Array and Polymorphism questions.

[Eoin Brosnan]

Hi,
I am working my way through some SCJP questions and I have come across two that I don't understand.
First one if from ExamLab.

Which is valid:

A: int[] it1 = new int[0]{};
B: new int[2];
C: int[] it2 = new int[][]{{1}}[0];
D: int[][] it3 = new int[]{0}[0][0];
E: int it5 = new [2]{}[];

Ok, I know why A&B are false, but the syntax for C&D&E is completely new to me. C is the only one that is true, but I have no idea why.

- second question removed until the source is cited -

[Rob Spoor]

I'll move this thread to our SCJP forum, but not before answering your first question.

Eoin Brosnan wrote:Hi,
I am working my way through some SCJP questions and I have come across two that I don't understand.
First one if from ExamLab.

Which is valid:

A: int[] it1 = new int[0]{};
B: new int[2];
C: int[] it2 = new int[][]{{1}}[0];
D: int[][] it3 = new int[]{0}[0][0];
E: int it5 = new [2]{}[];

Ok, I know why A&B are false, but the syntax for C&D&E is completely new to me. C is the only one that is true, but I have no idea why.

A is invalid because you either create an array with a size (new int[0]) or with its values ({}), not both.

B is invalid because it's impossible to only create an array without doing anything with it. I was actually a bit surprised by this, because it is possible to call "new Object();". Apparently the compiler is smart enough to see that an array without a reference is useless, whereas an object can still do things in its constructor.

C uses two "techniques". The first is create an array while immediately assigning its values: new int[][]{{1}}. Then its first element is accessed using [0]; the result is an int[] since the array itself is an int[][].

D tries to do the same, but the dimensions are wrong. new int[]{0} creates an array with one element with value 0. The first [0] returns that element, an int. The second [0] is invalid, since this is only allowed for arrays, not for ints.

E is just wrong.

[Eoin Brosnan]

Rob, thanks for the explanation. However isn't option C similar to B in that it creates an array but doesn't assign it to anything?
For example:

int[] it2 = new int[][]{{1,3,4}, {5,3}}[1];  

int i  = new int[]{1,2,3,4,5}[3];
System.out.println(it2[1] + " " + i);

Thanks again.

[Rob Spoor]

In example C, you still use the array. You don't assign it to a variable, but you access it nonetheless. And that's the difference with example B, where you create an array and ignore it immediately.

[Arun Arun Kumar]

what is the difference between

int[][] array1 = new int[][] {{1,2,3}, {4,5,6}};
int[][] array2 = {{1,2,3}, {4,5,6}};

[gihan dissanayaka]

arun,i think this two array initialisations is same .only two ways of expression.