Array : Multi-dimensional

[Arnb Sen]

This compiles and runs

public class multiarray {
public static void main(String args[]) {
 
int[][] i = new int[2][2];
 
i[0][0] = 0;
i[1][0] = 0;
i[1][1] = 0;
 
System.out.println("array output >> "+i);
}
}
 

This compiles but throws NullPointer exception

public class multiarray {
public static void main(String args[]) {
 
int[][] i = new int[2][];
 
i[0][0] = 0;
i[1][0] = 0;
i[1][1] = 0;
 
System.out.println("array output >> "+i);
}
}
 

Can someone please explain this ?

[Henry Wong]

In the first case, you created an array of int arrays -- and created the int array for each element.

In the second case, you just created an array of int arrays -- but did not create the int array for each element.

Henry

[Arnb Sen]

How can you say that "did not create the int array for each element" ? Did not understand.

[Henry Wong]

Originally posted by Arnb Sen:
How can you say that "did not create the int array for each element" ? Did not understand.

That is how it works... when you say...

   int[][] i = new int[2][];

Java will create you an array of int arrays but will not create each element. To do that, you must...

   i[0] = new int[3];
   i[1] = new int[5];

And also notice that each array element does not have to be an int array of the same size.

Henry