[0][1] = 1;  
          
        array2[1][0] = 2;  
        array2[1][1] = 3;  
          
        array2[2][0] = 4;  
        array2[2][1] = 5;  
          
        for(int l=0; l < array2.length; l++){  
            for(int c=0; c < 2; c++){  
                System.out.print(array2[l][c] + " ");  
            }  
            System.out.println("\n");  
        }  
    }  
}  

Being more especific...

Is this

byte[][] array1 = new byte[3][2];  

the same of it?

byte[][] array2 = new byte[3][];  
  
        // Now allocate the columns  
        array2[0] = new byte[2];  
        array2[1] = new byte[2];  
        array2[2] = new byte[2];  

Internaly that declaration byte[][] array1 = new byte[3][2]; creates an array of arrays?

Thank you very much

[Jeanne Boyarsky]

Bruno,
Yes. THose two are equivalent. The first is obviously easier to read and write.

The reason you only see the later in a certification book is that people don't generally use that approach in real life. The certification is testing a deeper understanding to see if you know what the first approach (convenience shortcut) is doing.

[Bruno Sant Ana]

Hi Jeanne,

Thanks for the explanation, I understood it. If appears one more doubt related to this subject I'll post here.