Sierra Bates SCJP 6 Ch. 3 No. 3

[Tod Stroszer]

Sierra Bates SCJP 6 Ch. 3 No. 3

Help me understand this question.

Given:
class Dims {

public static void main(String[] args) {

int[][] a = {{1,2,}, {3,4}};

int[] b = (int[]) a[1];

Object o1 = a;

int[][] a2 = (int[][]) o1;

int[] b2 = (int[]) o1;

System.out.println(b[1]);
 } }
		 What is the result?

Obviously there is an error in the code at line 4

int[] b = (int[]) a[1];

int[] a is a 2 dimensional array and cannot be cast to the size of a one dimensional array.

But the choices given are

A. 2
B. 4
C. An exception is thrown at runtime
D. Compilation fails due to an error on line 4
E. Compilation fails due to an error on line 5
F. Compilation fails due to an error on line 6
G. Compilation fails due to an error on line 7


If you choose D you are wrong - the code compiles, but an exception is thrown at runtime - so choice C is correct.

How can we tell that the error at line 4 will result in a runtime error vs. a compilation error? On the test we will not have the option of being able to compile this code ourselves, we just have to determine the result from the code.

[Prasad Kharkar]

   int[][] a = {{1,2,}, {3,4}};  

it is ok that this is two dimensional array
consider it array of arrays
so every elements in this array is itself a single dimensional array right?
now
we can assign a single dimensional array to another single dimensional array
so

int[] b = (int[]) a[1];     

here we are creating a single dimensional array and assigning a[1](which is single dimensional array to in[] b)
so this does not give compiler error
can you find the cause for the runtime Exception?

[Abimaran Kugathasan]

int[] b = (int[]) a[1];     

There is no problem with this line. There will be a ClassCastException in some where else in your code. Could you spot it? You can't cast an array of one dimension to other dimension.

[Dieter Quickfend]

a[1] is an element of a two-dimensional array. Elements of a two-dimensional array are one-dimensional arrays.

so if you have

int[][] a = new int[3][];

you can go:

a[0] = new int[2];
a[1] = new int[5];
a[2] = new int[1];

all the elements of a, namely a[0], a[1] and a[2], will be int arrays, NOT ints.

the elements of the elements of a will be ints.

meaning a[1][0] will be 0.

[Prithvi Sehgal]

Hello,

   
Object o1 = a;  
int[][] a2 = (int[][]) o1;  

Look at these two lines, line 3 is assigning a single dimensional array to object which is legal as array is an object.
Now when you see at line 4, you are basically trying to convert a single dimensional array to double dimensional
array which is not possible, so the compilation with fail because of the error at line 4.

Hope this helps,

[Tod Stroszer]

Prithvi Sehgal wrote:Hello,

   
Object o1 = a;  
int[][] a2 = (int[][]) o1;  

Look at these two lines, line 3 is assigning a single dimensional array to object which is legal as array is an object.
Now when you see at line 4, you are basically trying to convert a single dimensional array to double dimensional
array which is not possible, so the compilation with fail because of the error at line 4.

Hope this helps,

I interpreted the above code like this:

a is a 2 dimensional array

int[][] a = {{1,2,}, {3,4}};

so here you are changing its type to Object


Object o1 = a;

and here you are trying to cast an Object to a 2 dimensional array

int[][] a2 = (int[][]) o1;

-so trying to cast an Object to a 2 dimensional array will give you a ClassCast exception and runtime error?

[Abimaran Kugathasan]

As I told you, the problem is with the line below. Check the error message, when you compile.

int[] b2 = (int[]) o1;

You are assigning a two dimensional array into one dimensional array. Here is the problem. Check it with your compiler.

[Tod Stroszer]

I see it now, thank you!

[Abimaran Kugathasan]

Tod Stroszer wrote:I see it now, thank you!

So, did you solved it?

[Tod Stroszer]

Abimaran Kugathasan wrote:

Tod Stroszer wrote:I see it now, thank you!

So, did you solved it?

Object o1 = a;  
  
int[][] a2 = (int[][]) o1;  
  
int[] b2 = (int[]) o1;  

First o1 is an Object, then it is cast to a 2 dimensional array, then the code attempts to cast a 2 dimensional array to a 1 dimensional array - this causes the runtime exception.


Yes, if you assign an array to a previously declared array reference the array must be of the same dimensions as the array reference you're assigning it to.

[Abimaran Kugathasan]

And also, have look on your original code, line number 8. Got the difference!

[muthuraj sennakesavan]

ya that is correct ...
int[][]b2 =(int [][])01 ..then it will not show any exception.

[Prithvi Sehgal]

Hey,
I guess i was too busy looking at the correct source code and didn't answer properly.

Best Regards,

[Nikhil Arora]

I am unable to understand how the output for b[1] would be 4 if there is no error at line 7?